CMOS Transistors and Gates · 2009. 1. 30. · 2 gate oxide (good insulator, ! ox = 3.9)...
Transcript of CMOS Transistors and Gates · 2009. 1. 30. · 2 gate oxide (good insulator, ! ox = 3.9)...
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell
CMOS Transistors and Gates
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Simple electronicsOhm’s Law - V = IR
voltage (V) equals current (I) times resistance (R)
Hydraulic AnalogyCharge ⇒ liquidCurrent ⇒ flow rateVoltage ⇒ water pressureResistance ⇒ related to length and radius of pipe (kL/r4)
Hydraulic pictures fromhttp://hyperphysics.phy-astr.gsu.edu
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Hydraulic analogyVoltage ⇒ water pressure
Current ⇒ flow rate
Volume flow rate in m3/sec, etc. Current flow rate in coulombs/sec = amps
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Hydraulic analogy
Resistance ⇒ related to length and radius of pipe (kL/r4)
Ground ⇒ reservoir
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Hydraulic analogyResistances in series
Resistances in parallel
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CMOS Transistors
Need circuits to represent 2 discrete values1,0 for binary representationsTrue, False for Boolean logic
Let high voltage (Vdd) represent 1, or trueLet low voltage (0 volts or gnd) represent 0, or falseIf we have some switches to control whether or not thesevoltages can propagate through a circuit, we can build acomputer with them
Note, the earliest digital computers were electromechanical, madeout of relays, so this is hardly a new idea
Our switches will be CMOS transistors
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1 (Vdd)
Two kinds of transistorsN-type
1 (Vdd)
P-type
1 (Vdd)
1 (Vdd)source
draingate
s
dg
s
dg
s
dg
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0 (gnd)
Two kinds of transistorsN-type
0 (gnd)
P-type
0 (gnd)
0 (gnd)
s
dg
s
dg
s
dg
source
draing
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How they work as switches
N-type
n+ n+
p-type body
W
L
tox
SiO2 gate oxide
(good insulator, !ox
= 3.9)
polysilicon
gate
+-
Vgs
= 0
n+ n+
+-
Vgd
p-type body
b
g
s d
When gate is not at higher voltage than source• no excess electrons in channel under gate• so no current can flow• switch is open
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How they work as switches
N-type
When Vgs > Vth ,the threshold voltage• excess electrons attracted into channel• current flows and switch is closed• drain voltage cannot be more than source voltage = Vg-Vth• this is at most Vdd-Vth• Vdd-Vth is still considered a 1, but a weak 1• if source voltage is 0, then drain voltage is too, so 0 still strong
+-
Vgs
> Vt
n+ n+
+-
Vgd
= Vgs
+-
Vgs
> Vt
n+ n+
+-
Vgs
> Vgd
> Vt
Vds
= 0
0 < Vds
< Vgs
-Vt
p-type body
p-type body
b
g
s d
b
g
s dIds
CMOS transistor pictures fromUT ECE VLSI course slides
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CMOS circuit rules
Never create a path from Vdd to gndDon’t pass weak values
N-type transistors pass weak 1’s (Vdd - Vth)N-type transistors pass strong 0’s (gnd)Use N-type transistors only to pass 0’s (n to negative)Conversely for P-type transistors
Pass weak 0’s (Vth), strong 1’s (Vdd)Use P-type transistors only to pass 1’s (p to positive)
Never leave a wire undrivenMake sure there’s always a path to Vdd or gnd
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Example CMOS gate - inverter
0110
OutIn
Truth table Circuit
Note how all 3 design rules are obeyedCircuit amplifies weak input 1 or 0