ME 4447 / 6405Student Lecture

Transistors

Abiodun OtolorinMichael Abraham

Waqas Majeed

Lecture Overview• Transistor?• History• Underlying Science• Properties• Types of transistors

– Bipolar Junction Transistors (BJT)– Field Effect Transistors (FET)– Power Transistors

• Applications

Transistor ?• Transistor = transconductance + varistor• 3 terminal electronics device.• Solid state Semiconductor material.• Part and parcel of almost every circuit (digital / analog)

Transistor ?

• Two main categories:– Through-Hole– Surface-Mount

• Packaging Materials:– Plastic, Glass, metal or ceramics

* Power transistors packages clamped to heat sinks.

HistoryVacuum Tube (Predecessors)Edison effect in the Light Bulb (1879-83).John Fleming implements it in a diode.Lee DeForest extends it using third electrode. This triode

was used as both an amplifier and a switch (1906).

Diode Triode

First Transistor

William Shockley, John Bardeen, and Walter Brattain @ Bell Labs in 1947

Replaced vacuum tubesSmaller, robust, durable, no warm upsSolid state1956 Nobel Prize

Current Transistorssemiconductor

• The world's first single chip microprocessor2,300 MOS (metal oxide semiconductor) transistors Equivalent 18,000 vacuum tubes contained in 3,000 ft3

Nov. 1971the Intel 4004

• Today’s microprocessor has 2B transistors.• SD cards has 50B transistors.• Brought revolution like IC, Chips.

Current Transistor

Underlying Science

• FREE ELECTRONS are required for current flow.• Conductors have abundant of them.• Insulators & semiconductors have scarcity.• TEMPERATURE increase produces free electrons

but not possible.• SMALLER BAND GAPS of semiconductors than

insulators help yield free electrons.• Hence, DOPING is done.

Underlying Science• 8 electrons for stable valence shell.

GIII > P-dopant has 3 valence shell electrons (e.g. B, Ga).GIV > Semiconductor has 4 valence shell electrons (e.g. Si, Ge).GV > N-dopant has 5 valence shell electrons (e.g. P, As)

Underlying Science

• Covalent bonding/pair sharing.GIII + GIV => -1 electron => Hole > P-typeGV + GIV => +1 electron => Electron > N-type

DopantsSemiconductor

Properties

• Forward Bias (on)– Current flows from P

to N.– Vd = 0.7 V to start

conduction.

• Reverse Bias (off)– No Current flows

(ideally).– Excessive voltage or

heat can cause degradation to diode.

Properties

V threshold

Properties

PNP Transistor

NPN Transistor

Properties

No current flows

Collector current controlled by the collector circuit(Switching)

In full saturation VCE=0.2V

(Variable resistor)

Collector current IC proportional to Base current IB (Amplification + Regulation)

Properties

No current flows

Collector current controlled by the collector circuit(Switching)

In full saturation VCE=0.2V

(Variable resistor)

Collector current IC proportional to Base current IB (Amplification + Regulation)

Bipolar Junction Transistor (BJT)

• 3 layers of Semiconductor• 3 Terminal (Base,

Collector & Emitter)– Each terminal may act as

Input, Output or Common

• 3 possible configurations– Common Emitter

• Both Current and Voltage gain

– Common Base • Voltage gain but no current

gain

– Common Collector• Current gain but no voltage

gain

NPN Common Emitter

BJT Types• NPN

– Base is energized to allow current flow

– High potential at collector– Low potential at emitter– Allows current flow when the

base is given a high potential

• PNP– Base is connected to a lower

potential to allow current flow– High potential at emitter– Low potential at collector– Allows current flow when base

is connected to a low potential

BJT Characteristics• Cut-off Region: VBE < VTH, IB=0

– off switch• Active Linear Region:

– VBE=VTH, IB≠0, IC=βIB– current amplifier– Current gain (β)

• 100 for most transistors– Voltage Gain = β(IC/IB)

• Saturation Region: – VBE=VTH, IB>IC,max/ β– on switch

Operation region overview

Operation Operation RegionRegion

IB or VCE Char.

BC and BEBC and BEJunctionsJunctions

ModeMode

Cutoff IB = Very small

Reverse & Reverse

Open Switch

Saturation VCE = Small Forward & Forward

Closed Switch

Active Linear

VCE = Moderate

Reverse &Forward

Linear Amplifier

Break-down VCE = Large Beyond Limits

Overload

Types of BJT Circuits

• Major BJT Circuits– Transistor Switch– Common-Emitter Amplifier– Emitter Follower– Current Source

NPN Transistor Switch•Vin(Low ) < 0.7 V

•BE junction is notforward biased•Cutoff region•No current flows•Vout = VCE = Vcc

•Vout = High

•Vin(High)•BE junction forward biased (VBE=0.7V)•Saturation region•VCE small (~0.2 V for saturated BJT)•Vout = small•IB = (Vin-VB)/RB

•Vout = Low

•Linear Active Region•Significant current Gain

Example:•Let Gain, β = 100

•Assume to be in active region -> VBE=0.7V

•Is device in active region?

NPN Common Emitter Amplifier

NPN Common Emitter Amplifier

V

VRIRIVVmAII

mARR

VVI

IIIIVV

BEEECCCCCB

BC

EB

BEBBB

BCBE

BE

93.37.0)0107.0*101)(2()07.1)(3(10

**07.10107.0*100*

0107.0402

7.05101*

)1(7.0

==−−−=

=−−−====

=−

=+−

=

+=+==

β

β

VCB>0 so the BJT is in active region

Power Across BJT

• PBJT = VCE * ICE

• Should be below the rated transistor power• Should be kept in mind when considering

heat dissipation• Reducing power increases efficiency

Darlington Transistors

• Two BJT devices combined– BE junction in series

• Allow for much greater gain in a circuit

• β = β1 * β2

• VBE=VBE1 + VBE2

Field Effect Transistors (FET)• Analogous to BJTs• FETs switch by voltage rather

than by current• FETs have 4 terminals (except

for J-FET which have 3)

BJT FETCollector Drain

Base Gate

Emitter Source

N/A Body

B

S

G

D

FET: The Basic Idea• Current flow is controlled via the “field effect”• Electrons gather when a field is formed• Current flows when an electron bridge is created

Semi-conductor

Plate

Types of FETs

•MOSFET (Metal-Oxide-Semiconductor)•JFET (Junction Gate)•MESFET (Metal-Semiconductor)•MODFET (Modulated-Doping)•HFET (Hetero-structure)•HEMT (High Electron Mobility Transistor)

(Most common are MOSFET and JFET)

Boba Fett

The Two Most Common TypesMOSFET

2 varieties: enhancement mode (shown)

depletion mode

JFET

P

N sourcedrain

gate

P

sourcedrain

gate

NN

Both MOSFETs and JFETs can be n-channel orp-channel depending upon the doping of the drain and source

Junction Gate FET (JFET)•Source and Drain are connected to n-doped material (or p-doped material for p-channel)

•Gate is connected to p-doped material

•When a negative biased voltage is applied to the Gate, the size of the depletion layer increases and impedes current flow from the source to the drain.

•JFETs can thus behave as voltage-controlled variable resistors

•JFETs work by “depletion”

p-doped n-doped depletion layer

MOSFET

Depletion Mode Enhancement Mode

n-Channel Enhanced MOSFET

•n-Channel because source and drain are connected to n-doped regions•With no voltage applied, the MOSFET behaves as shown (no channel is formed between the source and drain)•Because a channel is being created where none existed, this is an “enhancement mode” MOSFET

•When a voltage is applied, the resulting field causes electrons from the p-doped region to collect near the gate and a channel is created that connects the source and the drain.•The size and shape of the channel vary depending upon the amount of voltage applied.

n-Channel Enhanced MOSFET

Circuit Symbols

• In practice the body and source leads are almost always connected

• Most packages have these leads already connected

B

S

G

D

B

S

G

D

S

G

D

MOSFET

JFET

Performance CharacteristicsCurrent flow

B

S

G

D

Performance RegionsCurrent flow

B

S

G

D

Region Criteria Effect on Current

Cut-off VGS < Vth IDS=0

Linear VGS > Vth

AndVDS <VGS-Vth

Transistor acts like a variable resistor, controlled by Vgs

Saturation VGS > Vth

AndVDS >VGS-Vth

Essentially constant current

JFET vs MOSFETCurrent flow

B

S

G

D

MOSFET JFETHigh switching speed

Will operate at VG<0

Can have very low RDS

Better suited for low signal amplification

Susceptible to ESD

More commonly used as a power transistor

Power Transistors

• Additional material for current handling and heat dissipation

• Can handle high current and voltage

• Functionally the same as normal transistors

Applications

• Switching• Amplification (Voltage / Current)• Variable Resistor (VDR)• Voltage Regulation

Switching

Switching

DC motor• Power to motor is proportional to duty cycle

• MOSFET transistor is ideal for this use

Analog signal (e.g. sensor, audio, etc.).

Op-amps does same but suitable for voltage amplification and this is for current/power amplification.

Amplification

• Transistors can be used in series to produce a very high current gain

Amplification

• FET operating in Linear or OhmicMode

• Lesser usage

Variable Resistor

• Transistors can be used in regulating voltage for high power devices.

• Inefficient• Power supplies

Voltage Regulation

Application Examples• Digital logic circuits• Microprocessors, microcontrollers, chips (TTL)• Photo-transistors• Replaces normal switches, mechanical relays.• ADC• Opamp• Encoders• Multiplexers• Power supplies

References• http://www.owlnet.rice.edu/~elec201/Book/images/img95.gif• http://nobelprize.org/educational_games/physics/transistor/

function/p-type.html• http://www.electronics-for-

beginners.com/pictures/closed_diode.PNG• http://people.deas.harvard.edu/~jones/es154/lectures/lecture_

3/dtob.gif• http://en.wikipedia.org/wiki/Image:IvsV_mosfet.png• http://en.wikipedia.org/wiki/Transistor• http://www.physlink.com/Education/AskExperts/ae430.cfm• http://www.kpsec.freeuk.com/trancirc.htm

References (contd.)

• Sabri Cetinkunt; MechatronicsJohn Wiley and sons; 2007

• Mechatronics by D. Bradley• Mechatronics System Design by Shetty• Mechatronics : Principles and Application

by Onwubolu• Applied Mechatronis by A. Smaili et al

Questions?