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From analog to digital circuits

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					               From analog to digital circuits

                       A phenomenological overview




Bogdan Roman
Outline                                                                                          2


•   Insulators, conductors and semiconductors
•   Semiconductor diodes: the p-n junction
•   The Field Effect Transistor (FET):
    – The Junction FET (JFET)
    – The Metal Oxide Semiconductor FET (MOSFET)
•   The MOS Inverter
    – Resistive and same MOS type
    – Complementary MOS (CMOS) technology
•   Elementary gates
•   Flip-flops
•   Examples
                                    Loosely based on the IA and 3B Engineering dept. courses
                                               (Linear Circuits and Devices, Digital Circuits,
                                      Information Processing, Integrated Digital Electronics)
Insulators and conductors                                                      3




                                              Current I given by: I  ne u A
                                                         V          ne A
                                              u  E       I          V
                                                          L           L
                                                   conductivity  ne
                                                                    L
                                                  R  resistance 
                                                                   A
                 Mean velocity of electron:
                          u  E                  Ohm's Law: I 
                                                                     V
                    where  = mobility                               R
Semiconductors: Intrinsic silicon                                                          4




                                         At room temperature, the thermal energy kT
  At low temperatures, silicon is an     ~ 1/40 eV is enough to break a few covalent
  insulator since there is not enough    bonds to produce free electrons. This also
  thermal energy to break the covalent   leaves holes (i.e. positive net charges left by
  bonds.                                 the broken covalent bond).

                                         Both electrons and holes contribute to
                                         current flow.
Semiconductors: Extrinsic silicon                                          5




 electron concentration n               hole concentration p
 electron mobility n                   hole mobility  p
  n-type conductivity  n  nen        p -type conductivity  p  pe p


                        when n  p then  n  p
The p-n junction: The diode                           6


                              Reverse biased diode:




                              Forward biased diode:
The Diode (contd.)                                7


  Reverse biased diode:   Forward biased diode:
The Junction Field Effect Transistor (JFET)   8



  - Proposed by Shockley in 1951
  - First made by Teszner in 1958 in
  France




  JFET Interactive (opens browser)
Metal Oxide Semiconductor FET (MOSFET)          9


- First made in 1960 at Bell Laboratories in
the USA by Atalla and Kahng.
- Offers extremely high component density
in integrated circuits.
- Very high input resistance, low noise,
simpler fabrication than bipolar transistors.




  MOSFET Interactive (opens browser)
The NMOS inverter                                                                    10


              Resistive load:                           NMOS load:




  -When input is low (0) then T1 is off,   - T2 has the gate tied to its drain and
  hence output goes high (1) (i.e. VDD)    is always on (and in saturation). Acts
  -When VIN = high (1) then T1             as a pseudo-resistor load.
  conducts (linear region) and brings      - Similar operation to the resistive
  the output low (0), depending on RL      load inverter
  -High RL = low logic zero and low        - Smaller area on silicon (so easier to
  power consumption but large area on      manufacture) and faster switching but
  silicon and slow switching =>            has a lower high logic voltage (VDD –
  compromise                               VT), and high power consumption
                                           when input high.
Complementary MOS (CMOS) inverter             11




  In CMOS technology, the output is
  clamped to one of the power rails by a
  conductive (on) device, while the other
  device serves as a load of effectively
  infinite resistance (off). This leads to
  static properties that approximate those
  of the ideal inverter.
  - The PMOS devices is slower (lower
  mobility of holes) so it has to be larger
  to compensate. It is also more complex
  to manufacture.
NOR and NAND gates                                                             12

                                                               +V
     A                             +V
                     A+B
     B                                                     B
         NOR Gate


The 74HC02 IC has                                A+B       A
four 2-input NOR gates
                           A            B                                A+B

                                                           A        B




     A                                      +V                          +V
                     A·B
     B
         NAND Gate                                     A        B

  The 74HC00 IC has four                         A·B
                                                                             A·B
  2-input NAND gates
                               B
                                                       B


                               A
                                                       A
Flip-flops and clocked flip-flops           13




                                        S   Q

                                   =   E
                                        R   Q




    S                         Q
    Ck   Master       Slave

    R                         Q
Multiplexer                                              14



  A crucial circuit, vital for implementing functions:
Binary Counter                                15




                 Current state   Next state
                  (ABCD)         (ABCD)+
                    0000          0001
                    0001          0010
                    0010          0011
                     ….             ….
                    1111          0000
Real stuff   16