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EE314
IBM/Motorola Power PC620
IBM Power PC 601
Motorola MC68020
Field EffectTransistors
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Chapter 12: Field Effect Transistors
1.Construction of MOS2.NMOS and PMOS3.Types of MOS 4.MOSFET Basic Operation5.Characteristics
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The MOS Transistor
Polysilicon Aluminum
JFET – Junction Field Effect Transistor
MOSFET - Metal Oxide Semiconductor Field Effect Transistor
n-channel MOSFET (nMOS) & p-channel MOSFET (pMOS)
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The MOS Transistor
n+n+
p-substrate
Field-Oxide
(SiO2)
p+ stopper
Polysilicon
Gate Oxide
DrainSource
Gate
Bulk Contact
CROSS-SECTION of NMOS Transistor
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Switch Model of NMOS Transistor
Gate
Source
(of carriers)
Drain
(of
carriers)
| VGS |
| VGS | < | VT | | VGS | > | VT |
Open (off) (Gate = ‘0’) Closed (on) (Gate = ‘1’)
Ron
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Switch Model of PMOS Transistor
Gate
Source
(of carriers)
Drain
(of carriers)
| VGS |
| VGS | > | VDD – | VT | | | VGS | < | VDD – |VT| |
Open (off) (Gate = ‘1’) Closed (on) (Gate = ‘0’)
Ron
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MOS transistors Symbols
D
S
G
D
S
G
G
S
D D
S
G
NMOS Enhancement NMOS
PMOS
Depletion
Enhancement
B
NMOS with
Bulk Contact
Channel
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JFET and MOSFET Transistorsor
L = 0.5-10 mmW = 0.5-500 mm
SiO2 Thickness = 0.02-0.1 mm
Device characteristics depend on L,W, Thickness, doping levels
Symbol
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MOSFET Transistor Fabrication Steps
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Building A MOSFET Transistor Using Silicon
http://micro.magnet.fsu.edu/electromag/java/transistor/index.html
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It is done. Now, how does it work?
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n-channel MOSFET Basic Operation
Operation in the Cutoff region
Schematic
pn junction: reverse bias
iD=0
for vGS<Vt0
When vGS=0 then iD=0 until vGS>Vt0 (Vt0 –threshold voltage)
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n-channel MOSFET Basic Operation
Operation in the Triode Region
For vDS<vGS-Vt0 and vGS>Vt0 the NMOS is operating in the triode region
Resistor like characteristic(R between S & D, Used as voltage controlled R)
For small vDS, iD is proportional to the excess voltage vGS-Vt0
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n-channel MOSFET Basic Operation
Operation in the Triode Region
2
02 DSDStGSD vvVvKi
2
KP
L
WK
Device parameter KP for NMOSFET is 50 mA/V2
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n-channel MOSFET Basic Operation
Operation in the Saturation Region (vDS is increased)
Tapering of the channel- increments of iD are smaller when vDS islarger When vGD=Vt0 then the channel
thickness is 0 and
2
0tGSD VvKi
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n-channel MOSFET Basic Operation
Example 12.1
An nMOS has W=160 mm, L=2 mm, KP= 50 mA/V2 and Vto=2 V.
Plot the drain current characteristic vs drain to source voltage
for vGS=3 V.
2
KP
L
WK
2
02 DSDStGSD vvVvKi
2
0tGSD VvKi
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n-channel MOSFET Basic Operation
Characteristic
2
DSD Kvi
Example 12.1
Channel lengthmodulation
id depends on vDS in saturation region
(approx: iD =const in saturation region)
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p-channel MOSFET Basic Operation
It is constructed by interchanging the n and p regions of n-channel MOSFET.
SymbolCharacteristic
How does p-channel MOSFET operate?-voltage polarities-iD current-schematic