Chapter 2 Semiconductor Materials

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Chapter 2 Semiconductor Materials Powered By Docstoc
					Chapter 2 :
Semiconductor Materials & Devices (I)




2005 SOC   論
                                        1
                     Reference


1. SemiconductorManufacturing Technology: Michael
   Quirk and Julian Serda (2001)


3. ULSI Technology : C. Y. Chang, S. M. Sze (1996)
4. Semiconductor Physics and Devices- Basic Principles
   (3/e) : Donald A. Neamen (2003)
5. Semiconductor Devices - Physics and Technology (2/e)
   : S. M. Sze (2002)
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                                                     2
                 Semiconductor Materials
            - The Crystal Structure of Solids

     •
     •
     •
     •
     •


     •                                                :
      104 ~10-10 (Ωcm)-1
     •                                   (impurity)

     •                     (carrier)

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              What is Semiconductor
    Conductivity between conductor and insulator




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                                              4
                  Element Semiconductors




   Why Silicon Can Dominate the IC Industry ?
   Silicon devices exhibit better properties at room temperature, and high-quality silicon
   dioxide can be grown thermally. There is also an economic consideration. Device-grade
   silicon costs much less than any other semiconductor material.
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•
•


•




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                   6
                     Silicon Structure
                                              Shared electrons
                             Covalence band


Silicon Atom
    Quadrivalent element
        Four valence electron
    Atomic number = 14
        4 valence electron
    Covalent bond




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                Silicon Structure




Diamond Structure




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Schematics of three general types of crystals:(a) amorphous, (b) polycrystalline, (c) single crystal.
Amorphous materials have order only within a few atomic or molecular dimensions.
Polycrystalline materials have a high degree of order over many atomic or molecular dimensions. These ordered
regions, or single-crystal regions, vary in size and orientation with respect to one another. The single-crystal regions are
called grains and are separated from one another by grain boundaries.
Single-crystal materials, ideally, have a high degree of order, or regular geometric periodicity, throughout the entire
volume of the material. The advantage of a single-crystal material is that, in general, its electrical properties are superior
to those of a non-single crystal material, since grain boundaries tend to degrade the electrical characteristics



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Preparation of Single Crystal Silicon Wafers

                        Polysilicon   Seed crystal
                                                      6. Edge Rounding
                                  Crucible
1. Crystal Growth

                                      Heater

                                                      7. Lapping

2. Single Crystal Ingot



                                                      8. Wafer Etching

3. Crystal Trimming and
   Diameter Grind
                                                                                        Polishing
                                                                             Slurry     head


                                                      9. Polishing
4. Flat Grinding
                                                                      Polishing table




5. Wafer Slicing                                     10. Wafer Inspection


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       General Characteristics of Silicon Wafer




Diameter: 10~30cm, 20cm(8-inch)

Thickness: 400~600µm

Resistivity: 0.05~0.1 cm




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                  SiO2
        Si            HCl              SiHCl3

                       Si

                                Bridgman)
         (Czochralski)

      > 50% market for growing GaAs




Schematic of a horizontal Bridgman growth system Simplified schematic drawing of the Czochralski
         2005 SOC        論                       puller. Clockwise (CW), counterclockwise (CCW).
                                                                                        12
                        Czochralski Growth




Time lapse sequence of boule
being pulled from the melt in a
     Czochralski growth

     2005 SOC   論                 A 200-mm silicon growth facility
                                                           13
                   Carriers of Semiconductor

                                      Conduction electron


Electron and Hole
  Covalent band is broken at room
  temperature
                                               Hole
     Produce the free electron
     Empty position – hole
  Both electron and hole are called
  “carriers”




   2005 SOC    論                                Covalent band broken
                                                                   14
                Carriers of Semiconductor


 Electrons – negative charge
 Holes – positive charge
      The movement of carriers cause current in semiconductor




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                       Ways of Doping

      Intrinsic semiconductor
           Bad conductivity
      Doping
           Substitutional impurity
           Interstitial impurity
           Interstitial-Substitutional impurity




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                       Doping Type

      Extrinsic semiconductor
           Doped the impurities into intrinsic semiconductor
      Acceptor
           p-type
      Donor
           n-type




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                 p-type Semiconductor

   Acceptor
       Adding the element of Group III (B, Al)
          Accept electron
       Majority carrier – holes


                                                 Acceptor




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                       n-type Semiconductor

   Donor
           Adding the element of Group V (P, As)
              Supply electron
           Majority carrier – electrons



                                                   Donor




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                          Energy Band Diagram




Schematic energy band representations of (a) a conductor with two possibilities (either the
partially filled conduction band shown at the upper portion or the overlapping bands shown at
the lower portion), (b) a semiconductor, and (c) an insulator.
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                    Energy-Band Diagram
               Donor & Acceptor Energy State


       Ed : the energy state of the
       donor electron



      The energy-band diagram showing (a) the discrete donor energy state and
                    (b) the effect of a donor state being ionized.



       Ea : the energy state of the
       acceptor electron




2005 SOC     論 Energy-band diagram showing (a) the discrete acceptor
                 energy state and (b) the effect of a acceptor state being      21
                        Mobility and Resistivity
  Mobilities and diffusivities in Si and GaAs at   Resistivity versus impurity
  300 K as a function of impurity concentration.   concentration for Si and GaAs.

  3900
  1900




  1350
  480

  8500


  400
                                                        1                1
                                                   ρ≡       =
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                                                        σ       q ( nµ n + pµ p )
                                                                                    22
                           Measurement of Resistivity


  J drf = q (µ n n + µ p p )E = σ E
       1                1
  ρ≡       =                          ( Ω - cm)
       σ       q ( nµ n + pµ p )
     I     V        L
  J = ; E = ⇒ V = ( ) I = IR
     A     L       σA




Current conduction in a uniformly
                                                  Measurement of resistivity using a
doped semiconductor bar with length
                                                  four-point probe.
L and cross-sectional area A.
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             Semiconductor Devices
    - Components on Printed Circuit Board




           Circuit types: Analog & Digital Circuits
             Component types: Passive & Active
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                  Passive Component Structures
      - IC Resistor Structures: Parasitic Resistor Structures


Integrated circuit resistors.
All narrow lines in the large
square area have the same
width W, and all contacts are
the same size.
                                                               0.65 □
   1 L ⎛1⎞
R≡ = ⎜ ⎟
   G W ⎜g⎟   ⎝ ⎠
where 1/g : sheet resistance (Ω/□)
Example: L=90 µm; W=10 µm;
1/g=1 kΩ/□
R=(9+0.65*2)* 1 kΩ/□=10.3 kΩ
       2005 SOC    論                 Examples of Resistor Structures in ICs
                                                                        25
                                Resistor


    Polysilicon resistor
           is doped on an IC chip
           Linear
              Resistance is determined by length, area, and the
              resistivity of the material type
                   Silicide Block


                                                     Poly
                                                                  FOX


                   Silicided Poly               P-substrate


                       l             area=A


                                                  symbol
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                   Resistor


      Interconnect Resistance




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             Passive Component Structures
           Examples of Capacitors Structures in ICs
(a) Integrated MOS capacitor. (b) Integrated p-n junction capacitor.
                                ∈ox
                             C=         ( F / cm2 ) ; ∈ox =∈r ∈0 = 3.9 ∈0
                                 d
                             where∈ox : dielectricpermittivi of SiO2
                                                           ty
                             ∈r : dielectricconstantof SiO2 = 3.9
                                                              ×
                             ∈0: permittivi of free space(8.85 10-14 F/cm)
                                          ty




                               Increase the
                               dielectric
                               constant
2005 SOC      論       Si 3 N 4 (∈ r = 7 ) or
                      Ta 2 O 5 (∈ r = 25 )                           28
                           Capacitor

      Charge storage device
           Memory Devices, esp. DRAM
           Two boards of semiconductor material as a capacitor
      Capacitances
           are proportional to the area (A=h*l)
           are inverse proportional to the distance
                       l

                                                         hl
                                                C =∈r ∈0         (F )
                   d
                             h
                                    symbol
                                                         d

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                      Capacitor


           Poly–poly (double poly process)
             Middle value
             Better noise immunity
           MMC (metal/metal capacitor)

                                              M5

                                             VIA
                                                    MMC Metal
                                                      M4
                                              VIA
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                                                     M3
                                                          30
                      Passive Component Structures
                    Examples of Inductors Structures in ICs


            Quality factor: Q = Lω/R
The higher the Q values; the lower the loss from resistance,
hence the better the performance of the circuits.
There are some approaches to improve the Q values:
(1) Reduce Cp : use low Єox material
(2) Reduce R1 : use thick film metal (e.g. Cu, Au)
(3) Reduce Rsub loss : use insulating substrate (SOI, quartz)

An estimated      L ≈ µ 0 n 2 r ≈ 1.2 ×10 −6 n 2 r
inductance of the
square planar     where µ 0 : permeability in vacuum
spiral inductor :              = (4π ×10 −7 H / m)
                                                                (a) Schematic view of a spiral
                            n : the number of turns             inductor on a silicon substrate.
         2005 SOC       論                                       (b) Perspective view along A-A’
                                                                                         31
           Active Component Structures



    The pn Junction Diode
    The Bipolar Junction Transistor (BJT)
    The Metal-Oxide-Semiconductor FET (MOSFETs)
    Complementary MOSFET (CMOS)




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                           PN Junction


Diode
  p region
     Doped with acceptor impurities
     The positive charges atoms left
  n region                             Metallurgical junction
     Doped with donor impurities
     The negative charges atoms left
  Space charge region in thermal
  equilibrium
     Also called depletion region
     No mobile carrier exists
     Two forces exactly balance each
     other
         No current

   2005 SOC   論
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                                              PN Junction




(a)   Simplified geometry of a pn junction;           The space charge region, the electric field,
                                                      and the forces acting on the charged carriers.
(b)                        ideal uniformly doped pn
      Doping profile of an 論
           2005 SOC
      junction                                                                              34
       Energy Band Diagram of the PN Junction




                                                      Drift
                                                         •
                                                                                  Diffusion
                                                                                  •

                                                     °
                                                Diffusion
                (a)                                  (b)               Drift
(a) Uniformly doped p-type and n-type semiconductors before the junction is  °
formed. (b) The energy band diagram of a p-n junction in thermal equilibrium.
                                             kT ⎛ N a N d    ⎞          ⎛N N      ⎞
   2005 SOC    論 Build-in   voltage : V bi =    ln ⎜
                                                   ⎜ n2      ⎟ = V t ln ⎜ a 2 d
                                                             ⎟          ⎜ n       ⎟
                                                                                  ⎟
                                              q    ⎝  i      ⎠          ⎝   i     ⎠   35
       Space Charge Density & Electric Field/Potential
                          uniformly doped pn junction

                                                  dφ ( x ) − ρ ( x )    dE ( x)   (b)
(a)                         Poisson' s Eq. :              =          =−
                                                   dx 2      ∈s          dx

                                       ρ ( x)
                            E =    ∫    ∈s
                                                 dx

                                   − qN
                               =             a
                                                 ( x + x p ) ; (− x p ≤ x ≤ 0)
                                     ∈s

                               =
                                   − qN      d
                                                 ( xn − x ) ; (0 ≤ x ≤ xn )             φ ( x) = − ∫ E ( x)dx
                                     ∈s

(a) The space charge density in a uniformly doped                                 (c)
pn junction assuming the abrupt junction
approximation
(b) Electric field in the space charge region in a
uniformly doped pn junction
         2005 SOC      through the space charge
(c) Electric potential 論
region in a uniformly doped pn junction                                                                 36
       Space Charge Density & Electric Field/Potential
                           One-sided abrupt junction


(a) One-sided abrupt junction (with NA
    >> ND) in thermal equilibrium.
(b) Space charge distribution.
(c) Electric-field distribution.
(d) Potential distribution with distance,
    where Vbi is the built-in potential.
                                   1/ 2
        ⎧ 2 ∈s Vbi ⎡ N a + N d ⎤ ⎫
     W =⎨          ⎢           ⎥⎬
        ⎩ q ⎣ N a Nd ⎦⎭
                 2 ∈s Vbi
        ≅ xn =
        2005 SOC
                   qN
                  論 d
                                                       37
             The Uniformly Doped pn Junction Diode
                                   1/ 2                                       1/ 2                                     1/ 2
   ⎧ 2 ∈ s V bi ⎡ N a + N d   ⎤⎫             ⎧ 2 ∈s (Vbi + VR ) ⎡ N a + N d ⎤ ⎫          ⎧ 2 ∈ (V − Va ) ⎡ N a + N d ⎤ ⎫
W =⎨            ⎢             ⎥⎬          W =⎨                  ⎢           ⎥⎬       W = ⎨ s bi          ⎢           ⎥⎬
   ⎩    q       ⎣ NaNd        ⎦⎭             ⎩        q         ⎣  Na Nd ⎦⎭              ⎩       q       ⎣  Na Nd ⎦⎭




Schematic representation of depletion layer width and energy band diagrams of a
        2005
p-n junction SOC    論
             under various biasing conditions. (a) Thermal-equilbrium condition.
                                                                                                                 38
          Ideal Current-Voltage Relationship of Diode




(a) A pn junction with an applied forward-     Excess minority carrier concentrations at
    bias voltage showing the directions of      the space charge edges generated by the
    the electric field induced by Va and the              forward-bias voltage
    space charge electric field.
(b) Energy-band diagram of the forward-
         2005 SOC     論
    biased pn junction                                                             39
                                       Ideal PN Junction Current




Steady-state minority carrier concentrations                  Ideal electron and hole current components through
in a pn junction under forward bias                           a pn junction under forward bias.
               qD p p n 0              qV a
J p ( xn ) =                [exp(           ) − 1]
                  Lp                   kT
                                                                                                      1
                 qD n n p 0             qV a                                                     rd =
J n (− x p ) =                [exp(          ) − 1]                                                 slope
                     Ln                 kT
                                                                                                    dV    V
J D = J p ( xn ) + J n (− x p )                                                      JQ            = a = t
                                                                                                    dI D I Q
        qD p p n 0        qD n n p 0            qV a          Js: the diode reverse         Diffusion resistance
   =[                +                 ][exp(        ) − 1]   saturation current density
           Lp                 Ln                kT
               2005qV a
                    SOC            論             qV a
   = J s [exp(            ) − 1] ≈ J s exp(           )                                                  40
                     kT                          kT              Ideal I-V characteristic of a pn junction diode
                       PN Junction Diode


                                        P   N
Switch
  Forward biased (Va > 0)
     Short                    Ohm contact   Ohm contact
          Current
  Reverse biased (Va < 0)
     Open
          No current




   2005 SOC    論
                                                 41
Fabrication Processes of
   PN Junction Diode



(a) The wafer after the development.
(b) The wafer after SiO2 removal.
(c) The final result after a complete
    lithography process.


(d) A p-n junction is formed in the
    diffusion or implantation process.


(e) The wafer after metallization.
(f)   A p-n junction after the compete
      process.

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                                         42
                    Application of Diode


    Diode
           Used for protection circuit
              In substrate
                  Remain reverse biased
              In n-well
                  Prevent forward pn junction
                      Substantial current flow



                       p+       n+               p+   n+

                                                      n-well
2005 SOC      論
                  p-substrate                                  43