SJTU Zhou Lingling1 Chapter 2 Field-Effect Transistors(FETs)
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Transcript of SJTU Zhou Lingling1 Chapter 2 Field-Effect Transistors(FETs)
SJTU Zhou Lingling 1
Chapter 2
Field-Effect Transistors(FETs)
SJTU Zhou Lingling 2
Outline
• Introduction
• Device Structure and Physical Operation
• Current-Voltage Characteristics
• MOSFET Circuit at DC
• The MOSFET as an amplifier
• Biasing in MOS Amplifier Circuits
• Small-signal Operation and Models
• Single-Stage MOS amplifier
• The MOSFET Internal Capacitance and High-Frequency Model
• The depletion-type MOSFET
SJTU Zhou Lingling 3
Introduction
• CharacteristicsFar more useful than two-terminal deviceVoltage between two terminals can control the current
flows in third terminalQuite smallLow powerSimple manufacturing process
SJTU Zhou Lingling 4
Introduction
• Classification of MOSFET MOSFET
P channel Enhancement type Depletion type
N channel Enhancement type Depletion type
JFET P channel N channel
• Widely used in IC circuits
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Device Structure and Physical Operation
• Device structure of the enhancement NMOS
• Physical operation
• p channel device
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Device Structure of the Enhancement-Type NMOS
Perspective view
Four terminals
Channel length and width
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Device Structure of the Enhancement-Type NMOS
Cross-section view.
L = 0.1 to 3 m
W = 0.2 to 100 m
Tox= 2 to 50 nm
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Physical Operation
• Creating an n channel
• Drain current controlled by vDS
• Drain current controlled by vGS
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Creating a Channel for Current Flow
The enhancement-type NMOS transistor with a positive voltage applied to the gate.
An n channel is induced at the top of the substrate beneath the gate.
Inversion layer
Threshold voltage
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Drain Current Controlled by Small Voltage vDS
An NMOS transistor with vGS > Vt and with a small vDS applied.
The channel depth is uniform.
The device acts as a resistance.
The channel conductance is proportional to effective voltage.
Drain current is proportional to (vGS – Vt) vDS.
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vDS Increased
Operation of the enhancement NMOS transistor as vDS is increased.
The induced channel acquires a tapered shape.
Channel resistance increases as vDS is increased.
Drain current is controlled by both of the two voltages.
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Channel Pinched Off
• Channel is pinched offInversion layer disappeared at the drain pointDrain current isn’t disappeared
• Drain current is saturated and only controlled by the vGS
• Triode region and saturation region
• Channel length modulation
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Drain Current Controlled by vGS
• vGS creates the channel.
• Increasing vGS will increase the conductance of the channel.
• At saturation region only the vGS controls the drain current.
• At subthreshold region, drain current has the exponential relationship with vGS
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p Channel Device
• Two reasons for readers to be familiar with p channel device Existence in discrete-circuit. More important is the utilization of CMOS circuits.
• Structure of p channel device The substrate is n type and the inversion layer is p type. Carrier is hole. Threshold voltage is negative. All the voltages and currents are opposite to the ones of n channel
device. Physical operation is similar to that of n channel device.
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Complementary MOS or CMOS
The PMOS transistor is formed in 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.
CMOS is the most widely used of all the analog and digital IC circuits.
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Current-Voltage Characteristics
• Circuit symbol
• Output characteristic curves
• Channel length modulation
• Characteristics of p channel device
• Body effect
• Temperature effects and Breakdown Region
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Circuit Symbol
(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.
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Output Characteristic Curves
(a) An n-channel enhancement-type MOSFET with vGS and vDS applied and with the normal directions of current flow indicated.
(b) The iD–vDS characteristics for a device with k’n (W/L) = 1.0 mA/V2.
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Output Characteristic Curves
• Three distinct region
Cutoff region
Triode region
Saturation region
• Characteristic equations
• Circuit model
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Cutoff Region
• Biased voltage
• The transistor is turned off.
• Operating in cutoff region as a switch.
tGS Vv
0Di
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Triode Region
• Biased voltage
• The channel depth from uniform to tapered shape.
• Drain current is controlled not only by vDS but also by vGS
tGSDS
tGS
Vvv
Vv
DStGSn
DSDStGSnD
vVvL
Wk
vvVvL
Wki
)('
2
1)(' 2
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Triode Region
• Assuming that the drain-t-source voltage is sufficiently small.
• The MOS operates as a linear resistance
OVn
tGSnVvD
DSDS
VL
Wk
VVL
Wki
vr
GSGS
'
1
)('1
OVDS Vv 2
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Saturation Region
• Biased voltage
• The channel is pinched off.
• Drain current is controlled only by vGS
• Drain current is independent of vDS and behaves as an ideal current source.
tGSDS
tGS
Vvv
Vv
221 )' tGSnD Vv
L
Wki (
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Saturation Region
The iD–vGS characteristic for an enhancement-type NMOS transistor in saturation
Vt = 1 V, k’n W/L = 1.0 mA/V2
Square law of iD–vGS characteristic curve.
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Relative Levels of the Terminal Voltages
The relative levels of the terminal voltages of the enhancement NMOS transistor for operation in the triode region and in the saturation region.
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Channel Length Modulation
• Explanation for channel length modulation
Pinched point moves to source terminal with the voltage vDS increased.
Effective channel length reduced
Channel resistance decreased
Drain current increases with the voltage vDS increased.
• Current drain is modified by the channel length modulation
)1)' 221
DStGSnD vVvL
Wki +((
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Channel Length Modulation
The MOSFET parameter VA depends on the process technology and, for a given process, is proportional to the channel length L.
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Channel Length Modulation
• MOS transistors don’t behave an ideal current source due to channel length modulation.
• The output resistance is finite.
• The output resistance is inversely proportional to the drain current.
D
A
Dconstv
DS
Do I
V
Iv
ir
GS
1
.
1
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Large-Signal Equivalent Circuit Model
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
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Characteristics of p Channel Device
(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.
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Characteristics of p Channel Device
The MOSFET with voltages applied and the directions of current flow indicated.
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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Characteristics of p Channel Device
Large-signal equivalent circuit model of the p-channel MOSFET in saturation, incorporating the output resistance ro. The output resistance models the linear dependence of iD on vDS
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The Body Effect
• In discrete circuit usually there is no body effect due to the connection between body and source terminal.
• In IC circuit the substrate is connected to the most negative power supply for NMOS circuit in order to maintain the pn junction reversed biased.
• The body effect---the body voltage can control iD
Widen the depletion layer Reduce the channel depth Threshold voltage is increased Drain current is reduced
• The body effect can cause the performance degradation.
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Temperature Effects and Breakdown Region
• Drain current will decrease when the temperature increase.
• BreakdownAvalanche breakdownPunched-throughGate oxide breakdown
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MOSFET Circuit at DC
a. Assuming device operates in saturation thus iD satisfies with iD~vGS equation.
b. According to biasing method, write voltage loop equation.
c. Combining above two equations and solve these equations.
d. Usually we can get two value of vGS, only the one of two has physical meaning.
e. Checking the value of vDS
i. if vDS≥vGS-Vt, the assuming is correct.
ii. if vDS≤vGS-Vt, the assuming is not correct. We shall use triode
region equation to solve the problem again.
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MOSFET Circuit at DC
The NMOS transistor is operating in the saturation region due to
tGD VV
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MOSFET Circuit at DC
Assuming the MOSFET operate in the saturation region
Checking the validity of the assumption
If not to be valid, solve the problem again for triode region
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The MOSFET As an Amplifier
Basic structure of the common-source amplifier.
Graphical construction to determine the transfer characteristic of the amplifier in (a).
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The MOSFET As an Amplifier and as a Switch
Transfer characteristic showing operation as an amplifier biased at point Q.
Three segments:
XA---the cutoff region segment
AQB---the saturation region segment
BC---the triode region segment
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Biasing in MOS Amplifier Circuits
• Voltage biasing scheme Biasing by fixing voltage
Biasing with feedback resistor
• Current-source biasing scheme
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Biasing in MOS Amplifier Circuits
The use of fixed bias (constant VGS) can result in a large variability in the value of ID.
Devices 1 and 2 represent extremes among units of the same type.
Current becomes temperature dependent
Unsuitable biasing method
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Biasing in MOS Amplifier Circuits
Biasing using a fixed voltage at the gate, and a resistance in the source lead
(a) basic arrangement;
(b) reduced variability in ID;
(c) practical implementation using a single supply;
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Biasing in MOS Amplifier Circuits
(d) coupling of a signal source to the gate using a capacitor CC1;
(e) practical implementation using two supplies.
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Biasing in MOS Amplifier Circuits
Biasing the MOSFET using a large drain-to-gate feedback resistance, RG.
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Biasing in MOS Amplifier Circuits
(a) Biasing the MOSFET using a constant-current source I.
(b) Implementation of the constant-current source I using a current mirror.
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Small-Signal Operation and Models
• The ac characteristic Definition of transconductance
Definition of output resistance
Definition of voltage gain
• Small-signal model Hybrid π model
T model
Modeling the body effect
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The ac Characteristic
Conceptual circuit utilized to study the operation of the MOSFET as a small-signal amplifier.
Small signal condition
OVgs Vv 2
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The ac Characteristics
• The definition of transconductance
• The definition of output resistance
• The definition of voltage gain
OVn
VvGS
Dm V
L
Wk
v
ig
GSGS
'
D
A
IiD
DSo I
V
i
vr
DD
Dmi
ov Rg
v
vA
SJTU Zhou Lingling 49
The Small-Signal Models
(a) neglecting the the channel-length modulation effect
(b) including the effect of channel-length modulation, modeled by output resistance ro = |VA| /ID.
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The Small-Signal Models
(a) The T model of the MOSFET augmented with the drain-to-source resistance ro.
(b) An alternative representation of the T model.
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Modeling the Body Effect
Small-signal equivalent-circuit model of a MOSFET in which the source is not connected to the body.
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Single-Stage MOS Amplifier
• Characteristic parameters
• Basic structure
• Three configurations
Common-source configuration
Common-drain configuration
Common-gate configuration
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Characteristic Parameters of Amplifier
This is the two-port network of amplifier.
Voltage signal source.
Output signal is obtained from the load resistor.
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Definitions
• Input resistance with no load
• Input resistance
• Open-circuit voltage gain
• Voltage gain
LRi
ii i
vR
i
iin i
vR
LRi
ovo v
vA
i
ov v
vA
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Definitions(cont’d)
• Short-circuit current gain
• Current gain
• Short-circuit transconductance gain
0
LRi
ois i
iA
i
oi i
iA
0
LRi
om v
iG
SJTU Zhou Lingling 56
Definitions(cont’d)
• Open-circuit overall voltage gain
• Overall voltage gain
LRsig
vo v
vG 0
sigv v
vG 0
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Definitions(cont’d)
Output resistance of amplifier proper
0
ivx
xo i
vR
Output resistance
0
sigvx
xout i
vR
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Definitions(cont’d)
Voltage amplifier
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Definitions(cont’d)
Voltage amplifier
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Definitions(cont’d)
Transconductance amplifier
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Relationships
• Voltage divided coefficient
sigin
in
sig
i
RR
R
v
v
oL
Lvov RR
RAA
omvo RGA
oL
Lvo
sigin
inv RR
RA
RR
RG
vosigi
ivo A
RR
RG
outL
Lvov RR
RGG
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Basic Structure of the Circuit
Basic structure of the circuit used to realize single-stage discrete-circuit MOS amplifier configurations.
SJTU Zhou Lingling 63
The Common-Source Amplifier
Common-source amplifier based on the circuit of basic structure.
Biasing with constant-current source.
CC1 And CC2 are coupling capacitors.
CS is the bypass capacitor.
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Equivalent Circuit of the CS Amplifier
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Equivalent Circuit of the CS Amplifier
Small-signal analysis performed directly on the amplifier circuit with the MOSFET model implicitly utilized.
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Characteristics of CS Amplifier
• Input resistance
• Voltage gain
• Overall voltage gain
• Output resistance
Gin RR
)////( LDomv RRrgA
)////( oLDmsigG
Gv rRRg
RR
RG
Doout RrR //
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Summary of CS Amplifier
• Very high input resistance
• Moderately high voltage gain
• Relatively high output resistance
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The Common-Source Amplifier with a Source Resistance
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Small-signal Equivalent Circuit with ro Neglected
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Characteristics of CS Amplifier with a Source Resistance
• Input resistance
• Voltage gain
• Overall voltage gain
• Output resistance
Gin RR
Sm
LDmv Rg
RRgA
1
)//(
Sm
LDm
sigG
Gv Rg
RRg
RR
RG
1
)//(
Dout RR
SJTU Zhou Lingling 71
Summary of CS Amplifier with a Source Resistance
• Including RS results in a gain reduction by the factor (1+gmRS)
• RS takes the effect of negative feedback.
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The Common-Gate Amplifier
Biasing with constant current source I
Input signal vsig is applied to the source
Output is taken at the drain
Gate is signal grounded
CC1 and CC2 are coupling capacitors
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The Common-Gate Amplifier
A small-signal equivalent circuit of the amplifier in fig. (a).
T model is used in preference to the π model
Neglecting ro
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The Common-Gate Amplifier Fed with a Current-Signal Input
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Characteristics of CG Amplifier
• Input resistance
• Voltage gain
• Overall voltage gain
• Output resistance
min gR 1
)//( LDmv RRgA
sigm
LDmv Rg
RRgG
1
)//(
Dout RR
SJTU Zhou Lingling 76
Summary of CG Amplifier
• Noninverting amplifier• Low input resistance• Has nearly identical voltage gain of CS
amplifier, but the overall voltage gain is smaller by the factor (1+gmRsig )
• Relatively high output resistance• Current follower• Superior high-frequency performance
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The Common-Drain or Source-Follower Amplifier
Biasing with current source
Input signal is applied to gate, output signal is taken at the source.
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The Common-Drain or Source-Follower Amplifier
Small-signal equivalent-circuit model
T model makes analysis simpler
Drain is signal grounded
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Small-Signal Analysis Performed Directly on the Circuit
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Circuit for Determining the Output Resistance of CD Amplifier
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Characteristics of CD Amplifier
• Input resistance
• Voltage gain
• Overall voltage gain
• Output resistance
Gin RR
mLo
Lov
gRr
RrA
1//
//
11
//
//
mLo
Lo
sigG
Gv
gRr
Rr
RR
RG
mo
mout g
rg
R1
//1
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Summary of CD or Source-Follow Amplifier
• Very high input resistance
• Voltage gain is less than but close to unity
• Relatively low output resistance
• Voltage buffer amplifier
• Power amplifier
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Summary and Comparisons
• The CS amplifier is the best suited for obtaining the bulk of gain required in an amplifier.
• Including resistance RS in the source lead of CS amplifier provides a number of improvements in its performance.
• The low input resistance of CG amplifier makes it useful only in specific application. It has excellent high-frequency response. Can be used as a current buffer.
• Source follower finds application as a voltage buffer and as the output stage in a multistage amplifier.
SJTU Zhou Lingling 84
The MOSFET Internal Capacitance and High-Frequency Model
• Internal capacitancesThe gate capacitive effect
Triode region Saturation region Cutoff region Overlap capacitance
The junction capacitances Source-body depletion-layer capacitance drain-body depletion-layer capacitance
• High-frequency model
SJTU Zhou Lingling 85
The Gate Capacitive Effect
• MOSFET operates at triode region
• MOSFET operates at saturation region
• MOSFET operates at cutoff region
oxgdgs WLCCC 21
032
gd
oxgs
C
WLCC
oxgb
gdgs
WLCC
CC 0
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Overlap Capacitance
• Overlap capacitance results from the fact that the source and drain diffusions extend slightly under the gate oxide.
• The expression for overlap capacitance
• Typical value
• This additional component should be added to Cgs and Cgd in all preceding formulas.
oxovov CWLC
LLov 1.005.0
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The Junction Capacitances
• Source-body depletion-layer capacitance
• drain-body depletion-layer capacitance
o
SB
sbsb
VV
CC
+1
0
o
DB
dbdb
VV
CC
+1
0
SJTU Zhou Lingling 88
High-Frequency Model
SJTU Zhou Lingling 89
High-Frequency Model
(b) The equivalent circuit for the case in which the source is connected to the substrate (body).
(c) The equivalent circuit model of (b) with Cdb neglected (to simplify analysis).
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The MOSFET Unity-Gain Frequency
• Current gain
• Unity-gain frequency
)( gdgs
m
i
o
CCs
g
I
I
)(2 gdgs
mT CC
gf
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The Depletion-Type MOSFET
• Circuits symbol
• Structure
• Characteristic curves
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Circuit Symbol for the n-Channel 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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Physical Structure
• The structure of depletion-type MOSFET is similar to that of enhancement-type MOSFET with one important difference: the depletion-type MOSFET has a physically implanted channel.
• There is no need to induce a channel.• The depletion MOSFET can be operated at both
enhancement mode and depletion mode.
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Characteristic Curves
Transistor with current and voltage polarities indicated.
Typical value for discrete transistor: Vt = –4 V and k
n(W/L) = 2 mA/V2
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The Output Characteristic Curves
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The iD–vGS Characteristic in Saturation
the iD–vGS characteristic in saturation.
Expression of characteristic equation
Drain current with
221 )' tGSnD Vv
L
Wki (
0GSv
2
21 ' tnDSS V
L
WkI
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The iD–vGS Characteristic in Saturation
Sketches of the iD–vGS characteristics for MOSFETs of enhancement and depletion types
The characteristic curves intersect the vGS axis at Vt.