MOSFET - CMOS by trigyy

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                                 Digital Electronics II

                              Integrated Circuit Technology
                                Tocci-Widmer, Chapter 8.
                                      Part II : CMOS



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                                             Objectives
      • Read and understand digital IC terminology.
      • Compare characteristics of standard logic.
      • Interpret IC’s data sheets.
      • Determine fan-out, noise margins, etc.
      • Have an appreciation towards cutting-edge technology
        currently dominating/dictating our daily lives.
      • Get you at least a little bit interested in the field of
        system engineering/design.



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                                           What is IC?
      • Integrated circuit: We integrate circuits, which would normally take large
        space on your breadboards into a single IC, which takes much less space.
      • We categorize these ICs as follows: SSI, MSI, LSI, VLSI, ULSI, and GSI.

                      IC Category               Size (# of gates)
                      SSI                       < 12
                      MSI                       12 – 99
                      LSI                       100 – 5,000
                      VLSI                      5,001 – 100,000
                      ULSI                      100,001 – 1,000,000
                      GSI                       > 1,000,000
                      TSI                       > 1,000,000,000
                                                                                                           IC/CMOS -   3
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                           Digital Systems Evolution

 1905         1947 1958 1960 1968 1970                     1980          1993       2000 2001 2007

                                             Dominated by CMOS
                      Silicon                                                                      Dual Core
First Vacuum                             MSI                                                       Itanium 2
                        IC                                 VLSI         Pentium
                                        12-99                                                      (1720m)
     Tube                                                 > 5,000      Evolution
                                SSI            LSI                       Begins
            Bipolar                         100-5,000                                    2001: Itanium (25m)
                                < 12
           Transistor                                                                    2003: Itanium 2 (220m)
           Invented                                                                      2004: It 2 9M L3 (592m)
                                         1970: Calculator
                                         1971: Microprocessor
                                                                   P1: 3,100,000 (1993)
                                         Intel’s 4004 (2300 x’tor)
                                                                   P2: 7,500,000 (1997)
                         1959: MOS transistor                      P3: 9,500,000 (1999)
                               invented                            P4: 42,000,000 (2000)
                         1959: First IC
                         1962: TTL developed
                         1963: CMOS invented
                                                                                                           IC/CMOS -   4
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                                      Moore’s Law (2004)
                                                                                      1,000,000,000




                                                                                                        2007

http://www.intel.com/research/silicon/mooreslaw.htm                                                        IC/CMOS -   5
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                                Moore’s Law (2008)




                                                                                                           IC/CMOS -   6
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                                        What is MOS?
      • Short for MOSFET - Metal-oxide Semiconductor Field-effect Transistor.
      • NMOS – n-type source & drain
                                                       0V
                                                      +5V
                                              GATE

    +VDD                                                                  OXIDE                       GND
                                              +   +   +   +   +   +
                                              +   +   +   +   +   +
                    -    -        -     -     -   -   -   -   -   -      -       -        -       -
                          -                   -   -   -   -   -   -                           -
                    -                                                        -
                        -                       current flow                         -
                    -                                                                      -
                   SOURCE                      SILICON                                   DRAIN

                                                                                                           IC/CMOS -   7
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                            MOSFET Characteristics
      • The MOSFET family features theoretically infinite input (gate) impedance.
        This allows one gate output to drive many inputs. This is particularly useful
        in memory systems.
      • Figure (b), when VGS = 0V, the MOSFET switch is OFF and the resistance
        between the drain and source is, for all practical purposes, an open circuit.
      • Figure (c), when VGS = +5V, the MOSFET switch is ON and the resistance
        between the drain and source is low.




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                                       NMOS Inverter
      •   The gate of NMOS transistor (Q1) is connected to VDD (all the time). Therefore, it
          is always ON. However, its resistance is approximately 100k-Ohm
      •   When Q2 is OFF (resistance, 1010 Ohm), VOUT is pulled-up to HIGH logic
      •   When Q2 turns ON (resistance, 1k-Ohm), VOUT is pulled-down to LOW logic.




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                                     NMOS & PMOS
      • NMOS – uses n-type semiconductor
            – ON when gate voltage is HIGH
      • PMOS – uses p-type semiconductor
            – ON when gate voltage is LOW




                                                                                                          IC/CMOS -   10
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                                       CMOS Inverter
      •   Now that modern complementary MOSFET (CMOS) circuits are as fast as
          TTL, they are also as popular as TTL circuits. In fact, you will find equivalent
          CMOS devices for many TTL numbers.
      •   For example the 74HC00 (high-speed CMOS) or 74HCT00 can replace the
          74LS00 IC.
      •   CMOS uses one N-channel and one P-channel MOSFET to create the inverter in
          Figure 8-22




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                                   CMOS Advantages
      • The CMOS output switches much closer to +5V and ground than TTL.
      • CMOS has a Fan-out of (about) 50, compared to about 10 for TTL.
      • CMOS input current is a few nanoamperes, compared to 0.4 µA for LS
        TTL.
      • Power requirements for CMOS are far less, even when many gates are on.
      • CMOS switching speed is as high as its equivalent TTL device.
      • CMOS circuits are usually smaller, since they do NOT require resistors as
        TTL does.




                                                                                                          IC/CMOS -   12
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                                        CMOS NAND
      • The CMOS NAND gate consists of two P-channel MOSFETs in parallel
        and two N-channel MOSFETs in series.
      • CMOS circuits are easy to analyze. Just remember that a high input (+5V)
        turns an N-channel MOSFET on, and a low (ground or 0) turns it off.
        Likewise, a low (0V) turns a P-channel MOSFET on and a high (+5V) turns
        it off.
      • CMOS integrated circuits are simpler and more compact in terms of
        integrated circuit fabrication. For one thing, CMOS gates don’t need
        Schottky diodes to eliminate the storage problem. CMOS transistors have
        no depletion zone and, therefore, no storage-time problems to fix.




                                                                                                          IC/CMOS -   13
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                Getting To Know CMOS NAND…

                         OFF
                         ON                            OFF
                                                       ON
       HIGH
       LOW

                                                                            LOW
                                                                            HIGH

                                               ON
                                               OFF



       HIGH
       LOW                                     ON
                                               OFF
                                                                      Q: How many MOSFETS are in a
                                                                         3-input CMOS NAND gate?

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                                          CMOS NOR
      •   The CMOS NOR gate is made by adding a series P-channel MOSFET and a parallel
          N-channel MOSFET to the basic CMOS inverter circuit.



                 LOW
                 HIGH                            OFF
                                                 ON


                 LOW
                 HIGH                            OFF
                                                 ON

                                                          LOW
                                                          HIGH


                      OFF
                      ON                                OFF
                                                        ON

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                           CMOS Power Dissipation
      • The acronym MOS stands for “metal-oxide-semiconductor” and describes
        how the transistor is constructed. There is a metal (gate) layer, a silicon
        dioxide (glass) layer, and a silicon substrate. Two conductors separated by
        an insulator form a very small capacitor.
      • A major advantage of CMOS is that it demands input current only when the
        device is switching from one state to the other. The CMOS input circuit is a
        capacitor and a steady state dc on the input results in no input (capacitor)
        current.
      • When the input must switch from low to high, the input capacitor demands
        some charging current. The driving gate must deliver charging current to as
        many input capacitors as there are gate inputs. The demand for charging
        current creates current spikes as shown in Figure 8-25.




                                                                                                          IC/CMOS -   16
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              Current Spikes due to Load Charging
      • Only occurs during LOW-to-HIGH switching at the output.




                                                                                                          IC/CMOS -   17
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                                              CMOS Power Dissipation 2
      • Another power-dissipating phenomenon in CMOS circuits is due to the
        unequal ON-to-OFF and OFF-to-ON switching speed.
            – Transistors turn ON faster than they turn OFF.
      • This causes a period when the two output transistors are partially ON.
      • Currents will flow through the low-resistance transistors from VCC to
        GND.
      • When switching frequency increases, power dissipation increases.
                      Power dissipation (J)




                                                                                Switching frequency (Hz)
                                                                                                          IC/CMOS -   18
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                                       CMOS Fan-out
      • CMOS input draws theoretically ZERO current.
      • Fan-out is determined by the load capacitance, which increases switching
        delay of a CMOS output.
      • Typically, about 50 loads—much larger than TTL fan-out.




                                                                                                          IC/CMOS -   19
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                             CMOS Major Weakness
      • CMOS is very vulnerable to static electricity.
      • Should never leave CMOS input floating…
      Warning:
      • Allowing a CMOS gate input to float is asking for trouble. The floating
        input capacitor can accumulate a charge from the energy in the
        environment. Charge accumulation is not predictable, so the input state will
        be unpredictable. All inputs in a CMOS package, even those of unused
        gates, must be tied to a valid logic level. Unwanted charge accumulation can
        cause the IC to overheat.
      • In TTL, if you have an entire gate in the package that is not used, you need
        not connect the inputs of the unused gate to a valid logic level. They can
        float.




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