EE130/230M Review Session 1.Small Signal Models for MOSFET/BJT 2.MOS Electrostatics

BJT Small Signal Model A small change in I B or V BE will result in a small change in I C and V CE input current = output current = the cutoff frequency (  T = 2  f T ) is defined at input voltage = output voltage = =

Example A BJT is biased at I C = 1 mA and V CE = 3V.  dc = 90,  F = 5ps, T = 300K. Find (a) g m, (b) r , (c) C . Solution: (a) (b) r  =  dc / g m = 90/0.039 = 2.3 k  (c) EE130/230M Spring 2013 Lecture 27, Slide 3

MOSFET Small Signal Model A small change in V G will result in a small change in I D and V DS input current = output current = the cutoff frequency (  T = 2  f T ) is defined at input voltage = output voltage = =

Summary of Small Signal Models Inputs/Outputs Linearized equivalent circuits Key components Performances  Gain  Cut-off frequency

MOS Capacitor Energy Band Diagram Problem: Solution:

Flat-band Condition

Equilibrium Condition

Accumulation Condition

Strong Inversion Condition

Charge distribution Charge amount Capacitance Equivalent circuits

MOS Capacitor Charge vs. Gate Voltage AccumulationDepletionInversion VTVT Flat-band voltage Maximum depletion charge

MOS Capacitance vs. Gate Voltage Accumulation Depletion Inversion VTVT Flat-band voltage Minimum capacitance