07-11-2012, 06:15 PM
CMOS Transistor Theory
CMOS.ppt (Size: 516.5 KB / Downloads: 28)
Introduction
So far, we have treated transistors as ideal switches
An ON transistor passes a finite amount of current
Depends on terminal voltages
Derive current-voltage (I-V) relationships
Transistor gate, source, drain all have capacitance
I = C (DV/Dt) -> Dt = (C/I) DV
Capacitance and current determine speed
Also explore what a “degraded level” really means
nMOS Saturation
Channel pinches off
Ids independent of Vds
We say current saturates
Similar to current source
I-V Characteristics
In Linear region, Ids depends on
How much charge is in the channel?
How fast is the charge moving?
pMOS I-V
All dopings and voltages are inverted for pMOS
Mobility mp is determined by holes
Typically 2-3x lower than that of electrons mn
120 cm2/V*s in AMI 0.6 mm process
Thus pMOS must be wider to provide same current