1. ECE2030
Introduction to Computer Engineering
Lecture 3: Switches and CMOS
Prof. Hsien-Hsin Sean Lee
School of Electrical and Computer Engineering
Georgia Tech
2. 2 2
Basic Switch
• A path exists when the Switch Control is closed
– If (Open) OUTPUT = unknown ; Switch is open (OFF)
– Else OUTPUT = INPUT ; Switch is closed (ON)
INPUT OUTPUT
Switch Control
3. 3 3
The Analogy of A Transistor
Cross Section
An N-Channel Metal-Oxide Semiconductor Field Effect Transistor (MOSFET)
INPUT OUTPUT
Switch Control (Gate)
4. 4 4
Transistor Characteristics
• Cut-off Region
– Vgs – Vt 0
– No current (Ids) between drain and source
• Linear (or Ohmic) Region
– 0 < Vds < Vgs – Vt
– Ids is a function of Vgs and Vds
– Ids = β*[(Vgs-Vt)*Vds – Vds*Vds/2]
• Saturation Region
– 0 < Vgs – Vt < Vds
– Ids is independent of Vds
– Ids = (β/2)*(Vgs-Vt)2
– β = process factor * (W/L)
• Vt : Threshold voltage, a function of
materials, doping, insulator thickness, etc.
Gate
Drain
Source
Ids
Vds
Vgs
N-type MOS Transistor
20. 20 20
CMOS Signal Transfer Property
Gate Path
0 Closed
1 Open
Gate
Drain
Source
Gate
Source
Drain
Gate Path
0 Open
1 Closed
pMOS
nMOS
• Transmits 1 well
• Transmits 0 poorly
• Transmits 0 well
• Transmits 1 poorly
21. 21 21
CMOS Transmission Gate
• Transmit signal from INPUT to OUTPUT when
Gate is closed
Gate (complementary of Gate)
Source Drain
Gate
INPUT OUTPUT
Gate pMOS nMOS OUTPUT
0 OFF OFF Z
1 ON ON INPUT
Z : High-Impedance State,
consider the terminal is “floating”
22. 22 22
High Impedance
• When a path exists
– Impedance is low to
allow ample flow of
current
• When no path
– Impedance is high
allowing almost no
current flow between
two terminals
Gate=1
Drain
Source
<< 10K
>> 100M
Closed
Gate=0
Drain
Source
Open