MOS Field-Effect Transistors
(MOSFETs)
EEZ 305 - Elektronik II
Microelectronic Circuits – Fourth Edition
Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press
Dr. Mehmet Siraç Özerdem
Elektrik Elektronik Müh. Bölümü
Dicle Üniversitesi
MOS Field-Effect Transistors
(MOSFETs)
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1
The n-channel MOSFET
Physical structure of the enhancement-type NMOS transistor:
(a) perspective view;
(b) cross-section. Typically L = 0.1 to 3 mm, W = 0.2 to 100 mm,
and the thickness of the oxide layer (tox) is in the range of 2 to
50 nm.
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The n-channel MOSFET
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Sedra/Smith
Operation with no Gate Voltage
• Back-to-back diodes exist in series between D and S
• The path between D and S has a high resistance
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The n-channel MOSFET
Creating a Channel
The enhancement-type NMOS transistor with VGS > 0
An n channel is induced at the top of the substrate beneath
the gate.
When VGS > Vt (threshold voltage), channel is induced
1V < Sedra/Smith
Vt < 3V
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The n-channel MOSFET
Applying a small VDS
Specifically, the channel conductance is proportional to vGS – Vt’ and
thus iD is proportional to (vGS – Vt) and vDS.
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The n-channel MOSFET
The iD–vDS characteristics of the MOSFET
The device operates
as a linear resistor controlled by vGS.
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The n-channel MOSFET
Operation as VDS is increased
The induced channel acquires a tapered shape, and its resistance
increases as vDS
is increased.
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The n-channel MOSFET
Increasing vDS causes the channel to acquire a tapered shape.
Eventually, as vDS reaches vGS – Vt’ the channel is pinched off at
the drain end. Increasing vDS above vGS – Vt has little effect
(theoretically, no effect) on the channel’s shape.
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The n-channel MOSFET
iD versus vDS for an enhancement-type NMOS transistor
operated with
vGS >Circuits
Vt - Fifth Edition Sedra/Smith
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Derivation of the iD–vDS Relationship
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The n-channel MOSFET
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The n-channel MOSFET
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The n-channel MOSFET
iD current for Triode region
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The n-channel MOSFET
For obtaining iD current for saturation region, take the
voltage, VDS, as follows,
iD current for saturation region
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}
Process transconductance parameters
The n-channel MOSFET
Aspect ratio
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The p-channel MOSFET
P-channel enhancement type MOSFET (PMOS)
Carrier : Holes
VDS, Vt, VGS negative values
Disadvantages of PMOS.
NMOS and PMOS in CMOS circuits
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Complementary MOS (CMOS)
Note that the PMOS transistor is formed in a separate n-type
region, known as an 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.
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The n-channel MOSFET
(a) Circuit symbol for the NMOS.
(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
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operation isMicroelectronic
unimportant.
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The n-channel MOSFET
(a) An n-channel enhancement-type MOSFET with vGS and vDS
applied and with the normal directions of current flow indicated.
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(b) The iD–vDS characteristics
for a device with k’n (W/L)
= 1.0 mA/V2.
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10
The n-channel MOSFET
For Triode region
Linear correlation (linear resistance)
rDS can
be controlled
VGS voltage
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The n-channel MOSFET
For saturation region
Vt = 1 V
k’n (W/L) =1.0 mA/V2
The iD–vGS characteristic for an enhancement-type NMOS
transistor in saturation
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11
Large-signal equivalent-circuit model of an n-channel
MOSFET operating in the saturation region.
Ideal model
The n-channel MOSFET
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For the broken line characteristic
vGS - Vt = vDS
substitute it in the either triode region equation or
the saturation region equation.
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12
The relative levels of the terminal voltages of the enhancement
NMOS transistor for operation in the triode region and in the
saturation region.
The n-channel MOSFET
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The n-channel MOSFET
Increasing vDS beyond vDSsat causes the channel pinch-off
point to move slightly away from the drain, thus reducing
the effective channel length (by ΔL).
0.03 > λ > 0.005
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200V > VA > 30V
Effect of vDS on iD in the saturation region. The MOSFET
parameter VA depends on the process technology and, for a
given process, is proportional to the channel length L.
The n-channel MOSFET
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The n-channel MOSFET
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Large-signal equivalent circuit model of the n-channel
MOSFET in saturation, incorporating the output resistance ro.
The n-channel MOSFET
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Characteristics of the p-channel MOSFET
(a) Circuit symbol of PMOS
(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 iD flows
out of the drain terminal.
The p-channel MOSFET
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Characteristics of the p-channel MOSFET
VGS ,VDS ,Vt and λ are all negative.
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Characteristics of the p-channel MOSFET
The relative levels of the terminal voltages of the
enhancement-type PMOS transistor for operation in
the triode region and in the saturation region.
The p-channel MOSFET
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The n-channel MOSFET
The p-channel MOSFET
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The depletion Type MOSFET
(a) Circuit symbol for the n-channel depletion-type
MOSFET. (b) Simplified circuit symbol applicable for
the case the substrate (B) is connected to the source (S).
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The depletion Type n-channel MOSFET
Vt = –4 V and kn(W/L) = 2 mA/V2
(a) transistor with current and voltage polarities indicated;
(b) the iD–vDS characteristics
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The depletion Type n-channel MOSFET
The iD–vGS characteristic in saturation.
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18
The depletion Type n-channel MOSFET
The relative levels of terminal voltages of a depletiontype NMOS transistor for operation in the triode and the
saturation regions. The case shown is for operation in
the enhancement mode (vGS is positive).
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The depletion Type MOSFET
Sketches of the iD–vGS characteristics for MOSFETs of
enhancement and depletion types, of both polarities (operating
in saturation). Note that the characteristic curves intersect the
vGS axis at Vt. Also note that for generality somewhat different
values of |Vt| are shown for n-channel and p-channel devices.
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Example
For a depletion_type NMOS transistor
Vt = - 2V
kn(W/L)=2m A/V2
a) Find the minimum VDS required to operate in the
saturation region, when VGS=1V
b) iD=?
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Example
For a depletion_type NMOS transistor
Vt = - 2V
kn(W/L)=4m A/V2
Neglect the effect of VDS on iD in the saturation region
Vs = ?
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The MOSFET as an amplifier
Conceptual circuit utilized to study the operation of
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the MOSFET
as a small-signal
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The MOSFET as an amplifier
To find the dc bias or operating point of the MOSFET,
we set the signal vgs = 0 (assume that λ=0).
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The MOSFET as an amplifier
vgs ≠ 0 (assume that λ=0).
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Small-signal operation of the enhancement MOSFET amplifier
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The other expression of gm is given below
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Voltage Gain
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Total instantaneous
voltages vGS and vD
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Small-signal equivalent circuit models
(a) neglecting the dependence of iD on vDS in saturation (the
channel-length modulation effect);
(b) including the effect of channel-length modulation,
modeled by output resistance ro = |VA| /ID.
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Example
Small signal voltage gain (vo/vi) = ?
Input resistance (Ri) = ?
Amplifier
circuit
Equivalent-circuit model
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Development of the T equivalent-circuit model for the MOSFET.
For simplicity,
ro has been
omitted
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Development of the T equivalent-circuit model for the MOSFET.
ro can be added between D and S in the T model
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Biasing in MOS Amplifier Circuits
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The High_Frequency MOSFET Model
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The High_Frequency MOSFET Model
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The MOSFET Unity-Gain Frequency (fT)
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