Metal Oxide Semiconductor
Field-Effect Transistors
(MOSFETs)
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Device
Structure
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N-Channel MOSFET
Providing
electrons
Pulling electrons
(makes current flow)
+ + +
• Apply positive voltage to gate:
– Drives away “holes” and attracts electrons in body under
gate
– Creates “n-channel” between the source and drain and
now current can flow
https://www.youtube.com/watch?v=tz62t-q_KEc
EE204 - 2015
Electronic Devices: Field Effect Transistors
3
Operation with Zero Gate Voltage
Two back-to-back diodes, High Resistance (Giga Ohms), No
Current
Flow Copyright © 2010 by Oxford University Press, Inc.
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Microelectronic Circuits, Sixth Edition
Creating a Channel
for Current Flow
-vGS (gate to source voltage)
-Vt: threshold voltage (vGS
which channel starts
conducting)
-Current flowing when vDS
applied
-Effective voltage (or overdrive
voltage):
vGS  Vt  vOV  veff
Microelectronic Circuits, Sixth Edition
Charge in the channel:
Q  Cox (WL)vOV
-Oxide capacitance:
Cox 
 ox
tox
-Gate to source capacitance:
C  CoxWL
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Applying a small vDS
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Operation as
vDS is increased
Channel becomes
more tapered and its
resistance increases
Microelectronic Circuits, Sixth Edition
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Microelectronic Circuits, Sixth Edition
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Operation for
vDS>>VOV
(channel pinchoff, saturation
mode of operation)
1 ' W 2
i D  k n ( )vOV
L
2
vOV  vGS  Vt
1 ' W
i D  k n ( )(vGS  Vt ) 2
2
L
k 'n   nCox
Microelectronic Circuits, Sixth Edition
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P-Channel
MOSFET
Microelectronic Circuits, Sixth Edition
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P-Channel MOSFET
Threshold voltage
vGS  Vtp
Use absolute value
vGS  Vtp
P-Channel transistor process transconductance parameter
k ' p   p Cox
P-Channel transistor transconductance parameter
k p   p Cox (
Microelectronic Circuits, Sixth Edition
“Formulae are the same,
switch sign of voltages”
W
)
L
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Complementary MOS or CMOS
Figure 5.10 Cross-section of a CMOS integrated circuit.
The PMOS transistor is formed in a separate n-type region (n well). Another
arrangement is also possible in which an n-type body is used and the n device
is formed in a p well. Not shown are the connections made to the p-type body
and to the n well; the latter functions as the body terminal for the p-channel
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Microelectronic Circuits, Sixth Edition
device.
Current-Voltage Characteristics
Figure 5.11 (a) Circuit symbol for the n-channel enhancement-type MOSFET.
(b) Modified circuit symbol with an arrowhead on the source terminal to distinguish
it from the drain and to indicate device polarity (i.e., n channel). (c) Simplified
circuit symbol to be used when the source is connected to the body or when the
effect of the body on device operation is unimportant.
Microelectronic Circuits, Sixth Edition
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Table 5.1 Regions of Operation of the
Enhancement NMOS Transistor
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Microelectronic Circuits, Sixth Edition
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The iD—vGS
Characteristic
iD 
1 ' W
k n ( )(vGS  Vt ) 2
2
L
in term of vOV:
1
W 2
i D  k n' ( )vOV
L
2
Microelectronic Circuits, Sixth Edition
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Large-signal equivalent-circuit model of an n-channel
MOSFET operating in the saturation
Microelectronic Circuits, Sixth Edition
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Finite Output Resistance in Saturation
Channel-length
modulation:
1 ' W
i D  k n ( )(vGS  Vtn ) 2 (1  v DS )
2
L
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1 ' W
iD  k n ( )(vGS  Vtn ) 2
2
L
VA
1
r0 
 r0 
I D
ID
Microelectronic Circuits, Sixth Edition
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Large-signal equivalent circuit model
1
r0 
I D
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Characteristics of the p-Channel MOSFET
Figure 5.19 (a) Circuit symbol for the p-channel enhancement-type
MOSFET. (b) Modified symbol with an arrowhead on the source lead. (c)
Simplified circuit symbol for the case where the source is connected to the
body.
Microelectronic Circuits, Sixth Edition
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Table 5.2 Regions of Operation of the
Enhancement PMOS Transistor
Microelectronic Circuits, Sixth Edition
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Example 5.3. MOSFET Circuits at DC (p.259)
Given:
ID=0.4mA, VD=0.5V,
Vt=0.7V, nCox=100 A/V2,
L=1 m, W=32 m, =0
Find:
R D , RS
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Example 5.6 (p. 263)
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Example 5.7 (p. 264)
Given:
ID=0.5mA
VD=3V
Vtp=-1V
PMOS
Find:
RG1
RG2
Microelectronic Circuits, Sixth Edition
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