High Speed GaAs Integrated Circuits

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					PROCEEDINGS OF THEIEEE, VOL. 70, NO. 1 , JANUARY 1982                                                                                    35

                   High Speed GaAs Integrated Circuits

                                                                 Invited Paper

  A6muct-Much   interest                          n
                           has been expressed i the use of GaAs          While density was not of importanceforthe           discreteFET,
MESFET’s for high speed digital integrated CiTCuits (IC‘s). Ropagdtion   which generally was rather large inareaand used relatively
delays i the 6@ to 9O-ps/gate range have been demonstrated by several
        n                                                                high pinchoff voltage active channel layers, it is of importance
laboratories on SSI and MSI logic circuita Recently, large scale digital
IC’s with over lo00 gates have been demonstrated in GpAg In this re-     in an IC because of the desire to maximize yield of functional
                            i u,
                            r t
view paper, thedevice, cc i and processingapproachespresently            circuitsand    to press circuit complexity   beyond  the     small
being explored for high speed GPAS digital circuits are presented. The   scale region (20 gates). Therefore, planar fabrication methods,
present performance status of high speed circuits and L S I circuits is  similar in concept and apperance to well-established silicon IC
                                                                         approaches, have been developed. The planar approach employs
                                                                         selective ion implantation to localize active devices in the semi-
                                                                         insulating GaAs substrate, and gate densities over 100 000/cm2
                          I. INTRODUCTION                                have been reported on very dense circuit structures [ 5 I .
         VER THE PAST DECADE, GaAs Metal-Semiconductor                      The development of planar fabricationmethods and high
         FET’s (MESFET’s) have been developed and extensively speed logic gate structures with modest power dissipation (<1
         utilized for high-frequency low-noise amplification. The mW/gate) and h g h density has created interest in the          extension
maximum available gain, noise figure, and bandwidth capabil-             of GaAs digital IC’s into the large or very large scale of integra-
ities of analog microwave FET amplifiers have been unequaled             tion range. The achievement of LSI or VLSI is important if
by any other semiconductor device in the 2- to 18GHz range. very high speed components for signal processors, computers,
These amplifiers have been widely utilized in commercial and             and memory are to be feasible in GaAs IC’s. The high com-
governmentsystems.Reliability           studieson low-noise FET’s        plexity of such systems makes it very desirable t o include large
have proven that these devices aresutiable for mostsystem                numbers of components on a single chip to reduce the large
applications. These discrete GaAs devices have also been shown quantity of high speed interconnections. Trends in this direc-
to be readily manufacturable in large quantities despite their tion are also quite evident in silicon IC implemented signal
0.5-   and 1.0-pm gate lengths.                                          processing and computational hardware 61, [ 71.
   More recently, application of GaAs MESFET’s for high speed               Very recent demonstration of high speed LSI parallel multi-
 digital circuits has also been emphasized in several laboratories plier chipsin GaAs MESFET logic [81 has proven that such
 because of the high switching speed and transconductance of              chip complexities (over 1000 gates) are feasible in a well-con-
 these devices. Propagation delays as low as 30 ps/gate at room          trolled IC process. While further development and optimization
temperature and 17.5 ps/gate at 77 K have been reported on               will certainly provide improved results, the initial observation
 minimum-area lightly loaded GaAs ring oscillator circuits [ 11. of 1SO-ps/gate propagation delays in the 1000 gate 8 bit multi-
 Even on morecomplexcircuits,such             as MSI binary divider plier structure is quite encouraging. Also, even thoughthe
 circuits,delays in the 60- to 90-ps/gate range have beenob-              1000 gate complexity level (about 3000 transistors and 3000
 tained in several laboratories using various circuit implementa-         diodes) permits a wide variety of “random” logic circuits or
 tions and processing approaches. As a consequence, frequency sequential circuits t o be developed, memory                  requirements
 dividers have been built in GaAs MESFET logic which function            (fast access RAM) will mandate even greater complexity if
 at input frequencies of 5.5 GHz [2].                                     memory chips are to be large enough to permit fast processor
   These promising high speed results have placed emphasis on integration in small volumes (needed to minimize the effect of
 achieving fabrication methodswhich result in high density FET interconnect delay on system clock speeds).
 logic gate structures with highly uniform device characteristics.          Inthis paper,avariety     of device, circuit,andfabrication
                                                                          approaches currently being employedfor high speed GaAs
                                                                          digital IC’s arepresentedand discussed. The performance of
   Manuscript received July 8, 1981;revised October 1, 1981.
   B. M. Welch, R. Zucca, P. M. Asbeck, C. P. Lee, C. Kirkpatrick, F. S. both ring oscillation and frequency divider “benchmark”-type
Lee, and G . R. Kaelin are with Rockwell International Microelectronic    IC’s is compared followed by a brief summary of the LSI status
Research and Development Center, Thousand Oaks, CA 91360.                 of GaAs IC’s.
   S. I. Long was with Rockwell International Microelectronics Research
and Development Center, Thousand Oaks, CA 91360. He is now with
the University of California, Santa Barbara, CA.                            11. GAAS DIGITAL DEVICE
                                                                                                IC          AND CIRCUIT APPROACHES
  R. C. Eden waswith RockwellInternational Microelectronics Research
and Development Center, Thousand Oaks, CA. He is now with Gigabit             The Schottky barrier gate field effect transistor (MESFET)
Logic, Culver City, CA.                                                    is the main active device used in GaAs IC’s. Fig. l ( a ) shows a

                                              0018-9219/82/0100-0035$00.75 O 1982 IEEE
36                                                                          PROCEEDINGS OF THEIEEE, VOL. 7 0 , NO. 1, JANUARY 1982

                               lwn                                           3 x 1017
           DIELECTRIC-     GATE
                                                                                                         IMPLANTATION ~ H R O U G H Z S A
                                                                                                                                        S~N,, LAYER

                                                                                                           SILICON                                       SILICON



                          7          rSECON0-LEVEL
                                                                                               400 KcV. 2.2 x 10l2 an-2

                                                                                                     IMPLANT           SOFL IC DEVICE
                                                                                                      SPECIES           APPLICATION

                                                                                               ION       DUESFET
        SOURCE                                    DRAIN

                  - 1 x 1018
                  - 1 x10l6             n-GA      4mA
                  - 6 ~ 1 0 n-Aln&+-rA.
                             ~ ~                  260A

                                                                                1015   I0.02   I     I     I   ,   ,    ,   ,
                                                                                                                                             I   I         1   I   ,   ,   ,
                                                                                                                                                                               1 .o
                         S.I. GaAs SUBSTRATE
                                                                                                                                 DEPTH Gm)
                                                                          Fig. 2. Activelayerdopantprofdes      for the n-FET channelSelenium
Fig. 1. Device cross-sectional view of (a) MESFET, (b) JFET [ 11 1, and    implant and then+ high-speed      switchingdiode   sulfur implant.
              (c) rnodulation-doped [ 161 device structures.               silicon implant can also be used for both the n-and n+ regions.

cross-sectional diagram of a typical planar       ion-implanted           stricted to a narrower range (generally 0 V to 0.7 V, the for-
MESFET fabricated by localized implantationinto a semi-                   ward gate conduction voltage for a MESFET) than was the
insulating GaAs substrate. The channel implant species, energy,           depletion-mode FET device. This restriction results in smaller
and dose are chosen so that the peak position and depth of the            I& for a given device area of an enhancement-mode FET than
doping profile (as shown in Fig. 2) result in a device with the           a depletion-mode FET, and therefore, the propagation delay is
desired pinchoff ( 5 )   voltage. If the device channel is conduc-        generally higher and powerdissipationlower        for GaAs IC’s
tive at Vgs= 0, the pinchoff voltage is negative and a depletion-         implemented with normally OFF transistors.
mode (normally ON) device is obtained.The logic voltage                      JFET devices, using a p+ gate stripe formed by selective ion
swing can extend from below - Vp to the onset of gate conduc-             implantation, have also been successfully employed in GaAs
tionduetothe        forward-biasedgate-source junction. Larger            digital IC’s [ 111. A diagram of a GaAs JFET is illustrated in
logic swings will produce higher speed circuitsfor agiven device          Fig. l(b). The JFET is somewhat more difficult to fabricate
type since Ids (Y ( vgs V p ) 2 when vgs vp< 1.2 V for GaAs
                        -                  -                              than a MESFET because of the additional p+ implant process
FET’s. Therefore, higher Ids per device area will be available            steps and the precise control of the p+ junction depth neces-
with increasing gate bias for charging load capacitances. Fur-            sary to control the pinchoff voltage of the device. However,
ther increase of logic voltage will result in velocity saturation         sufficient control has been obtained, at least for SSI circuits.
limited currents (Ids (Y (vgs V,)) and no further improvement
                               -                                          Gate lengths in the 1- to 2-pm range are readily achievable with
in speed shouldbeobserved. The speed-power product will,                  optical lithography. The greater built-in potential of the p+-n
however, be greatly increased by the higher logic voltage swing           junction provides a higher forward-bias gate conduction limit
[ 91. Thus a superior approach for higher speed circuit perfor-           (approximately 1.1 V) which should provide a significant speed
mance would be to increase gm/Cin (or the currentgain-band-               advantage for  enhancement-mode        JFET’s over MESFET’s,
width product) of the switching transistor. This can       be done        assuming that source and gate     resistances are minimized on
by reducing source resistance and gate length through process             this structure [ 121. However, the speed performance reported
improvements(or by substitution of a higher performance                   t o date on thesedevices has not yet equaled the MESFET. The
device type as discussed in [ l o ] ).                                    larger available logic voltage swing will also be beneficial in
   If the built-in potential of the Schottky gate metal capable
                                                         is               increasing noise margins for normally OFF JFET logic over
of preventing current flow in the channel & = 0, an enhance-
                                               at                         normally OFF MESFET logic.
ment-mode (normally OFF) device results. This device requires                The choice of a particular type of FET device is associated
forward gate bias (v,, > 0) to enable flow of I d s . Therefore,                 a
                                                                          with choice                     fabrication
                                                                                             of circuit and                  The
the logic voltage swing of an enhancement-mode FET is r e                 depletion-modemetal-semiconductor (Schottky barrier) FET
LONG et al.: HIGH SPEED GaAs IC’s                                                                                                       31

(D-MESFET) is the most widely used device, and also is the
one that has given the highest performance to date. Circuits
employing depletion-mode MESFET’s pose the least fabrication
problems (see Section 111) because Schottky barriers on GaAs
                                            F=A+B+C                                         A0
are easier to fabricate than p-n junctions, and the larger (typ-
ically 1-V) logic swings associated with D-MESFET circuits
avoid excessively stringent requirements for FET    pinchoff volt-
age uniformity. Because any regions of the source-drain chan-
nel not under the gate are conductive in D-MESFET’s, precise
gatealignments are notrequired,nor are special gate recess
etch processes or other means to avoid parasitic source and
drain resistances necessary. The MESFET fabrication simplicity
makes it considerably easier to achieve high yields than with
more complex device structures. On the other hand, however,
logic gates employing depletion-mode active devices necessarily                                                             F=X

require some form of voltage level shifting between FET drains
 and gates to meet turnoff requirements, and usually require
two power supplies, imposing some penalty in terms of wafer
area utilization. An exception to the two    power supply require-
mentsfor D-MESFET circuits is the enhancement-depletion
logic approach [ 131 which uses 0.4-V < Vp < 0.1-V MES-
 FET’s with diode level shiftingin single power supply logic

    Enhancement-mode MESFET’s (E-MESFET’s) offer circuit
 simplicity because the logic gates   :require only  one power
 supply, but the permissible voltage swing is rather low because
 Schottky barrier gates on GaAs cannot be forward biased above
                                                                               :                                          F=A+B+C

 0.6 t o 0.8 V without drawing excessive currents. A 0.5-V swing
 is a desirable goal for the operating range of ultra low power
 circuits, but very tight control is required in order to fabricate
                                                                    Fig. 3. Enhancement-mode JFET or MESFET circuits [ 111, [ 121,
 uniform, very thin active layers, so that they are totally de-                   Simpledirect-coupled
                                                                      [ 1 8 ] . (a)                     FET logic (DCFL) NOR gatewith
 pleted at zero gate bias voltage and yet give good device trans-     resrstor load.(b)Pseudocomplementarybuffered       inverter gate.(c)
 conductance when the device is turnedon.For            reasonable    combination of source-follower logic with the circuit of (b) to give a
                                                                      buffered NOR gate. This type of approach has been extended to two-
 noise margins and good dynamic performance, standard devia-          level gates as well.
 tions of FET pinchoff voltage of the order of 25 mV could be
 required-a very difficult goal for GaAs FET’s.
    Implementation of a MOSFET or MISFET (Metal Insulating          swings of only 100 mV or so. This would result in extremely
 Semiconductor FET) technology in GaAs would eliminate the low speed-power products. To utilizethese devices for LSI
 logic swing limitation completely, but attaining such      devices GaAs circuits,however, an effective means of fabrication of
 has provendifficult.     Some simple ring oscillators have been    large numbers of these transistors with nearly identical device
 fabricated using directly coupled FET logic implemented with characteristicsmust be developed. At the presenttime, only
 buried channel GaAs MOSFET’s and resistor loads [ 141. How- discrete transistors have been demonstrated.
 ever, at this point, stable oxides have not been‘achieved insuch
 circuits, so that gate threshold voltages shiftwithrespect to Enhancement-Mode Circuit Designs
 the prior input signal history. This limitation has constrained       A number of circuit designs for basic logic gate structures
 the demonstration of GaAs MOSFET’s to ring oscillators and         have been proposed or demonstrated utilizing normally OFF
 other simple circuits in which the input waveform has a sym-       FET’s in conjunctionwith resistor ordepletion loads               and
 metric(50-percentduty        cycle) nature,buttheperformance       Schottky-barrier level shifting diodes. These have mainly been
 observed to date does not supportMOSFET’s in general digital       orientedtoward use of the MESFET or JFET as the active
 circuit applications. Efforts to improve the stateof GaAs oxide switching devices. The following          subsection will summarize
 technology are continuing,    however.       The use of InP  for   and review several of these circuit approaches.
 MISFET devices may be more promising than GaAs [ 151.                 The simplest circuit approach, direct-coupled FET logic
   High mobility FET devices are also being developed for use       (DCFL), is illustrated for a 3-input (positive) NOR gate in Fig.
in GaAs IC’s. These devices take advantage of the greatly re-       3(a). Inthisapproach, a logic “0” corresponds to a voltage
duced ionized impurity scattering possible at 77 K in a lightly     near zero. A logic “1” correspondsto a positivevoltage capable
 doped n-GaAs channel when free carriers are introduced through of fully turning on the normally OFF FET’s, a value usually
a wide-gap n+-A1,Gal -,As heterojunction as shownin Fig.            limited by the onset of gate conduction in the FET; typically
 l(c). These structures have beenreferred t o as modulation-        on the order of 0.6 V to 1.4 V depending on what technology
 doped FET’s [ 16 ]or high electron mobilitytransistors (HEMT) is used (MESFET, JFET, or HJFET). It has been proposed to
 [ 171, and electronmobilities of 80 000 cm*/V * s at 77 K have     place input FET’s in series, generating the NAND function [ 181.
been reported [ 161. Such FET’s, fabricated with short gate         However, theimplementation of such design wouldappear
lengths, should achieve high g, and fr with very small logic        impractical because the on resistance of the conducting FET’s
 38                                                                     PROCEEDINGS OF THE IEEE, VOL. 7 0 , NO. 1, JANUARY 1982

 would cause larger threshold shiftsthan couldbe tolerated
 with the very low logic swings of E-MESFET logic. (It would
 probablywork      with E-JFET’s or H-JFET’s, however, and
 should represent no problem with MOSFET’s.)
   A significant improvement to the DCFL gate shown in Fig.
 3(a) would be to substitute forload resistor RL,an active load
current source made withanormally             ON (depletion-mode)
FET, with its gate tied to the source. Such a nonlinear load
would sharpen the                              and
                     transfer characteristic significantly
improve the speed and speed-power products of the circuits
(by perhaps a factor of 2). The fabrication of the depletion-
mode active load requires a dopant concentrationprofiie differ-
entfromthat      of theenhancement-mode devices. Although
enhancement-mode demonstration circuits have been fabricated
using a single active layer, the multiple localized implantation
fabrication technique used for the Schottky-diode FET logic
(SDFL)approach (discussed later in this section)could be
applied to such enhancement-mode circuits so that depletion-
mode active loads could be used. An additional nonlinear load
component which shows great promise for enhancement-mode
logic circuit is the saturated drift velocity resistor [ 121, [ 191    i.
                                                                      F g 4.  Single-supplyenhancement-mode     MESFET NOR gate   circuits
in which the current limiting action is enhanced by the use of         [ 13 1. (a) Quasi-normally off >input NOR gate. (b) 3-input NOR gate
                                                                       with pseudocomplementary buffer; note similarity to Fig. 3(c).
a shortchannel gateless FETstructure. Here, thecurrent is
limitedbyvelocity      saturation of the channel electrons, and
very low saturation voltages should be possible withshort             smaller than IL,while its “ON” current is well above ZL , This
sourcedrain gaps, thus reducing power dissipation.                    can be achieved in depletion-mode MESFET’s with reasonably
   From a static point of view, the fanout capability of DCFL         small pinchoff voltages (5w - 0.4 V) with zero or slightly
is excellent since it is determined by the very low gate leakage      positive gate voltages, so that only a single power supply is
currents. However, from a dynamic point of view, the switch-          required. For example, with Vp = - 0.4 V and %,(ON) = +0.7
ing speeds are reduced by the gate capacitance loadings by a          v, Gs(OFF) = +0.1 v, We have &(ON) = 4.84 x &(OFF), an
                                                                      ample margin for switching.
factor of approximately 1/N where N is the number of loading
gates, as in silicon MOS. In general, the current through the           A number of circuit    approaches       for single supply E-D
                                                                      MESFET logic have been proposedandanalyzed [ 131. Fig.
resistor RL, saturated resistor, or active load is kept fairly low
in DCFL in order to reduce static power and improve noise             4(a) shows the circuit diagram for an elemental 3-input NOR
margin byreducing theoutput “low” voltage of the FET.                 gate inthe most promising of these publishedapproaches.
Consequently, the output risetime under heavy fanout loading          This uses source follower logic to obtain the positive OR func-
conditions is very poor. This canbegreatlyimproved             with   tion, with single diode level shifting and a resistor pulldown Rs
thepseudocomplementary    outputbuffer           configuration of     to drive the output inverter FET. The analysis in [ 131 indicates
Fig. 3(b), at very little increase in static power dissipation, but   proper gate operation for MESFET pinchoff voltages in the
this circuit performs only logic inversion [ 121. By combining        - 0.4 < Vp < +O. 1-Vrange, which is several times the allowable
the inverting buffer with a source-follower positivwa input           range width for E-MESFET logic and much more reasonable
structure as shown in Fig. 3(c), a general multiple-input NOR
gate can be achieved whichhasexcellent fan-in and fan-out                  -                                          -
                                                                      in terms of practical fabrication control. The supply voltage
                                                                      (&a 3 v) and logic Voltage Swing ( Vout 0.2 v t o 2.4 v)
                                                                      values used are even larger than those used in the SDFL D -
drive capabilities at very modest static power levels [ 181. Un-
           this      source-oR/pseudocomplementary inverter           MESFET approach, so that very low PD7d products would not
gateconfiguration is also quite complex,requiring 7 FET’s             be expected. The gate output of Fig. 4(a) has the same drive
and 2 resistors for a 4-input NOR gate, which can be expected         problems as that of Fig. 3(a), but this should be improved for
to consumeconsiderablechiparea           and have significant self-                                                   of
                                                                      heavily loaded gates with the buffer structure Fig. 4(b). This
capacitance.                                                          is, of course, very similar (except for the two voltages shifting
                                                                      diodes) to the enhancement circuit of Fig. 3(c).
                                                                        At the present time, demonstration circuits containing up to
Enhancement-Depletion-Mode MESFET Logic                               15 gates have been fabricated using enhancement-mode FET’s.
   Because of the nonlinear, approximately square-law nature          MESFET-implemented circuits have exceeded the performance
of the FET Ids versus   V,  relationship, it is not always neces-     of JFET circuits in both speed and power even through the
sary to completely turn off theFET (i.e., make          V,  more      higher JFET logic swing should provide greater speed        as dis-
negative than Vp)in orderto obtain switching behavior.                cussed above. Complexity of enhancement-mode circuits has
   Drain dotting of many FET’s, as in Fig. 3(a), necessitates         probably been limited by fabrication technology and threshold
turning all of the FET’s nearly off so that the s u m of all of       uniformity. Comparisons of ring oscillators      and frequency
their drain currents is substantially less than the load current      divider demonstration circuits are made in SectionIV.
ZL through RL require to produce an output voltage near the
switching threshold of the next driven gate. However, if only         Depletion-Mode Logic Approaches
a single FET switches the load, it is only necessary to reduce         BufferedFET logic (BFL) [20] and Schottky Diode FET
its drain currentinthe     OFF stateto a value significantly          Logic (SDFL) [ 2 11 gate circuit approaches have been exten-
LONG et al.: HIGH SPEED GaAs IC’s                                                                                                                         39

BASIC INVERTER                         +45V
                   LOAD      +A
                                                       SOURCE                                                    F
                                                      FOLLOWEA                                                OUTWT


     -2v     u
           INPUT PULSE                                                OUTPUT PULSE
                                                                                                                               a1 SDFL NOR GATE

                          AMPLIFIER AND
                           LOGIC SECTION

                                               OUTPUT DRIVER
                                                AND VOLTAGE
                                                LEVEL SHIFTER


             NOR                                     NAND

                   60          2-A.B                                  z-   A.B
                                                                                                                             bl SDFL OWNAND GATE



                                                                                                                        c i SDFL ORINANDNIIRED-AND GATE

                                                                                     Fig. 6. Comparison o f 1- , 2- , and 3-level SDFL gate configurations
                    (d)                                         (e)
                                                                                       [ 2 2 ] . All FET’s are depletion-mode, typically -1.5  ’
                                                                                                                                               V  <   <
                                                                                                                                                    -0.5 V ;
Fig. 5. Basic circuit confiiurations for buffered FET logic [ 2 0 ] . (a)              unshaded diodes are very small high speed switching Schottky diodes
  Basic inverter circuit. (b)-(e) Options for the input section for NOR                while shaded diodes are larger area, higher capacitance voltage shift-
  SASD and combined NANDNOR functions.                                                ing diodes.

sively employedfordepletion-mode          GaAs IC’s. Circuit dia-                    applications requiring lower LSI complexity (200-500 gates).
grams for NOR gates formed by        these two approaches        are                 Demonstration circuits with about 20 gates complexity have
presentedin Figs. 5 and 6 . The BFL circuitemploys FET’s                             been reported in the literature using high pinchoff BFL gates
to perform a NOR (or 2-input NAND for a dual-gate FET) func-                         [20] ; however, larger BFL circuits ( a 2 0 0 gates) using low
tion at the input. The output     is driven by a source follower,                    pinchoff FET’s are currently in development.
with level shifting diodes to restore the required logic levels to                     The SDFL circuit approach, shown in Fig. 6, permits high
the +0.7-V (high) to - Vp (low) voltages required by the input                       speed operation comparable to the BFL approach, but results
FET’s. The source-follower output driver yields a gate struc-                        in considerable savings in area/gate (600 to 2000 pm’) and in
ture which has relatively low sensitivity to fanout loading and                      lowerpowerdissipation      (0.2to 2 mW/gate). SDFL utilizes
loadcapacitance.     Also nodcoutputcurrent           is required to                 clusters of small high-performance Schottky diodes to perform
drive subsequent BFL gate inputs. Fan-in is limited for practi-                      the logical positive-OR function on groups inputs which may
cal purposes to 3 for a NOR gate by the drain capacitances of                        then be further processed with the normal FET logic functions
the input transistors and thearea required by these devices and                      (series-NAND,wired-AND , etc.). Fig. 6 shows SDFL gate cir-
2 for the NAND gate because of voltage drop in the       series FET’s                circuits diagrams for single- , two- , and three-level logic gate
which results in threshold shift.                                                    configurations [ 221. Note that the SDFL gate structure allows
  Nearly all BFL circuits reported to date utilized relatively
                                            have                                     virtually unlimited fan-in at the first (positive-OR) logic level
high pinchoff voltages (- 2.5 V) and three level-shift diodes for                    (SDFL circuits with    up    to 8-input NOR gates have been
convenience in fabrication (since epi/implant-mesa approaches                        described in publications) 1231, but it has the same practical
provide suitable threshold control for   large logic voltage swings)                 restrictions to a fan-in of 2 at the second (series FET NAND)
and, therefore, have exhibited high power dissipation per gate                       and third (wired-AND) levels if dynamic performance is to be
(40 mW typical). However, since fabrication          methods and                     maintained.
pinchoff voltage control have been improvedwith ion-implanted                           The SDFL circuit approach offers large savings, not only in
planar approaches, there is no reason why low pinchoff (- 1-V)                       power, but also in circuit   area,    over previous D-MESFET
MESFET’s and two level-shift diodes should not be employed                           approaches. The circuit area savings comes about because of
for advanced BFLgates designs. These modifications should                            the simplicity of the gate design and replacement of (large)
reduce power dissipation to -5 mW/gate by allowing operation                         FET’s with very small (typically 1 pm X 2 pm) Schottkydiodes
at lower voltage and current levels with relatively little sacrifice                 for most logic functions. The fact that thediodes are 2-terminal
in speed and could make BFL circuits a possible candidate for                        devices also significantly reduces the number of vias and over-
40                                                                         PROCEEDINGS OF THE IEEE, VOL. 70, NO. 1, JANUARY 1982

crossings required in most circuits as compared to the vias and          as shown in Fig. 7(b) [ 25 I , [ 26 1 . Other similar processes not
overcrossingsneededwhen3-terminal        FET’s areused as the            illustrated [27], [28] have   used        both        and
                                                                                                                       epitaxial ion-
logic elements.                                                          implanted layers. Precisethicknesscontrol            has provenvery
  The input logic diodes require a lower carrier concentration,                                           IC’s
                                                                         difficult for GaAs E-MESFET utilizing vapor phase epitaxy.
lower sheet resistance implant than the FET channel to opti-             This has necessitated controlled thinning of the epitaxial layer
mize their reverse-bias capacitance and series resistance. Thus          using self-limiting anodization and stripping techniques which
SDFL circuits require two separate implant steps using     local-        donot     appear        for Workers
                                                                                         practical LSI.          employing   ion-
ized implantation into selected areas of the substrate.                  implanted layers [28] appear t o omit this step. However, all
  Fanout of the basic SDFL gate is limited 3 without buffer-             of these E-MESFET approaches requirethe use of recessed gate
ing or using wider channel widths in the driving gate. However,          structures. Recessed gates circumvent some of the difficulties
the propagation delay is not as sensitive to fanout loading as           associated withthe high series resistance        surface depletion
the direct coupled FET logic    (or NMOS) approach, since the            layers often observed on very thin FET channel layers. A deep
gate-source capacitance of the switching FET is discharged by            implant or thicker epitaxial layer      is initially provided (lower
the pulldown active load instead    of the preceding FET drain           sheetresistance),and the Schottky gate is recessed into the
current.                                                                 GaAs surface by using a chemical etchant or chemical anodiza-
                                                                         tion method. This     process approach providesimproved FET
     111. GAAS DIGITALIC FABRICATION         APPR0ACHF.S                                                              to
                                                                         characteristics by lowering source/drain gate series resistance,
  Inthe followingsection,a     reviewof fabricationmethods               but the uniformity, control, and yield of the resulting devices
currently in use on GaAs digital IC’s will be presented. These           for LSI applications are in serious question. For example, the
methods areschematicallyrepresented      in Fig. 7.While     the         difficulties encountered in obtaining adequate uniformity using
selection of processingtechniques to be discussed is by no               implanted layers and recessed gate structures for E-MESFET
means exhaustive,it     is representative section
                           a            cross      of                    devices has led workers to explore innovative, less demanding
approachessuitableforfabrication     of FET-based GaAs IC’s              (in terms of device uniformity) circuit concepts such as quasi-
with gate lengths as short as 1 pm for optical lithography or            normally OFF MESFET logic [ 13I .
3  pm for  electronbeam      or certaintypes     of self-aligned            Bothcontactphotolithography          [ 271 and EBL [28] have
procedures.                                                                                                             ,
                                                                         beenemployed in thefabrication of L = 1-pm circuits of
                                                                         this type. The fabrication of these E-MESFET circuits is quite
Mesa-Implanted D-MESFET                                                  similar to that of the mesa D-MESFET approach except for the
  InitialGaAs IC efforts werebased on the well-developed                 variationshown in Fig. 7(b)whichuses              the ohmiccontact
depletion-modeGaAsMESFETmesafabricationtechnology                        metallization for thefirst-level interconnects, with the Schottky
used in the production of low-noise microwave FET’s. Isola-              gate metallization also sewing for thesecond-level interconnects
tion between active devices is accomplished in this approach             1271.
by etchingthroughtheepitaxialorimplantedactivelayer.
Basically, as shown in Fig. 7(a), a discrete MESFET fabrication          Self-Aligned Epitaxial D-MESFET
process is used with the addition of a second layer of metal                The development of a planar technology          in Si IC’s marked
and a dielectric layer for interconnecting       the various circuit     theturningpointwhich            led to rapidprogresstowardLSI,
elements. First layer to second layer metal overcrossings have           suggesting that planar development in GaAs will also have a
also fabricated    using plated           &-bridges ratherthana          similar impact. Recent work toward the development of strua
dielectric. Initial work employing epitaxial techniques for the          tures hasled to thefabrication of D-MESFETGaAs IC’s shown
(-2000-& active layers         encountered difficulties in achieving     in Fig. 7(c) [30]. In this fabrication process, the structure can
the uniformityandreproducibilityrequiredfor              IC’s. Subse-    be made planar by replacing the mesa isolation step withselec-
quently, an implanted layerwas substituted for epitaxiallayer,           tive proton, oxygen, or boron bombardmentwhich renders the
withtheimplantationmadeintoa               high resistivity epitaxial    underlying epitaxial material semi-insulating. This fabrication
buffer layer or directlyinto the semi-insulating substrate. Both         method may use epitaxial or implanted layers. However, it is
techniques are currently being used in the mesa-implanted D-             still limited t o a single active layer. The qualityof the electrical
MESFET fabrication of buffered FET logic (BFL) circuits          [ 201            and long-term
                                                                         isolation the                       reliability in high temperature
shown in Fig. 7(a).                                                      operation of this high resistivity ion bombarded layerneeds t o
  While this mesa fabrication approach has the important ad-             be investigated in greater detail.
vantage of process simplicity, it has been, atleast in its present         Unique t o this fabrication approachis the use of a self-aligned
form, restricted to applications in which only a single active                                              not
                                                                         FET gate scheme which does require a Schottky gate              mask-
layer (implanted or epitaxial) is required, i.e., circuits in which      ing step. This process requiresthe use of A1 gates since alloyed
only a single type of device needs t o be optimized. The density         ohmic contacts are fabricated after the formation of the gates
and yield limitations associated with mesa structures may in-                   I
                                                                         and A provides a suitable Schottky barrier material capable of
hibit the extension of this technology to LSI/VLSI. Mostof               withstanding the subsequent 45OoC alloying cycle. Since the
the work using this approach has been directed toward             high   ohmic contacts are composed to AuGe and the gates are made
speed MSI logic, withexcellent results achievedusingl-pm                             o
                                                                         of Al, a M barrier layer is required t o separate the A1 and Au
contact photolithography [ 241.                                          based metallization systems, adding some complexity            t o the
                                                                         process and raising some concern regardingthe long-term relia-
Mesa Epitaxialllmplanted E-MESFET                                        bility of mixing A1 and Au metallizations. Circuits of this type
  Enhancement-mode GaAs MESFET devices have        also been             have been fabricated using contact photolithography and have
fabricated on epitaxiallayersandisolatedbymesaetching.                   yielded promising results in the lower MSI level of complexity,
This technique has provided Direct Coupled FET Logic circuits                                                        to
                                                                         with reasonable promise of extension larger circuits [ 21.
LONG er ai.: HIGH SPEED GaAs IC’s                                                                                          41

                                                                                   UF E
                                                                                  S R K OF I W L A T I N G

Planar Implanted E-JFET                                            7(d). Thisapproach uses a junction FET (JFET) to provide
   A planar enhancement-mode E-JFET fabrication technology                                                          and
                                                                   DCFL circuits with resistor loads. The FET channel heavily
[ 4 ] , [ 1 1 1 currentlyunderdevelopment is illustrated in Fig.   doped regions under the ohmic contacts are produced by using
42                                                                             IEEE,
                                                                        PROCEEDINGS OF THE             VOL. 70,NO. 1 , JANUARY 1982

multiple selective n-type implantation stepsand the gate region       ing interconnects) intest IC’s. MSI/LSI circuitswith up to
is fabricated using ap-type implant. Sinceisolation through           1000 gatecomplexities have been successfully demonstrated
mesa is no longer needed, a planar structure is obtained.             with this fabrication technology with gate densities as high as
   The principal interest in the E-JFET is in its larger allowable    350 gateslmm’. A complete description of this particular
voltage swing before the onset of gate conduction, although           planar approach has been previously reported [ 51.
the structure also offers potentially lower parasitic source and
drain channelresistances than the E-MESFET. The E-JFET                Electron-Beam Lithography
might also offer better control over gate threshold voltage than         Direct writing electron-beam lithography(EBL) has        been
the E-MESFET in that V, can be controlled both by the n-              utilized as an alternative to projection optical lithography when
implant and by controlling the depth of the pyn junction (ina         submicrometer gate lengths are required. The performance of
manner similar to controlling the emitter depth of a Si bipolar       the GaAs MESFET (current gain-bandwidth product) increases
transistor). This fabrication technology has been used to pro-        as linewidths below 1 /m are used, and very high speed GaAs
duce E-JFET devices withgatelengths of 1 pm, using a Mg               IC’s, which employ gate lengths in      the 0.5- to 0.7-pm range
implantforthep+       gate.Thiswork     is at an earlier stage of     have been demonstrated [ 21. Ring oscillators using 0.6 pm X
development than the D-MESFET or E-MESFET efforts, but                20 pm normally OFF GaAs MESFET’s were fabricated by EBL
some inverter ring oscillator results have been published with        [ 291, and propagation delays as low as30 ps/gate were observed
gate densities of 200 gateslmm’ with projected [ 4 ] , [ 11 ] gate    ata power dissipation/gate of 1.9 mW. These same devices
densities of 800 gateslmm’. In principle this appears to be an
                                                                      also were used in a divide-by-8 circuit with 66-ps/gate equiva-
attractive fabricationandcircuit     approachforenhancement
                                                                      lent delay.
logic. The main drawback for practical LSI results from the              Another advantage provided by direct-write EBL is the ability
yield limiting additional processing required for JFET’s in           to rapidly modify circuit and device designs to optimize circuit
comparison to D-MESFET’s. In particular theadditional p+              performance. Delays associated with photomask procurement
implant and subsequent self-aligned gate electrode pose diffi-        are thereby eliminated. The writing rate of all but the most
cult fabricationproblemsat the I-pm level and below. Also,            exotic EBL systems is, however, much too     slow to be considered
it would appear that a depletion-mode active load would be an         for LSI or VLSI circuit fabrication on large substates. Writing
attractive alternative to the currently used resistor load con-       rates are also limited by the sensitivity of electron-beam resists.
figuration, leading to additional processing in theform of            Application of direct-write EBL for LSI circuits will, for the
implants and Schottky barriers. Optimized      development of         most part, be conditional on the development of higher sensi-
planar JFET logic will possibly require one (or  more) additional     tivity electron resists or EBL systems with fasterbeam scanning
mask levels than D-MESFET fabrication approaches.                     and higher beam intensities.

Planar Implanted D-MESFET                                                           IV. PERFORMANCE       GAAS IC’s
  The planarimplanted D-MESFET GaAs IC fabrication a p                  In the following section, the high speed performance of
proach is illustrated in Fig. 7(e). Planar circuits are fabricated    various GaAs digital IC approaches will be presented and con-
as in the planar E-JFET approach by using multiple localized          trasted. This comparison will be based on reported speed and
ion implants directly into semi-insulating GaAs substrates [ 51.      power dissipation of GaAs IC ring oscillators and binary fre-
Hence, individual devices can be optimized by using different         quency dividers, demonstration circuits which nearly all of the
implants, and theunimplanted GaAs substrate directlypro-              approaches discussed in Sections I1 and 111 have successfully
vides isolation between devices. Very uniform MESFET device           fabricated andevaluated.      Inaddition, a brief description of
parameters have been obtained over 1-in GaAs IC wafers using          the statusof Large Scale Integration in GaAs will be presented,
the direct implant approach. Standard deviations of pinchoff          along with adescription of the performance of a 1000 gate
voltage as low as 34 mV have been observed, while 50 to 80            parallel multiplier circuit implemented in SDFL.
mV is routinely obtained [ 38 J . This fabrication method con-           Ring oscillators (RO)are a widely used, simple circuits con-
veniently complements the Schottky diode-FET logic (SDFL)             sisting of chains of an odd number (N) of inverters or logic
circuit approach [ 2 11, which requires the use of at least two       gates. If the loop gain exceeds 1 then an oscillation is obtained
different implantationsfor optimizing both D-MESFET and               with frequency f. The propagationdelay TD of the inverters or
high speed Schottky barrier   switching diodes. Also, planar          gates is related to f by TD = 1/(2fn3,and the dynamicswitching
devices can generally be located closer togetherthan mesa             energy PDTD is also provided by this measurement. The param-
devices, because space need not be allocated for mesa side            eters extracted    by the ring oscillator techniques represent
walls.                                                                nearly intrinsic speed and power since capacitive loading due
  Thefabrication process outlined in Fig. 7(e) is much less           to parasitics are generally minimized by a compact layout and
prone to surface related problems than other approaches be-           fanout of 1 is usually employed. Since the propagation delay
cause the GaAs substrate is totallyprotectedby           dielectric   of all logic families increases by varying degrees with fanout
layers throughout the fabrication   process; windows are opened       and capacitive loading,ring oscillator results are notnecessarily
in the dielectric only where ohmic contacts, Schottky barriers,       representative of the performance to be expected in larger,
or interconnect metallizationsarerequired.       One micron fea-      more realistic logic circuits. In spite of the above limitations,
tures are resolved using reduction projection photolithography        the R O is still a useful evaluator of the intrinsic speed of a
inconjunctionwithliftoff,      plasma etching,and ion milling         circuitdevice combination,and at least provides a lower bound
techniques. The metallizations used in these IC’s are AuGe for        on propagation delay and dynamic switching energy.
alloyed ohmic contacts and TiPtAu for    gates, first- and second-      Table I presents a summary of published ring oscillator data
level interconnections. At the present level of development                both
                                                                      from enhancement-     and           depletion-modeapproaches
this fabrication technology has demonstrated gate areas as low        employed in several laboratories. Propagation delays as low as
as 600 pm’lgate or circuit densitiesof -800 gateslmm’ (includ-        30 ps/gate are shown on a submicron gate length RO (1 7.5 ps/
 LONG e t d.:HIGH SPEED GaAs IC's                                                                                                                                                                                                                                                    43

                                 TABLE I                                                                                                                                                               TABLE I1
            RINGOSCILLATORS SPEEDPOWER PERFORMANCE FOR                                                             SEVERAL                                GaAs I FREQUENCY
                                                                                                                                                                C       DIVIDER
                          GaAs IC TECHNOLOGIES                                                                                                                                                                                                                                   ~

                                                                                                                                   6ak IC                   H e a c ul e d
                                                                                                                                                          T h e oi rseut r itta l                                                       Equivalent             P0-r
                                                                                                                               T e c h a p po g y c h
                                                                                                                                       nol roa                                          k x . T o g g l e F.   Pax.       Toggle F.                      Dlssipatlon             Pn7*
                                                           Gate L e n g t h                                                                                                                                                                                               ~~~~

                                                           6 Gaze Y l dPh o p a g a t i o n           Speed-Power             1 y m D/SOFL                 D.F.F. i 2                       115   T~             1.9 WZ                   105 PS         2. 5 M I g a t e 0.26 pJ
     Saurce                                                 (vm = F a n l n t r oadnuocut t e l a y
                                                                   "3) P F              D                                     R o c k r e l l [23]         (HOR U T E )
                                                                                           DE                 DJ

     Hughes [ 2 8 3           MESFET/BFL                   0.5              50            34              1.d          It1
                                                                                                                              0.7 v m o m t                n.F.F.          +   z            it5   T~             3.0                      67             40                2. 68
                                                                   )I                                                         TCSF [30]
     F.P. [201                D I E S F E T t E F L NOR    1   x   20                     86              3.9          2/2    1 urn O I B F L              D.F.F.          t    2           1/5   T~             2. 2                      91            78                7.1
                                                                                                                              Hughes 1333
     Rockwe11                 WIESFET.'SDFL NO09           1   x   10                   120               0.040        211
                                                                                         52               0.063        211    1 urn D/EFL                  NAANDiNOR           i    2       It2   T~             4.5                     111             40                4.4
     Thonson   PESFE7/ECL
          CSF[30]                                          0.75         x    20           68              2            1/1    H.P. [20]                    CM. CLKK
     FUJlTSU    Dl1           EMESFETID-LOAD               1.2     I        20          170               0.12         It1    0.6 v m D/BFL                NANDINOR            + 2          112   lo             5. 5                     91             40                3.6
     FUJITSU [32]             SELF P L I G N E t O C F L   1.5     I(       30            50              0.287               LEP 123                      C . CLOCK
     N.T.T.C2gl               EHESFETI3CFL                 0.6     x        20            30              0.057         It1   0.6 y m E/DCFL               O.F.F.          t   8            1/4   T~             3.8            0.079     66             1.2
                                                                                        17.5.             0.616        111    N.T.T.  [291                  (NU7)

                                                                                                                              1.2 urn EIOCFL               CMP. CLOCK                       1/4 T~               2.4                     100             3.9              0.39
     nco 321                  EJFETIPreudo                 1.0     x        13          150               0.06         It1    HEC 1341                     t 2 NOR
     Thonson EMESFETlquasl-
         CSF[131                                           1.0     X        35          105               0.23         111
                                                                                                                                                                                         FREWENCY DIVIDERPERFORMANCE
    'Measured     a t 77-K.

                                                   h                                     1                                                                                                        \                               \                                   \


                                                                                                                              Y            1\

                                                                                                                                                   ; 1"          10 x PD'\\                                           \                              \
                                                                                                                                                                                                                           \                              \

                      (b)         fmax = 115 r D                                                                                                                                             PDWER DISSIPATIONPER GATE

                                                                                                                              Fig. 9. Speed-power compahonof            a
                                                                                                                                                                        variety    of frequency divider
                                                                                                                                  approaches corresponding to Table 11. At the far left, for reference,
                                                                                                                                  is a Josephson Junction divider. Its power dissipation has been scaled
                                                                                                                                  byafactor or 10 to accountfortheinefficientheatdissipationin
                                                                                                                                  liquid He.
                              I                                                                       I
                      (c)         fmax 1/4 r D
                                                                                                                              capacitance also increases directly, the ratio of device capaci-
                                                                                                                              tance to parasitic capacitance increases and  thus the propagation
                                                                                                                              delay asymptotically approaches the intrinsic device perfor-
                  C                                                                                                           mance. Therefore, it is very important t o only compare data
                  c                                                                                                           for circuits with the same gate length FET's (and to a lesser
                                                                                                                              extent gatewidths) to obtain ameaningfulcomparison               of
                                                                                  C C
                                                                                                                                The performance of logic circuits such as a binary ripple fre-
                                                                                                                              quency divider is a more meaningful indicator of the overall
  F g 8.  Four circuit implementations of binary ripple frequency dividers
    (divide by 2 circuits) with different theoretical maximum toggle fre-                                                     performance of a particular circuit anddevice approach because
    quencies. These frequencies are expressed as inverse multiplesof logic                                                    fan-in, fanout, and capacitance loading are greater in an actual
    gatepropagationdelays.(a)       1/4 rd D-type  single-clocked FF. (b)
    1/5 7d D-typesingle-clocked FF. (c) 1/4 Td master-slave, compla
                                                                                                                              sequential or combinational circuit than in a ring oscillator.
    mentary-clocked FF, (d) 1/2 rd master-slave complementary-clocked                                                         Fig. 8 depicts the circuit diagrams of four types of frequency
    FF.                                                                                                                       dividers which have been implemented in GaAs. The theoret-
                                                                                                                              ical maximum toggle frequency of these dividers depends on
  gate at 77 K-a speed rivaling that of the Josephson Junction).                                                              the number of logic gates which must serially stabilize before
  However, the interpretation of the Table I data is complex;                                                                 the output reaches its correct state. This factor      ranges from
  performance is expected to vary with gate length and width                                                                  2Td for a   complementary-clocked        NAND/NOR implemented
  and with other factors, such as source resistance and parasitic                                                             flip-flop (Fig. 8(d)) to 57d for the D-type flip-flop shown in
  capacitance. At gate lengths in the 0.5- to 1.5-pm range, trans-                                                            Fig. 8(b). Thus equal gate delays will produce 2.5 times higher
  conductance shouldincrease roughly as l/Lgat smallgate                                                                      clock frequencies in the former    circuit than in the latter.
  biases abovethreshold (avoiding velocity saturation effects).                                                                  Table I1 and Fig. 9 present a comparison of speed and power
  Thus a shorter gate length device should provide higher current                                                             of a variety of frequency divider approaches. The propagation
  (Zds) at fixed logic swing and load capacitance, and therefore                                                              delays determinedfromthe          dividing frequencies of all the
  will reduce propagation delay. An increase in gate width (wg)                                                               depletion-mode circuits are fairly close to those obtained from
  will directly increase Zds at a given gate bias. While the device                                                           ring oscillator evaluation. This indicates    that the speed of the
44                                                                     PROCEEDINGS OF THE IEEE, VOL. 70,NO. 1 , JANUARY 1982

                                 Fig. 10. Photomicrograph of an 8 X 8 multiplier chip. The     chip,
                                         including bonding pads, covers a 2.7-mm X 2.25-mm area.

logic gate of the depletion-mode circuits is not greatly reduced                             TABLE 111
by fanouts of 2 or 3. The higher toggling frequencies (also at            COMPARISONGaAs LSI 8 X 8 BIT PARALLEL
                                                                                     OF                           MULTIPLIER
                                                                        P R O M N E WITH STATEOFTHEART
                                                                         E F R A C                     SILICON BIPOLAR
the expense of high power) demonstrated by the HP [ 201 BFL
circuits and the LEP [2] BFL circuits are primarily the conse-                MPX-8H3-1               IULTIPLV TIRE                POYER D I S S I P A T I O h   .IULTlPLV T I H E -
quence of the use of complementaryclocked master-slave flip                                            (NAYOSECONDSI
                                                                                                      (UATTS)                                                    POYER PRODUCT
                                                                               Rl ]                              5.2                          2.2                 11.4 ( N a n o s e c - U a t t )
flops implemented with    NAND/NOR gates. This complementary                   (GaAs)                          16.8                           0.9                 15. 1

clock design can also be implemented in SDFL by using the               6.4    M"T(21                            19                                               R4
oR/NAND gate discussed in Section 11.
                                                                        1.2   TRW(31                            45                                                54
  With enhancement-mode GaAs FET's, a divide by eight fre-                    *OX-WJ-1

quency counter (gate length of 0.6 pm) has demonstrated a                      pHn
                                                                                                                 45                           1.4                 63

maximum clock frequency of 3.8 GHz with a power dissipation                   MH i                             125
                                                                                                                 175                          1.4
of 1.2 mW/gate. This corresponds to a gate delay time of 66                    67558

ps and a speed-power product of 79 fJ. This is a significant                  ( I ) Under D e r e l o p n e n tM t e r i a l :
                                                                                                               :                      taAr
                                                                              ( 2 ) Uses Cdrry Lookahead
achievement. However, the enhancement-mode FET approach                       (3)    C o n t a i n sn i l t i p l i e r   b r a y and C u t p u t Latches m l y (No I n p u t Latches.)
hasa basic limitation on the permissible logic voltage swing                         Eftlnated t d t e Count = 900

"0.6 V, duetothe        onset of gateconduction.To        achieve
reasonable LSI/VLSIyields, further improvement gate thresh-
                                                   in                bit, requiredover 1000 NOR gates (about3000 FET's and
old voltage control is required. On the other hand,most of the       3000 Schottky diodes) for the complete circuit. This level of
BFL depletion-modeapproaches would likely be prohibited              complexity would clearly qualify this as an LSI GaAs digital
from achieving LSI/VLSIcomplexities duetothe relatively              circuit and also the most complex GaAs circuit to be success-
high power dissipation required and the lower packing density.       fully demonstrated to date.
  To date,theSDFLapproach          appears to be the only one           The best performance observed on the 8 bit multiplier corre-
meeting the powerdissipation,gate       density, and fabrication     sponds to a propagation delay of 150 ps/gate at a power dissi-
yield required for LSI. In fact, the first GaAs LSI circuits to      pation of about 2 mW/gate. At this speed, a full 16bit product
be reported have utilized the depletion-modelowpinchoff              would be available every 5.25 ns. Lower power operation 0.6
voltage SDFL approach. As shownby the results in Table I             mW/gate was also possible (using lower pinchoff voltages) at
and 11, the 1-pm SDFL circuits provide a middle ground be-           the still respectable multiply time of 16.8 ns. This performance
tween the potentiallylower power but more difficult to control       is summarized in Table 111 and contrasted to the reported per-
EIDCFL circuits and the higher power, higher pinchoff voltage,       formance of state-of-the-art silicon-based multipliercircuits.
BFL approaches.                                                      The two best Si multipliersutilize 2jlm. bipolar technology
  The LSI circuit recently demonstrated is a parallel multiplier     [35], [36]. The best speed of the 1-pm GaAs MESFET IC is
[8]. This circuit, consisting of NOR gate full adders and half       about 4 times better than that of the fastest Si 8 bit multiplier
adders in a regular array, forms the binary product of two 8         [36] employing input bit recording [37] and carry-look-ahead
bit         words. The planar,  localized-implant     fabrication    adders. Even faster GaAs multipliers (1.5 to 3 ns) would be
approach, described in Section 111, was used to process the          expected if these same circuit approaches were to beutilized.
2.25 x 2.7 chip shown in Fig. 10. This 8 X 8 multiplier, which         The successful fabrication of theSDFL 8 X 8 multipliers
also contained D-type flip-flop latches every input and output       brings the GaAs technology into the realm of LSI while ad-
LONG et al.: HIGH SPEED GaAs IC’s                                                                                                                              45

vancing the state-of-the-art for multiplier chips. The propaga-                         circuits,”Proc. IEEE, this issue, pp. 13-25.
tion delay of 150 ps/gate observed on the 8 X 8 multiplier is                   [ 111 R. Zuleeg, J. K. Notthoff, and K. Lehovec,“Femtojoule high-
                                                                                        speed planar GaAs E-JFET logic,” IEEE Trans Electron Devices,
in good agreementwith the results of much simpler GaAs SDFL                             VOl. ED-25, Qp. 628-639, June 1978.
circuits such as ring oscillators and frequency dividers.                       [ 121 K. Lehovec and R. Zuleeg, “Analysis of GaAs FET’s integrated
                                                                                        logic,” IEEE Trans Electron Devices,vol. ED-27, pp. 1074-1091,
  T i high speed of operation indicates that the extension of                           June 1980.
the planar SDFL circuit approach to the LSI level of complex-                   [ 131 G. Nuzillat, G. Bert, T. P. Ngu, and M. Gloanec, “Quasi-normally-
ity does not result in significant speed degradation.     The low                       off MESFET logic for high-performance GaAs               IC’s,’’ IEEE Trans.
                                                                                        Electron Devices, vol. ED-27, pp. 1102-1108, June 1980.
power dissipation on the 8 X 8 multipliers also indicates that                  [ 141 N. Yokoyama, T. Mimura, and M. Fukuta, “Planar GaAs MOSFET
the SDFL approach is a suitable candidate for theVLSI range                             integrated logic,” IEEE Trans. Electron Devices, vol. ED-27, pp.
of complexity if further modest reductions in power per gate                             1124-1127, June 1980.
                                                                                [ 151 L. Messick, “A dc to 16GHz Indium Phosphide MISFET,” Solid-
are achieved.                                                                           State Electron., vol. 23, pp. 551-555, 1980.
                                                                                [ 161 S. Judaprawira et aL, “Modulation doped MBE GaAs/nAl+a, - x
                           V. CONCLUSION                                                As MESFET’s,” IEEE Electron Device Lett., vol. 1, EDL-2, pp.
                                                                                        14-15, Jan. 19, 1981.
  GaAs has matured as a semiconductor material t o provide a
viable IC technology.Significant      advantages inperformance
                                                                                1171    T. Mimura, S. Hiyamizu, T. Fujii, and K. Nanbu, “A new field-
                                                                                        effect transistor  with           selectively doped GaAs/n-Al,Ga,
                                                                                        heterojunctions,” Japan. J. AppL P h y s L e t t , voL 19, no. 5,
over conventional silicon IC’s have been demonstratedwith                               pp. 225-227, 1980.
                                                                                        J. K. Notthoff and C. H. Vogelsang, “Gate design for DCFL with
the operation of GaAs circuits at high speed and low power.                             GaAs E-JFET’s,” in Research Abstracts of First Annual Gallium
Furthermore,theability       to achieve LSI complexities with                           Arsenide Integrated Circuit Symp., Lake Tahoe, Sept. 27, 1979,
GaAs (1000 gates)has also been demonstrated. Further im-                                Paper 10.
                                                                                        R. Zuleeg, Jap. J. AppL P h y s , vol. 19, pp. 315-318, 1980.
provements in the performance of GaAs IC’s are still possible                           R. L. VanTuyl, C. Liechti, R. E. Lee,and E. Gowen, “GaAs
throughthe use of heterojunctions ina new generation of                                 MESFET logic with 4-GHz clock rate,” IEEE J. SolidState Cir
devices.                                                                                cuits, vol. SC-12, pp. 485-496, Oct. 1977.
                                                                                        R. C. Eden, B. M. Welch, and R. Zucca, “Lowerpower GaAs
  Today,strong developmentprogramsexist           inthe U.S.A.,                          digital IC’s using Schottkydiode-FET logic,” 1978 Int.Solid
Europe, and Japan,and applications of GaAs logicinto systems                            State Circuits Con$ Dig. Tech Papers, pp. 68-69, Feb. 1977.
                                                                                        R.C. Eden, F. S. Lee, S. I. Long, B. M. Welch, and R. Zucca,
are anticipated soon. The first applicationsare expected to                             “Multi-level logic gate          implementation       in GaAs IC’s using
take place in the form of MSI circuits utilized at the front end                        Schottky diode-FET logic,” 1980 Int. Solid State Circuits Cons.
of high speed digital systems. In the near future, furtherappli-                        Dig. Tech. Papers, pp. 122-123, Feb. 1980.
                                                                                        S. I. Long, F. S. Lee, R. Zucca, B. M. Welch, and R.               C. Eden,
cations of MSI/LSI GaAs circuits are expected in high speed                             ‘“SI high-speed low-power GaAs IC’s using Schottky diode FET
signal processing. A scenario can be envisioned where success-                          logic,” IEEE Trans. Microwave Theory T e c h , vol. MTT-28, pp.
                                                                                        466-471, May 1980.
ful utilization of GaAs IC’s will stimulate further development,
                                                                                (241 C.A. Liechti, “GaAs FET logic,” 1976 Int. GaAs Symp., Inst.
which will, in turn, encourage further applications.                                    Phys Con$ Series33a, ch. 5, pp. 227-236, 1977.
  The potential for long range application of GaAs digital IC’s                  1251 H. Ishikawa, H. Kusakawa, K. Suyama,and M. Fukuta, “Normally-
                                                                                        off type GaAs MESFET for low-power high-speed logic circuits,”
also exists in computer mainframes. A forecast on whether, or                            1977 I n t Solid State         CircuitsCon$,       Dig. Tech. Papers, pp.
when and how this type of application may occur is presently                             200-201, Feb. 1977.
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