Upload
christine-nelson
View
227
Download
0
Embed Size (px)
DESCRIPTION
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 3 Hydraulic analogy Voltage water pressure Current flow rate Volume flow rate in m 3 /sec, etc.Current flow rate in coulombs/sec = amps
Citation preview
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell
CMOS Transistors and Gates
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 2
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
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 3
Hydraulic analogyVoltage water pressure
Current flow rate
Volume flow rate in m3/sec, etc. Current flow rate in coulombs/sec = amps
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 4
Hydraulic analogy
Resistance related to length and radius of pipe (kL/r4)
Ground reservoir
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 5
Hydraulic analogyResistances in series
Resistances in parallel
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 6
CMOS Transistors
Need circuits to represent 2 discrete values1,0 for binary representationsTrue, False for Boolean logic
Let high voltage (Vdd) represent 1, or true
Let low voltage (0 volts or gnd) represent 0, or falseIf we have some switches to control whether or not these voltages can propagate through a circuit, we can build a computer with them
Note, the earliest digital computers were electromechanical, made out of relays, so this is hardly a new idea
Our switches will be CMOS transistors
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 7
1 (Vdd)
Two kinds of transistorsN-type
1 (Vdd)
P-type
1 (Vdd)
1 (Vdd)source
draingate
s
dg
s
dg
s
dg
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 8
0 (gnd)
Two kinds of transistorsN-type
0 (gnd)
P-type
0 (gnd)
0 (gnd)
s
dg
s
dg
s
dg
source
draing
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 9
How they work as switches
N-type
n+ n+
p-type body
W
L
tox
SiO2 gate oxide(good insulator, εox = 3.9)
polysilicongatε
+-
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
QuickTime™ and a decompressor
are needed to see this picture.
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 10
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 d Ids
CMOS transistor pictures fromUT ECE VLSI course slides
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 11
CMOS circuit rules
Never create a path from Vdd to gnd
Don’t pass weak valuesN-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
University of Texas at Austin CS310 - Computer Organization Spring 2009 Don Fussell 12
Example CMOS gate - inverter
In Out0 11 0
Truth table Circuit
Note how all 3 design rules are obeyedCircuit amplifies weak input 1 or 0