4.2 Current-Voltage Characteristics
• In this section, we focus on the MOSFET Characteristics
at DC or low frequencies (Static Characteristics)
• High-frequency characteristics and high switching speeds
are covered in section 4.8 in Sedra (Out of the Scope of
this course)
Microelectronic Circuits - Fifth Edition
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Dr. Tamer ElBatt
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Circuit Symbol for npn enhancement-type MOSFET
(aka n-channel MOSFET)
• Spacing between two vertical lines indicates that the Gate is
insulated from the Body
• MOSFET is a symmetric device, however, its useful to designate
one terminal as Source (S) and the other terminal as Drain (D)
Microelectronic Circuits - Fifth Edition
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The Current-Voltage Characteristics
Family of curves, each measured at constant vGS
2
3
1
• From the curves, there are 3 distinct regions of operation:
1. Cutoff Region
2. Triode (Linear) Region
3. Saturation Region
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Dr. Tamer ElBatt
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The Triode (Linear) Region
• Characterized by:
- vGS ≥ Vt
- vDS < vGS – Vt
(induce a channel)
(keep the channel continuous)
• As we derived last time, the iD-vDS characteristics in the Triode
region is given by:
W
iD = k
L
'
n
1 2 

(
)
v
V
v
v
−
−
GS
t
DS
DS 

2

where kn’ = µn Cox
• For Small vDS: the iD-vDS relationship near the origin can be
approximated by :
iD ≈ k’n W/L (vGS - Vt) vDS
Linear resistor rDS
Dr. Tamer ElBatt
The Saturation Region
• Characterized by:
- vGS ≥ Vt
- vDS ≥ vGS – Vt
(induce a channel)
(pinched-off channel)
• The iD-vDS characteristics in the Saturation region is obtained
when vDS = vGS – Vt which yields:
[
1 'W
2
iD = k n
(vGS − Vt )
2 L
]
In saturation, iD becomes independent of vDS. It depends
only on vGS
Dr. Tamer ElBatt
iD-vGS characteristic for an ,MOS transistor in Saturation
iD = 0.5 k’n W/L (vGS - Vt)2,
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vGS > Vt
Dr. Tamer ElBatt
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Large-signal equivalent circuit model of an ,MOS
Transistor in Saturation
The saturated MOSFET behaves as an Ideal VoltageControlled Current Source (controlled by vGS)
Microelectronic Circuits - Fifth Edition
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Dr. Tamer ElBatt
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Terminal Voltages and Operation Regions for ,MOS
Transistors
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Channel Length Modulation (in Saturation Region)
• In practice, increasing vDS beyond vDSsat does affect the channel
• As vDS is increased, the channel pinch-off point is moved slightly, away
from the Drain, toward the Source
• The depletion region between the end of the channel and the drain region
widens as (vDS - vDSsat) increases and, hence, the channel length reduces from
L to L-"L (a phenomenon known as Channel Length Modulation)
Microelectronic Circuits - Fifth Edition
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Channel Length Modulation cont.
1 'W
2
iD = k n (vGS − Vt ) (1 + λvDS )
2 L
where λ is a process technology parameter
Microelectronic Circuits - Fifth Edition
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Dr. Tamer ElBatt
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Channel Length Modulation cont.
• The Output Resistance r0 is given by,
'
n
k W
2 −1
(vGS − Vt ) ]
ro = [λ
2 L
Figure 4.17 Large-signal equivalent circuit model of the n-channel MOSFET in saturation, incorporating the output
resistance ro. The output resistance models the linear dependence of iD on vDS and is given by Eq. (4.22).
Microelectronic Circuits - Fifth Edition
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Dr. Tamer ElBatt
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Characteristics of the p-Channel MOSFET
Figure 4.18 (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. (d) The MOSFET with voltages applied and
the directions of current flow indicated. Note that vGS and vDS are negative and iD flows out of the drain terminal.
Microelectronic Circuits - Fifth Edition
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Dr. Tamer ElBatt
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Terminal Voltages and Operation Regions for PMOS
Transistors
Figure 4.19 The relative levels of the terminal voltages of the enhancement-type PMOS transistor for operation in the triode region
and in the saturation region.
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