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)