Download Slide 1

Design and Implementation of VLSI Systems
(EN0160)
Prof. Sherief Reda
Division of Engineering, Brown University
Spring 2007
[sources: Weste/Addison Wesley – Rabaey/Pearson]
Summary of last lecture
NMOS transistor
PMOS transistor
Summary of transistor ideal (Shockley)
model
polysilicon
gate
W
tox
n+
L
n+
SiO2 gate oxide
(good insulator, ox = 3.9)
p-type body


0
Vgs  Vt


V
I ds    Vgs  Vt  ds Vds Vds  Vdsat
2
 
2


V

V
Vds  Vdsat

gs
t

2
for nMOS
for pMOS
cutoff
linear
saturation
DC transfer characteristics
PMOS on (linear), NMOS off
• Vin = 0
Vin0
Idsn, |Idsp|
Vin0
Vout
VDD
PMOS on (linear), NMOS on (saturation)
• Vin = 0.2VDD
Idsn, |Idsp|
Vin1
Vin1
Vout
VDD
PMOS on (linear ~ sat) and NMOS (sat)
• Vin = 0.4VDD
Idsn, |Idsp|
Vin2
Vin2
Vout
VDD
PMOS on (sat) NMOS on (linear)
• Vin = 0.6VDD
Idsn, |Idsp|
Vin3
Vin3
Vout
VDD
PMOS on (off ~ linear) and NMOS on (linear)
• Vin = 0.8VDD
Vin4
Idsn, |Idsp|
Vin4
Vout
VDD
NMOS on (linear) and PMOS cut off
• Vin = VDD
Vin0
Idsn, |Idsp|
Vin5
Vin1
Vin2
Vin3
Vin4
Vout
VDD
Summary of voltage transfer function
A
B
C
D
E
Noise margins
CMOS inverter noise margins
desired regions
of operation
What is the impact of altering the PMOS width in
comparison to the NMOS width on the DC char?
polysilicon
gate
Idsn, |Idsp|
W
tox
Vin3
n+
Vin3
Vin3
L
n+
SiO2 gate oxide
(good insulator, ox = 3.9)
p-type body
Vin3
Vout
VDD
If we increase (decrease) the width of PMOS compared to NMOS
 for the same input voltage, a higher (lower) output voltage is obtained
V
out
V
in
Impact of skewing transistor sizes on
inverter noise margins
 Increasing (decreasing) PMOS width to NMOS width
increases (decreases) the low noise margin and decreases
(increases) the high noise margin
Pass transistor DC characteristics
 As the source can rise to within a threshold voltage of the
gate, the output of several transistors in series is no more
degraded than that of a single transistor
Summary
• Today
– Covered DC transfer characteristics
• Next time
– Transistor non-ideal behaviour