comp541 transistors and all that… a brief overview
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COMP541
Transistors and all that…
a brief overview
Montek Singh
Sep 8, 2014
Transistors as switches At an abstract level, transistors are merely
switches3-ported voltage-controlled switch
n-type: conduct when control input is 1p-type: conduct when control input is 0
2
g
s
d
g = 0
s
d
g = 1
s
d
g
d
s
d
s
d
s
nMOS
pMOS
OFF ON
ON OFF
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-typeB 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 flowssource and drain: are what the current flows between
Figure 1.29 nMOS and pMOS transistors
nMOS Transistors Gate = 0
OFF = disconnectno current flows
between source & drain
Gate = 1ON= 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 disconnectGate = 0 connect
Summary:
6
g
s
d
g = 0
s
d
g = 1
s
d
g
d
s
d
s
d
s
nMOS
pMOS
OFF ON
ON OFF
CMOS Topologies There is actually more to it than
connect/disconnectnMOS: pass good 0’s, but bad 1’s
so connect source to GNDpMOS: pass good 1’s, but bad 0’s
so connect source to VDD
Typically use them incomplementary fashion:nMOS network at bottom
pulls output value down to 0pMOS network at top
pulls output value up to 1only one of the two networks must conduct at a time!
or smoke may be produced if neither network conducts output will be floating 7
pMOSpull-upnetwork
outputinputs
nMOSpull-downnetwork
Inverter
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VDD
A Y
GND
N1
P1
NOT
Y = A
A Y0 11 0
A Y
A P1 N1 Y
0 ON OFF 1
1 OFF ON 0
NAND
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A
B
Y
N2
N1
P2 P1
NAND
Y = AB
A B Y0 0 10 1 11 0 11 1 0
AB
Y
A B P1 P2 N1 N2 Y
0 0 ON ON OFF OFF 1
0 1 ON OFF OFF ON 1
1 0 OFF ON ON OFF 1
1 1 OFF OFF ON ON 0
3-input NOR Gate?
10
B
CY
A
2-input AND Gate?
11
AB
Y
Transmission Gates Transmission gate is a switch:
nMOS pass 1’s poorlypMOS pass 0’s poorlyTransmission gate is a better switch
passes both 0 and 1 wellWhen EN = 1, the switch is ON:
A is connected to BWhen EN = 0, the switch is OFF:
A is not connected to B
IMPORTANT: Transmission gates are not driverswill NOT remove input noise to produce clean(er)
outputsimply connect A and B together (current could even flow
backward!)
use very carefully!
A B
EN
EN
Logic using Transmission Gates Typically combine two (or more) transmission
gates Together form an actual logic gate whose output is
always driven 0 or 1Exactly one transmission gate drives the output;
all remaining transmission gates float their outputs
Example: XORwhen C = 0, TG0 conducts
F = Awhen C = 1, TG1 conducts
F = A’ therefore:
F = A xor C
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TG0
TG1
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 If on: output input If off: output is floating
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
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E A Y0 0 Z0 1 Z1 0 01 1 1
A
E
Y
A Y
EN
EN
Power Consumption Power = Energy consumed per unit time
Dynamic power consumptionStatic power consumption
Dynamic Power Consumption Energy consumed due to switching activity:
All wires and transistor gates have capacitanceEnergy 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 computerVDD = 1.2 VC = 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
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