Download - Lecture Metal-Oxide-Semiconductor (MOS) Field-Effect Transistors (FET) MOSFET Introduction 1
Lecture Metal-Oxide-Semiconductor (MOS)
Field-Effect Transistors (FET)MOSFET
Introduction
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Goals
• Describe operation of MOSFETs.
• Define MOSFET characteristics in operation regions of 1. cutoff, 2. triode and 3. saturation.
• Develop mathematical models for i-v characteristics of MOSFETs.
• Introduce graphical representations for output and transfer characteristic descriptions of electron devices.
• Define and contrast characteristics of enhancement-mode and depletion-mode FETs.
• Define symbols to represent MOSFETs in circuit schematics.
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MOS Field-Effect Transistors
– Primary component in high-density VLSI chips such as memories and microprocessors
Cha
nnel
I
I
Vcontrol
The control voltage determines the value of the current in the
channel I
Drain
Source
Gate
Symbolic View
How do we create the channel?
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Enhancement Type NMOS Transistor: Structure
• 4 device terminals: Gate(G), Drain(D), Source(S) and Body(B).
• Source and drain regions form pn junctions with substrate.
• vSB, vDS and vGS always positive during normal operation.
• vSB always < vDS and vGS to reverse bias pn junctions
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Definitions
• Vt: Threshold Voltage for MOS Transistor in general. The gate voltage required to form the channel between the source and drain
• VTN: Threshold Voltage for N-Channel Transistor.
• VTP: Threshold Voltage for P-Channel Transistor.
• The body (or bulk) current (iB) is always zero
• The gate current (iG) is always zero
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NMOS Transistor: Qualitative I-V Behavior
• VGS<<VTN & VGS<<0 : No Channel for current-conduction (2 back-back diodes), Only small leakage current flows, iD ≈ 0 .
• 0<VGS<VTN: Depletion region formed under gate merges with source and drain depletion regions. No current flows between source and drain.
• VGS>VTN: Channel formed between source and drain. If vDS>0,, finite iD flows from drain to source.
OFF
OFF
ON
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For vGS > VTN-- Triode Region Characteristics
Di
DSv
TNV
GSv
L
WnK
Di
DSv
DSv
2DS
vTNV
GSv
L
WnKDi
where, Kn= Kn’W/L
Kn’=μnCox’’ (A/V2)
Cox’’=εox/Tox
εox=oxide permittivity
(F/cm)
Tox=oxide thickness (cm)
for
TNV
GSV
DSV
TNV
GSV &
•A channel is induced between the source and drain, and current will flow•Electrons flow from the source to drainCurrent flows from drain to source
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NMOS Transistor: Triode Region Characteristics (contd.)
• Output characteristics appear to be linear.
• FET behaves like a (gate-source) voltage-controlled resistor between source and drain with
TNV
GSV
LW
nKD
iDS
vonRchannel
'1)( resistance
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MOSFET as Voltage-Controlled Resistor
Example 1: Voltage-Controlled Attenuator
TNVGG
VRnKRonRonR
svov
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If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then,
667.0V15.12000
2V
μA5001
1
sv
ov
To maintain triode region operation,
V)15.1(667.0 S
v or V750.0S
v
TNVGG
Vov orTNVGS
vDS
v
If Kn=500μA/V2, VTN=1V, R=2kΩ and VGG=1.5V, then,
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NMOS Transistor: Saturation Region
• If vDS increases above triode region limit, channel region disappears, also said to be pinched-off.
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VGS > VTN, and VGD < VTN
VDS > VGS-VTN
Channel pinches off iD is independent on VDS
Saturation region
2))(('2
1TNGSnD VV
L
WKi
• If vDS increases above triode region limit, channel region disappears ( pinched-off).
• Current saturates at constant value, independent of vDS.
• Saturation region operation mostly used for analog amplification.
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NMOS Transistor: Saturation Region (contd.)
2
2
TNVGS
vL
WnKDi
for TNVGS
vDS
v
TNVGS
vDSAT
v is also called saturation or pinch-off voltage
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Circuit Symbols for N-Channel MOSFET (Enhancement type)
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I-V Characteristics for N-Channel MOSFET (Enhancement type)
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iD
VT
Slope = KnVDSW/L
vGS
Small vds: triode region Higher vds; saturation region
iD-VGS characteristics
DSv
TNV
GSv
L
WnK
Di
2
2
TNVGS
vL
WnKDi
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Terminal Voltage Levels
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Depletion-Mode MOSFETS
• NMOS transistors with
• Ion implantation process used to form a built-in n-type channel in device to connect source and drain by a resistive channel
• Non-zero drain current for vGS=0, negative vGS required to turn device off.
0TNV
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Transfer Characteristics of MOSFETS
• Plots drain current versus gate-source voltage for a fixed drain-source voltage
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Enhancement-Mode PMOS Transistors: Structure
• P-type source and drain regions in n-type substrate.
• vGS<0 required to create p-type inversion layer in channel region
• For current flow, vGS< vTP
• To maintain reverse bias on source-substrate and drain-substrate junctions, vSB <0 and vDB <0
• Positive bulk-source potential causes VTP to become more negative
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Enhancement-Mode PMOS Transistors
Direction of current is opposite to n-channel
In figures (c) and (d) the body is connected to the source
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Enhancement-Mode PMOS Transistors: Output Characteristics
• For , transistor is off.
• For more negative vGS, drain current increases in magnitude.
• PMOS is in triode region for small values of VDS and in saturation for larger values.
TPVGS
V
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DC –Analysis of n-channel MOSFET
])(2)[('2
1DSTNGSnD VVV
L
WKi
Check VGS
VGS< VTN
Cutoff region
iD=0
VGS> Vt
VDS < VGS –VTN
Triode regionVDS > VGS –VTN
Sat. region2))(('
2
1TNGSD VV
L
WKi
we start the analysis by assuming certain operating region
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