COMP541
Transistors and all that…
a brief overview
Montek Singh
Sep 8, 2014
1
Transistors as switches
 At an abstract level, transistors are merely switches
 3-ported voltage-controlled switch
 n-type: conduct when control input is 1
 p-type: conduct when control input is 0
d
nMOS
pMOS
g=0
g=1
d
d
OFF
g
ON
s
s
s
s
s
s
g
OFF
ON
d
d
d
2
Silicon as a semiconductor
 Transistors are built from silicon
 Pure Si itself does not conduct well
 Impurities are added to make it conducting
 As provides free electrons  n-type
 B provides free “holes”  p-type
Figure 1.26 Silicon lattice and dopant atoms
MOS Transistors
 MOS = Metal-oxide semiconductor
 3 terminals
 gate: the voltage here controls whether current flows
 source and drain: are what the current flows between
Figure 1.29 nMOS and pMOS transistors
nMOS Transistors
 Gate = 0
 OFF = disconnect
 no current flows
between source & drain
 Gate = 1
 ON= connect
 current can flow between
source & drain
 positive gate voltage
draws in electrons to
form a channel
Figure 1.30 nMOS transistor operation
pMOS Transistors
 Just the opposite
 Gate = 1  disconnect
 Gate = 0  connect
 Summary:
d
nMOS
pMOS
g=0
g=1
d
d
OFF
g
ON
s
s
s
s
s
s
g
OFF
ON
d
d
d
6
CMOS Topologies
 There is actually more to it than connect/disconnect
 nMOS: pass good 0’s, but bad 1’s
 so connect source to GND
 pMOS: pass good 1’s, but bad 0’s
 so connect source to VDD
 Typically use them in
complementary fashion:
 nMOS network at bottom
 pulls output value down to 0
 pMOS network at top
pMOS
pull-up
network
inputs
output
nMOS
pull-down
network
 pulls output value up to 1
 only one of the two networks must conduct at a time!
 or smoke may be produced
 if neither network conducts  output will be floating
7
Inverter
NOT
A
VDD
Y
A
Y=A
A
0
1
P1
Y
N1
Y
1
0
GND
A
P1
N1
Y
0
ON
OFF
1
1
OFF
ON
0
8
NAND
NAND
A
B
Y
B
0
1
0
1
A B P1
0 0 ON
0 1 ON
P1
Y
Y = AB
A
0
0
1
1
P2
Y
1
1
1
0
A
N1
B
N2
P2
N1
N2
Y
ON OFF OFF 1
OFF OFF ON 1
1 0 OFF ON ON
1 1 OFF OFF ON
OFF 1
ON 0
9
3-input NOR Gate?
A
B
C
Y
10
2-input AND Gate?
A
B
Y
11
Transmission Gates
 Transmission gate is a switch:
 nMOS pass 1’s poorly
 pMOS pass 0’s poorly
 Transmission gate is a better switch
 passes both 0 and 1 well
 When
A
B
EN = 1, the switch is ON:
 A is connected to B
 When
EN
EN = 0, the switch is OFF:
EN
 A is not connected to B
 IMPORTANT: Transmission gates are not drivers
 will NOT remove input noise to produce clean(er) output
 simply connect A and B together (current could even flow backward!)
 use very carefully!
Logic using Transmission Gates
 Typically combine two (or more) transmission gates
 Together form an actual logic gate whose output is always
driven 0 or 1
 Exactly one transmission gate drives the output;
all remaining transmission gates float their outputs
 Example: XOR
 when C = 0, TG0 conducts
F = A
 when C = 1, TG1 conducts
TG0
 F = A’
 therefore:
 F = A xor C
TG1
13
Tristate buffer and tristate inverter
 When enabled: sends input to output
 When disabled: output is floating (‘Z’)
 Implementation:
 Tristate buffer using only a pass gate
E
Y
A
 If on: output  input
 If off: output is floating
EN
A
Y
EN
E
0
0
1
1
A
0
1
0
1
Y
Z
Z
0
1
 Tristate inverter
 Top half and bottom half are not fully
complementary
 Either both conduct: output  NOT(input)
– will act as a driver!
 Or both off: output is floating
14
Power Consumption
 Power = Energy consumed per unit time
 Dynamic power consumption
 Static power consumption
Dynamic Power Consumption
 Energy consumed due to switching activity:
 All wires and transistor gates have capacitance
 Energy required to charge a capacitance, C, to VDD is CVDD2
 Circuit running at frequency f: transistors switch (from 1 to 0
or vice versa) at that frequency
 Capacitor is charged f/2 times per second (discharging from 1
to 0 is free)
Pdynamic = ½CVDD2f
Static Power Consumption
 Power consumed when no gates are switching
 Caused by the quiescent supply current, IDD (also called the
leakage current)
Pstatic = IDDVDD
Power Consumption Example
 Estimate the power consumption of a wireless
handheld computer
VDD = 1.2 V
 C = 20 nF
 f = 1 GHz
 IDD = 20 mA

P = ½CVDD2f + IDDVDD
= ½(20 nF)(1.2 V)2(1 GHz) +
(20 mA)(1.2 V)
= 14.4 W