Lecture 3:
CMOS
Transistor
Theory
CMOS VLSI DesignCMOS VLSI Design 4th Ed.
3: CMOS Transistor Theory 2
Outline
Introduction
MOS Capacitor
nMOS I-V Characteristics
pMOS I-V Characteristics
Gate and Diffusion Capacitance
CMOS VLSI DesignCMOS VLSI Design 4th Ed.
3: CMOS Transistor Theory 3
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 (∆V/∆t) -> ∆t = (C/I) ∆V
–Capacitance and current determine speed
CMOS VLSI DesignCMOS VLSI Design 4th Ed.
3: CMOS Transistor Theory 4
polysilicon gate
(a)
silicon dioxide insulator
p-type body
+
-
V
g
< 0
MOS Capacitor
Gate and body form MOS
capacitor
Operating modes
–Accumulation
–Depletion
–Inversion
(b)
+
-
0 < Vg < Vt
depletion region
(c)
+
-
Vg > Vt
depletion region
inversion region
CMOS VLSI DesignCMOS VLSI Design 4th Ed.
3: CMOS Transistor Theory 5
Terminal Voltages
Mode of operation depends on Vg, Vd, Vs
– Vgs = Vg– Vs
– Vgd = Vg– Vd
– Vds = Vd– Vs= Vgs - Vgd
Source and drain are symmetric diffusion terminals
–By convention, source is terminal at lower voltage
–Hence Vds ≥0
nMOS body is grounded. First assume source is 0 too.
Three regions of operation
–Cutoff
–Linear
–Saturation
V
g
V
s
V
d
V
gd
V
gs
V
ds
+
-
+
-
+
-