TransistorsCamille CruzChase ThompsonTyler Nelson

September 26, 2013

Outline•Introduction•Transistors Types•Bipolar Junction Transistors•Field Effect Transistors•Power Transistors•Example

What is a Transistor?

Transistors

Different types and sizes

BJT (PNP) Electrical Diagram

First TransistorModern Electronics

FET and BJT Transistor

•Purpose▫To amplify and switch electronic signals on

or off (high or low)

•Modern Electronics

Microprocessor

Cell Phones

Motor Controllers

Vacuum tubes•Purpose

▫Used as signal amplifiers and switches▫Advantages

High power and frequency operation Operation at higher voltages Less vulnerable to electromagnetic pulses

▫Disadvantages Very large and fragile Energy inefficient Expensive

Invention• Evolution of electronics

▫ In need of a device that was small, robust, reliable, energy efficient and cheap to manufacture

• 1947▫ John Bardeen, Walter Brattain and William

Schockly invented transistor• Transistor Effect

▫“when electrical contacts were applied to a crystal of germanium, the output power was larger than the input.”

General Applications

Doping •Process of introducing impure elements

(dopants) into semiconductor wafers to form regions of differing electrical conductivity

Negatively charged Semiconductor

Positively charged semiconductor

Doping Effects

•P-type semiconductors▫Created positive charges, where electrons

have been removed, in lattice structure•N-type semiconductors

▫Added unbound electrons create negative charge in lattice structure

•Resulting material▫P-N junction

P-N junction

Forward BiasingReverse Biasing

•P-N junction▫Controls current flow via external voltage

•Two P-N junctions (bipolar junction transistor, BJT)▫Controls current flow and amplifies the

current flow

Transistor Categories

•Semiconductor material•Structure•Polarity•Maximum power rating•Maximum operating frequency•Application•Physical packaging•Amplification factor

Types of Transistors•Bipolar Junction Transistor (BJT)

•Field Effect Transistors (FET)

•Power Transistors

BJT Introduction

• Bipolar Junction Transistors (BJT) consists of three “sandwiched” semiconductor layers

• The three layers are connected to collector (C), emitter (E), and base (B) pins

• Current supplied to the base controls the amount of current that flows through the collector and emitter

BJT Schematic•NPN

▫BE forward bias

▫BC reverse bias

•PNP▫BE reverse

bias▫BC forward

bias

NPN

PNP

BJT Characteristic Curves

Transfer Characteristic• Characteristic curves can be drawn to show other useful

parameters of the transistor• The slope of ICE / IBE  is called the Transfer Characteristic (β)

BJT Characteristic CurvesInput Characteristic• The Input Characteristic is the base emitter current

IBE against base emitter voltage VBE

• IBE/VBE shows the input Conductance of the transistor.

• The increase in slope of when the VBE is above 1 volt shows that the input conductance is rising

• There is a large increase in current for a very small increase in VBE.

BJT Characteristic CurvesOutput Characteristic• collector current (IC) is nearly independent of the collector-

emitter voltage (VCE), and instead depends on the base current (IB)

IB1

IB2

IB3

IB4

BJT Operating Regions

Operating Region

Parameters Mode

Cut OffVBE < Vcut-in VCE > Vsupply

IB = IC = 0Switch OFF

LinearVBE = Vcut-in

Vsat < VCE < Vsupply

IC = β*IB

Amplification

Saturated

VBE = Vcut-in,VCE < Vsat

IB > IC,max, IC,max > 0

Switch ON

BJT ApplicationsBJT Switch• Offer lower cost and substantial reliability over

conventional mechanical relays. • Transistor operates purely in a saturated or cutoff state

(on/off)• This can prove very useful for digital applications (small

current controls a larger current)

BJT ApplicationsBJT Amplifier

BJT ApplicationsBJT Amplifier

Field Effect Transistors (FET)

Chase Thompson

FET Basics

•Electric Field

•Voltage Controlled

•FET includes three distinct pieces▫Drain▫Source▫Gate

FET versus BJT?Same:• Applications: amplifier,

switch, etc.• Relies on PNP or NPN

junctions to allow current flow

Difference:• Voltage vs Current Input• Unipolar vs Bipolar• Noise• Higher input impedance• Fragile and low gain

bandwidth

Types of Field-Effect Transistors

Type Function

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

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

JFET• Reverse Biased PN-

junction• Depletion mode

devices▫Creates a potential

gradient to restrict current flow. (Increases overall resistance)

http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/jfet.html

JFET• N-channel JFET

• P-channel JFET uses same principles but▫Channel current is positive due to holes instead

of electron donors▫Polarity of biasing voltage must be reversed

N-Type Characteristics

Characteristics and Applications of FETs

JFETs

• Simplest type of FET – easy to make

• High input impedance and resistance

• Low Capacitance• Slower speed in switching• Uses?

– Displacement sensor– High input impedance

amplifier– Low-noise amplifier– Analog switch– Voltage controlled resistor

MOSFET

•Similar to JFET ▫ A single channel of single doped SC

material with terminals at end▫ Gate surrounds channel with doping

that is opposite of the channel, making the PNP or NPN type

▫ BUT, the MOSFET uses an insulator to separate gate from body, while JFET uses a reverse-bias p-n junction

p-channel

n-channel

MOSFETenhanced mode

MOSFETdepleted mode

How does a MOSFET work?

Simplified Notation

No current flow “Short” allows current flow

No Voltage to Gate Voltage to Gate

Source Source DrainDrain

n n

MOSFETTriode Mode/Linear Region VGS > Vth and VDS < ( VGS - Vth )

VGS : Voltage at the gateVth : Threshold voltageVDS : Voltage from drain to sourceμn: charge-carrier effective mobilityW: gate width L: gate length Cox : gate oxide capacitance per unit areaλ : channel-length modulation parameter

Saturation/Active Mode

VGS > Vth and VDS > ( VGS - Vth )

Characteristics and Applications of FETs

MOSFETs

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

• Rapid switching• More noise than JFET• Uses?

• Again, switches and amplifiers in general

• The MOSFET is used in digital CMOS logic, which uses p- and n-channel MOSFETs as building blocks

• To aid in negating effects that cause discharge of batteries

Use of MOSFET in battery protection circuit

Power Transistors Concerned with delivering high power Used in high voltage and high current application

In generalFabrication process different in order to: Dissipate more heat Avoid breakdown

Different types: Power BJTs, power MOSFETS, etc.

Comparison

Property

BJT MOSFET

JFET

Gm Best Worst Medium

Speed High Medium Low

Noise Moderate

Worst Best

Good Switch

No Yes Yes

High-Z Gate

No Yes Yes

ESD Sensitivity

Less More Less

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Questions?