ME 6405 Student Lecture

Transistor

Cong ZhaoFeifei Qian

Kuan-Wen Tung

October 2, 2012Georgia Institute of Technology

Contents

Introduction to Transistor(Speaker: Cong Zhao)

Field Effect Transistor(Speaker: Kuan-Wen Tung)

Power Transistor(Speaker: Kuan-Wen Tung)

1

3

4

Bipolar Junction Transistor(Speaker: Feifei Qian)

2

Introduction

Amplifier and Electronic Switch are needed.

Amplifier and Electronic Switch are needed. Amplifier: any device that changes, usually increases, the amplitude of a signal. Electronic Switch: switch that the physical opening and closing is achieved by

applying appropriate electrical control signals.

Question #1: How can we control the TV with remote-controller?

Question #2: How can a computer recognize 0(off) and 1(on) for computing?

Introduction Early 20th century, vacuum tube was used for the amplifier and switch.

ENIAC, the first general-purpose electronic computer, contains 17,468 vacuum tubes.

Vacuum Tube Radio

However, Vacuum Tube is too big, fragile, and energy-consuming.

Transistor solved this problem.

Introduction – Invention of Transistor

Invention In 1947, John Bardeen, Walter Brattain,

and William Schockly, researchers at Bell Lab, invented Transistor.

They found Transistor Effect: “when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.”

In 1956, they were awarded the Nobel Prize in physics.

John Bardeen, Walter Brattain, and William Schockly

First model of Transistor, 1947

Introduction – Definition of Transistor

A transistor is a three-terminal semiconductor device that amplifies or switches the flow of current between two terminals by varying the current or voltage between one of the terminals and a third.

Introduction – Semiconductor

Semiconductor is a material that has an electrical resistivity between that of a

conductor and an insulator. has a few charge carriers(“holes” or free electrons). the conductivity increases by adding impurities or dopants(doping).

Silicon is used in most commercial

semiconductors

Introduction – Doping Doping: add neighboring elements to the semiconductor to

make it electropositive or electronegative P(positive)-type doping is adding Acceptor impurities (from Group

III) to the semiconductor in order to increase holes. Ex: Silicon doped with Boron

N(negative)-type doping is adding Donor impurities (from Group V) with more electrons in order to increase free electrons.

Ex: Silicon doped with Phosphorous

Add Group III(Boron) Add Group V (Phosphorous)

Introduction – P-N Junction

PN Junction is a junction formed by P-type and

N-type semiconductors together in very close contact.

Thin depletion region forms near junction at an equilibrium condition (Zero bias)

Near the p–n interface, electrons from N diffuses into P, leaving positively charged ions in N region; holes from P diffuses into N, leaving negatively charged ions in P region

Ions exhibit a force which inhibits further carriers flow across the P-N Junction unless a forward bias external voltage is applied.

What happens at the junction?

Introduction – Forward and Reverse Bias

Forward bias -V pumps electrons into the N-region. +V pumps more holes into the P-region. Excess of charge in P and N region will

apply pressure on the depletion region and will make it shrink.

→ current flows

Reverse bias -V sucked out electrons from N-region. +V sucked out holes from P-region. The depletion layer widens and it

occupies the entire diode(p-n). → current doesn’t flow

Introduction – Transistor

Transistor is comprised by two P-N junctions: Base-Collector junction and Base-Emitter junction.

Introduction – Types of Transistor

Transistor are categorized by Semiconductor material: germanium, silicon, gallium arsenide, etc. Structure: BJT, FET, IGFET (MOSFET), IGBT Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs) Maximum power rating: low, medium, high Maximum operating frequency: low, medium, high Application: switch, audio, high voltage, etc. Physical packaging: through hole, surface mount, ball grid array, etc. Amplification factor

General Purpose Transistors Bipolar Junction Transistor (BJT) Field Effect Transistors (FET) Power Transistors

NPN BJTn-channel

JFET

BIPOLAR JUNCTION TRANSISTOR

by: Feifei Qian

Introduction

• A bipolar junction transistor (BJT) is a three terminal

semiconductor device in which the operation depends

on the interaction of both majority and minority

carriers and hence the name Bipolar.

• It is used in amplifier and oscillator circuits, and as a

switch in digital circuit.

• It has wide applications in computers, satellites and

other modern communication system.

Structure and Symbols

Emitter is heavily doped so that it can inject a large number of charge carriers into the base.

Base is lightly doped and very thin, therefore it passes most of the injected charge carriers from the emitter into the collector.

Collector is moderately doped and larger in size, so that it collects the major portion of the majority carriers supplied by the emitter.

Three Regions in BJT

Transistor biasing

NPN: BE forward

biased BC reverse

biased

PNP: BE reverse

biased BC forward

biased

BJT Transistor Operation

http://www.learnabout-electronics.org/bipolar_junction_transistors_05.php

Regions of OperationBJT Switch “On”

BJT Switch “Off”

Linear mode

Transistor as a Switch When a transistor is used as a switch, it is usually required

to be brought alternatively in the saturation and cut-off conditions.

When in saturation condition, it should carry heavy current, so the voltage drop across the transistor is as near to zero as possible. It may be considered as “closed switch”.

When in cut-off condition, it should carry almost no current so that it may be considered to be an “open switch”.

Transistor as an Amplifier

Transistor Configurations

Parameter Common Emitter Common Collector Common Base

Voltage gain High (about 100) Unity (1) Medium (10-50)

Current Gain High (50 - 800) High (50 -800) Low (<1)

Transistor as an Amplifier

• Use small digital signal to turn coils on/off

Field Effect Transistor, FET

By Kuan-Wen Tung

What makes a FET?

• 3 terminal device with:• Drain• Source• Gate

• The body has contacts at the ends: the drain and source

• Gate surrounds the body and can induce a channel by use of electric field

FET BJT  Input voltage controls

output currentInput current controls

output current

Gate Base Controls flow of currentDrain Collector Current goes out here

Source Emitter Current comes in here

What is a FET?

• Semiconductor device that uses electric field to control current, thus, the channel conductivity.

• Unipolar transistor involving single carrier type operation, in contrast to BJT.

• Relies on PNP or NPN junctions to allow current flow

• Performs same functions as a BJT; amplifier and switch.

FET

How does FET work?

Basic operation: Assume a 2-terminal device with nonlinear I-V characteristics Add a third terminal to control the I-V curve by changing voltage

Result: change in the current flowing through the first 2 terminalsApplication:

Amplification : increase current id by increasing gate voltage. Switching : change gate voltage to switch device on, vice versa.

E=id*R+vd

Types of Field-Effect Transistors

Type Function Metal-Oxide-Semiconductor FET (MOSFET) Uses insulator (usu. SiO2) between gate and body

Junction Field-Effect Transistor (JFET)Uses reversed biased p-n junction to separate gate

from body

Insulated Gate Bipolar Transistor (IGBT) Similar to MOSFET, but different main channel

Organic Field-Effect Transistor (OFET) Uses organic semiconductor in its channel

Nanoparticle Organic Memory FET (NOMFET)Combines the organic transistor and gold

nanoparticles

NPNnMOS

PNPpMOS

NPNnMOS

PNPpMOS

NPNnMOS

PNPpMOS

Metal-Oxide-Semiconductor, MOSFET

Four terminal device: Source, gate, drain, and body

(substrate) that connects to source Enhancement mode of

operation Positive gate voltage required to

conducting channel for nMOS device to turn on.

Negative gate voltage for pMOS device to turn on.

nMOS

How does a MOSFET work?

Structure: Device formed on lightly

doped p-type substrate. Source and drain are heavily

doped with n-type. Oxide layer separates gate

from Si surface. Result: N-P-N type, nMOS.

nMOS device in enhancement mode

Equilibrium energyband diagram

Operation: VGS > Vth

Enhancement Mode of Operation

Triode Mode/Linear Region

VGS > Vth and VDS < ( VGS - Vth )

VGS : Voltage at the gateVth : Threshold voltageVDS : Voltage from drain to source

Saturation/Active Mode

VGS > Vth and VDS > ( VGS - Vth )

FETs vary voltage to control current.

Characteristics and Applications of MOSFET

Circuit LayoutSymbol: Not Function/ Inverter

• Oxide layer prevents DC current from flowing through gate• Reduces power consumption• High input impedance

• Rapid switching• Noisy due to undesired capacitive behavior• Applications:

• Used as switches and amplifiers in general• Used in digital CMOS logic, which uses p- and n-

channel MOSFETs as building blocks• CMOS: Complementary MOS

Power Transistors

• Built to carry and dissipate more power as opposed to tiny transistor• has low current gain• Used in high voltage and high current application• Used in high switching frequency of 100 kHz (switch in <1 μs)• Provides current limit protection• Cannot withstand reverse voltage

• Different types: Power BJTs, power MOSFETS, etc.• Application: RF and microwave power transistors, multiple

chip device• Limitation: limited to dc voltage for inverters

34

References

[1] “Solid State Electronic Devices" Ben G. Streetman.

[2] Student Lecture Fall 2010.

[3] Student Lecture Fall 2011.

[4] “Electronic Circuits – Handbook for Design and Applications” U. Tietze and Ch. Schenk.

[5] “Transistors." http://en.wikipedia.org/wiki/Transistor

[6] “FET.” http://en.wikipedia.org/wiki/Field-effect_transistor

[7] “JFET.” http://en.wikipedia.org/wiki/JFET

[8] “MOSFET.” http://en.wikipedia.org/wiki/MOSFET

[9] “MOSFET Animation.” http://www.youtube.com/watch?v=v7J_snw0Eng

[10] “Class Lecture #6 on Digital Arithmetic 2012.” Dr. Charles Ume

[11] “Class Lecture #4 on CMOS Inverter 2010.” Dr. Miltiadis Hatalis