Field Effect Transistors Unit 2 Field Effect Transistors

Objectives: BJT v/s FET JFET MOSFET JFET v/s MOSFET Handling and biasing MOSFETs FET applications CMOS IGBT

5.1BJT v/s FET: Current controlled – voltage controlled Bipolar devices – unipolar devices Zi < 1 MΩ - Zi very high (100s MΩ) Less temp. stable – more temp. stable Small size – smaller size More gain – less gain

5.2 JFET (junction FET): Intro.: JFET simpler device among family 3 – terminal device; 1 controls current through 2 Types – p - channel n - channel

Construction

Characteristic:

Characteristic:

Transfer characteristics:

The drain resistance (rd) in saturation region is given by: (1-VGS / VP)2 Where rd is drain resistance at VGS = 0 & rD is drain resistance at particular VGS

Relationship between output drain current & input gate-to-source voltage: VGS 2 ID = IDSS 1- ------------ VP

Effect of temperature: Carrier mobility ↓ ID ↓ Thus in above steps: T ↑ ID ↓ Negative temperature coefficient Better thermal stability ? JFET temperature ↑ Depletion region width ↓ & Increase channel width ↑ ID ↑ Thus in above steps: T ↑  ID ↑ Positive temperature coefficient

5.3 Metal oxide FET (MOSFET): What exactly metal oxide? -MOSFET is insulated from the semiconductor channel by very thin oxide (SiO2) layer These are also known as insulated gate (IG) FET MOSFET types / modes: Depletion MOSFET (De-MOSFET) Enhancement MOSFET (E-MOSFET)

DE-MOSFET: There is no p-type gate There is no direct electrical connection between gate & channel Capacitive effect exists between gate & channel

Symbols:

Circuit connection: Do not copy

Characteristics: Enhancement mode / region, +ve charge carriers from p-substrate contribute Depletion mode / region, Normal operation as MOSFET

Transfer characteristics:

Notice that the channel is not fabricated, E-MOSFET: ha Notice that the channel is not fabricated, It will be generated

Symbols:

Working: Capacitive effect induces electrons from p-substrate Creating a n-channel

Characteristics:

E-MOSFET Transfer characteristics:

Differences between JFET & MOSFET: Operational modes: JFET -depletion mode DE-MOSFET -depletion / enhancement modes E-MOSFET -enhancement mode MOSFET – input resistance high compared to JFET JFETs have higher drain resistance rd than MOSFETs Leakage current in MOSFET is less compared to JFET MOSFETs are easier to construct & widely used than JFETs

Handling MOSFETs: SiO2 layer is thin & prone to damage Due to static charges, potential difference between SiO2 can result in breakdown & establish conduction through it Precautions: Person handling ground himself properly Connect zener diodes, back-to-back as shown

Biasing MOSFETs: Biasing DE-MOSFETs: Same as JFETs Example: Fixed bias (page 183)

Biasing E-MOSFETs: Feedback biasing configuration:

KVL at input VDD –IG RG - ID RD - VGS = 0 VGS = VDD – ID RD -----(1) KVL at output VDS = VDD – ID RD -----(2)

Voltage divider bias: Assignment #1: Numerical examples 5.9 & 5.10 on pages 196 & 197

FET applications: Amplifier Analog switch Multiplexer Current limiter Voltage variable resistors Oscillators

Analog switch:

Multiplexer:

5.10 Testing FETs: Damaged: Between any two: 0  short circuit ∞  open circuit

5.13 CMOS devices: CMOS Inverter Operation: Vin=0  Q2 ON & Q1 OFF  Vout=1 Vin=1  Q1 ON & Q2 OFF  Vout=0

Insulated Gate Bipolar (IGBT): Have positive attributes of BJT & MOSFET Faster switching like MOSFET Lower ON – state voltage like BJT Application / Usage area: SMPS Motor control – as high voltage handling capacity Induction heating control

Questions: Explain JFET construction, biasing and characteristics, transfer characteristics How better stability is achieved in JFET? Explain DE-MOSFET (construction diagram, symbol, output & transfer characteristics, working) Explain E-MOSFET ( --”--) Difference between JFET & MOSFET Explain biasing methods of De- & E-MOSFETs

Example 5.8, page 193 Example 5.9, page 196 Explain Applications of FET Explain CMOS inverter -----o0o-----