Download CMOS FUNDAMENTAL

Introduction
to CMOS Transistor and
Transistor Fundamental
Introduction
 Integrated circuits: many transistors on one chip.
 Very Large Scale Integration (VLSI): very many
 Complementary Metal Oxide Semiconductor
– Fast, cheap, low power transistors
 Today: How to build your own simple CMOS chip
– CMOS transistors
– Building logic gates from transistors
– Transistor layout and fabrication
 Rest of the course: How to build a good CMOS chip
0: Introduction
CMOS VLSI Design
Slide 2
Silicon Lattice
 Transistors are built on a silicon substrate
 Silicon is a Group IV material
 Forms crystal lattice with bonds to four neighbors
0: Introduction
Si
Si
Si
Si
Si
Si
Si
Si
Si
CMOS VLSI Design
Slide 3
Dopants





Silicon is a semiconductor
Pure silicon has no free carriers and conducts poorly
Adding dopants increases the conductivity
Group V: extra electron (n-type)
Group III: missing electron, called hole (p-type)
0: Introduction
Si
Si
Si
Si
Si
Si
As
Si
Si
B
Si
Si
Si
Si
Si
-
+
+
-
CMOS VLSI Design
Si
Si
Si
Slide 4
p-n Junctions
 A junction between p-type and n-type semiconductor
forms a diode.
 Current flows only in one direction
0: Introduction
p-type
n-type
anode
cathode
CMOS VLSI Design
Slide 5
nMOS Transistor
 Four terminals: gate, source, drain, body
 Gate – oxide – body stack looks like a capacitor
– Gate and body are conductors
– SiO2 (oxide) is a very good insulator
– Called metal – oxide – semiconductor (MOS)
capacitor
Source
Gate
Drain
Polysilicon
– Even though gate is
SiO2
no longer made of metal
n+
n+
p
0: Introduction
CMOS VLSI Design
bulk Si
Slide 6
nMOS Operation
 Body is commonly tied to ground (0 V)
 When the gate is at a low voltage:
– P-type body is at low voltage
– Source-body and drain-body diodes are OFF
– No current flows, transistor is OFF
Source
Gate
Drain
Polysilicon
SiO2
0
n+
n+
S
p
0: Introduction
D
bulk Si
CMOS VLSI Design
Slide 7
nMOS Operation Cont.
 When the gate is at a high voltage:
– Positive charge on gate of MOS capacitor
– Negative charge attracted to body
– Inverts a channel under gate to n-type
– Now current can flow through n-type silicon from
source through channel to drain, transistor is ON
Source
Gate
Drain
Polysilicon
SiO2
1
n+
n+
S
p
0: Introduction
D
bulk Si
CMOS VLSI Design
Slide 8
pMOS Transistor
 Similar, but doping and voltages reversed
– Body tied to high voltage (VDD)
– Gate low: transistor ON
– Gate high: transistor OFF
– Bubble indicates inverted behavior
Source
Gate
Drain
Polysilicon
SiO2
p+
p+
n
0: Introduction
CMOS VLSI Design
bulk Si
Slide 9
Power Supply Voltage
 GND = 0 V
 In 1980’s, VDD = 5V
 VDD has decreased in modern processes
– High VDD would damage modern tiny transistors
– Lower VDD saves power
 VDD = 3.3, 2.5, 1.8, 1.5, 1.2, 1.0, …
0: Introduction
CMOS VLSI Design
Slide 10
Transistors as Switches
 We can view MOS transistors as electrically
controlled switches
 Voltage at gate controls path from source to drain
d
nMOS
pMOS
g=1
d
d
OFF
g
ON
s
s
s
d
d
d
g
OFF
ON
s
0: Introduction
g=0
s
CMOS VLSI Design
s
Slide 11
Complementary CMOS
 Complementary CMOS logic gates
– nMOS pull-down network
– pMOS pull-up network
inputs
– static CMOS
Pull-up OFF
Pull-up ON
Pull-down OFF Z (float)
1
Pull-down ON
X (crowbar)
0: Introduction
0
CMOS VLSI Design
pMOS
pull-up
network
output
nMOS
pull-down
network
Slide 12
Series and Parallel




nMOS: 1 = ON
pMOS: 0 = ON
Series: both must be ON
Parallel: either can be ON
a
a
0
g1
g2
(a)
(b)
a
g1
g2
(c)
a
g1
g2
b
0: Introduction
CMOS VLSI Design
0
1
b
b
OFF
OFF
OFF
ON
a
a
a
a
0
1
1
1
0
1
b
b
b
b
ON
OFF
OFF
OFF
a
a
a
a
0
0
b
1
b
0
b
1
1
0
g2
a
b
a
g1
a
0
0
b
(d)
a
0
1
1
0
1
1
b
b
b
b
OFF
ON
ON
ON
a
a
a
a
0
0
0
1
1
0
1
1
b
b
b
b
ON
ON
ON
OFF
Slide 13
CMOS Inverter
A
VDD
Y
0
1
A
A
Y
Y
GND
0: Introduction
CMOS VLSI Design
Slide 14
CMOS Inverter
A
VDD
Y
0
1
OFF
0
A=1
Y=0
ON
A
Y
GND
0: Introduction
CMOS VLSI Design
Slide 15
CMOS Inverter
A
Y
0
1
1
0
VDD
ON
A=0
Y=1
OFF
A
Y
GND
0: Introduction
CMOS VLSI Design
Slide 16
CMOS NAND Gate
A
B
0
0
0
1
1
0
1
1
Y
Y
A
B
0: Introduction
CMOS VLSI Design
Slide 17
CMOS NAND Gate
A
B
Y
0
0
1
0
1
1
0
1
1
0: Introduction
ON
ON
Y=1
A=0
B=0
CMOS VLSI Design
OFF
OFF
Slide 18
CMOS NAND Gate
A
B
Y
0
0
1
0
1
1
1
0
1
1
0: Introduction
OFF
ON
Y=1
A=0
B=1
CMOS VLSI Design
OFF
ON
Slide 19
CMOS NAND Gate
A
B
Y
0
0
1
0
1
1
1
0
1
1
1
0: Introduction
ON
A=1
B=0
CMOS VLSI Design
OFF
Y=1
ON
OFF
Slide 20
CMOS NAND Gate
A
B
Y
0
0
1
0
1
1
1
0
1
1
1
0
0: Introduction
OFF
A=1
B=1
CMOS VLSI Design
OFF
Y=0
ON
ON
Slide 21
CMOS NOR Gate
A
B
Y
0
0
1
0
1
0
1
0
0
1
1
0
0: Introduction
A
B
Y
CMOS VLSI Design
Slide 22
3-input NAND Gate
 Y pulls low if ALL inputs are 1
 Y pulls high if ANY input is 0
0: Introduction
CMOS VLSI Design
Slide 23
3-input NAND Gate
 Y pulls low if ALL inputs are 1
 Y pulls high if ANY input is 0
Y
A
B
C
0: Introduction
CMOS VLSI Design
Slide 24