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Field‐Effect Transistors (FETs)
Junction FET, Metal‐Oxide Semiconductor FET
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FETs: Outline
‐ Introduction
‐ Advantage of FET
‐ 1. Junction FET‐ JFET Electrical Characteristics of JFET
‐ 2. Metal‐Oxide Semiconductor FET‐ MOSFET
Electrical Characteristics of MOSFET
‐ Summary
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Introduction
‐ BJTs are current‐controlled devices ‐ FETs are voltage‐controlled devices
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Advantages of FET
‐ Field‐ Effect Transistors or FETs are majority –carrier devices ‐> high speed
‐ 2 main groups of FETs are;
>> Junction FETs or JFETs
>> Metal‐Oxide Semiconductor FETs or MOSFET
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1. Junction FET: Types
‐ 2 types of JFET: n channel & p channel
‐ Three terminals of FET : Drain (D), Gate (G), & Source (S)
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Symbols of JFETs
‐ The arrow on the gate points "in" for n channel & "out" for p channel. 6
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Construction of n‐channel JFETs
‐Water analogy provides a picture for the JFET control at the gate terminal. Gate controls flow from source to drain.
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JFET in the VGS = 0 V and VDS > 0 V
Varying reverse‐bias potentials across the p‐n junction of an n‐channel JFET
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Pinch‐off (VGS = 0 V, VDS = VP)
VDS is increased to a level such that the two depletion regions would “touch” (Pinch‐off Voltage; VP )
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Current source equivalent for VGS = 0 V, VDS > VP
IDSS is the maximum drain current for a JFET and is defined by the conditions
VGS = 0 V and VDS > |VP|
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JFET; VGS < 0 V
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V‐I Characteristic of JFET
- n-Channel JFET characteristics with IDSS = 8 mA and VP = -4 V
- 3 regions of operation : ohmic, saturation or constant current
and cut off
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Voltage Controlled Resistor
‐The region to the left of the pinch‐off locus is referred to as the ohmic or voltage‐controlled resistance region
‐ Can be employed as a variable resistor (used in automatic gain control or AGC)
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P‐Channel JFET
‐ Same construction with n‐channel JFET except reversal of p‐type and n‐type
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Summary of JFET
Max. current
Switch off
Voltage Controlled Current Source
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JFET Transfer Characteristics
‐ BJTs have linear relation but JFETs have nonlinear one
‐> Shockley’s equation
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Transfer Curve of JFET
‐ Obtaining the transfer curve from the drain characteristics
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JFET Forward Transconductance
‐ Or Transfer conductance gm
‐ Depend on DC operating point
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JFET: Operating region
‐ Operating region for linear amplifier
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Comparison between BJT and JFET
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2. MOSFETs
The most popular FETs
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MOSFETs : SMD package
Most hand-held electronic equipment employ SMD
enhancement MOSFETs.
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2.1 Enhancement MOSFET: Symbols
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Enhancement‐Type MOSFET: Construction
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Enhancement‐type MOSFET: Channel Formation
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E‐type MOSFET : Pinch‐Off
‐There is a pinch‐off near the Drain terminal.
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E‐type MOSFET : Drain Characteristic
There are minimum gate‐source voltages called Threshold Voltage, VT
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E‐type MOSFET : Transfer Characteristic
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E‐type MOSFET: If VGS > VT
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P‐channel enhancement MOSFET
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Complementary MOSFET : CMOS
‐ This is a very useful device as an Inverter Gate.
‐ Power consumption is very low.
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2.2 Depletion‐Type MOSFET: Symbols
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Depletion‐Type MOSFET: construction 33
D‐type MOSFET: Diffuse Channel
D‐type MOSFET can conduct the current at VGS = 0 V
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Drain & Transfer Characteristic of
Depletion-type MOSFET
D-type MOSFETs have partly JFET and
partly E-type MOSFET transfer characteristic
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Summary of FET Important Characteristics
JFET
D‐type MOSFET
E‐type MOSFET
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FET : Summary
‐ 3‐terminal Devices : Drain, Gate & Source
Majority‐Carrier
Carrier Devices
Devices
‐ Majority
‐ 2 types : n‐channel & p‐channel
‐ 3 regions : Ohmic, Saturation & Cut off
‐ The most popular FET is enhancement‐type MOSFET.
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