6.8 Static Analysis using SR model

• SR circuit model for invertor gate:

• SR circuit model of NAND Gate

6.7 Physical Structures of MOSFET

• The on-resistance of MOSFET depends on its physical properties (ie. geometry)
• Silicon doped with material rich in electrons is called n-type semiconductor (n+) 
• Silicon doped with material rich in holes is called p-type semiconductor (p+)

 

• Ron is the resistance of n channel
• Rn is resistance per square of n channel MOSFET in on state.
• Ron = (Rn*L)/W
• In VLSI technology, there is min fabricatable value for MOSFET channel length

SR Model of MOSFET

• S model is a gross simplification of MOSFET
• A practical MOSFET displays a non-zero resistance between its D and S terminals when it is on
• A more accurate model of MOSFET uses resistance Ron between D & S.

• SR model is still a gross simplification of MOSFET behavior.( Ron is not fixed but a function of VGS)
• Fixed resistance model is simpler and suffices for analyzing some aspect of digital circuit because the gate voltage is bimodal.
• SR model is valild only when VDS <= VGS - VT and when there is only one value for gate voltage when input is high ( VGS = VS)

6.5 Static Analysis using S model

6.4 MOSFET Switch Implementation of Logic Gates

• S circuit model of NAND Gate

• S circuit model of n-channel MOSFET inverter

• MOSFET inverter

6.3 MOSFET and its S model

• MOSFET belongs to transistor class, with three terminals: control, input & output
• Control = Gate, Input = Drain, Output = Source

• VGS, IDS, VDS

• S model of MOSFET

A typical value for VT of n channel MOSFET is 0.7 V

6.2 Logic Functions using Switches

Switches implement a form of digital logic called steering logic

6.1 The Switch