BJT

Introduction to Transistors

Balaji

Overview

Introduction & History

Semiconductors Operation of

Transistors Transistor Types Applications Examples Questions Conclusion

Background

Invented at Bell Laboratories in 1947. John Bardeen, Walter Brattain, and William Schockly

received Nobel Prize in Physics in 1956 for Inventing Transistors.

First application: telephone signal amplification Replaced cumbersome and inefficient vacuum tubes

Transistors can now be found on a single silicon wafer in most common electronic devices

Background

Model of First Transistor

What are Transistors?

Versatile three lead semiconductor devices whose applications include electronic switching and modulation (amplification)

Transistors are miniature electronic switches. Configuration of circuit determines whether the transistor will

serve a switch and amplifier Building blocks of the microprocessor, which is the brain of the

computer. Have two operating positions- on and off. Binary functionality of transistors enables the processing of

information in a computer.

Semiconductors

Silicon Basic building material of most integrated circuits Has four valence electrons, which allow it to form

four covalent bonds. Silicon crystal is an insulator-- no free electrons.

Semiconductors

Resistance to current flow in the silicon crystal is reduced by adding small amounts of foreign impurities, which is referred to as doping.

Doping transforms a silicon crystal from a good insulator into a viable conductor; hence, the name semiconductor.

Semiconductors

Two Dopant Types N-type (Negative) –Free flowing electrons are

added to the silicon crystal structure. Examples include Group V elements including

Phosphorous, Arsenic, and Antimony. P-type(Positive)- Lack electrons and serve as

potential slots for migrating electrons. Examples include Group III elements such as Boron,

Aluminum, and Gallium

Comparison of Energy Bands

Semiconductor resembles an insulator, but with a smaller energy band.

Small energy band makes it a marginal conductor

Simple Semiconductors: Diodes

Diode is the simplest semiconductor. Allows current to flow in one direction only.

Diode Sign Conventions

Power dissipated by a load = (+) quantity

Current flows from (+) (-) Forward Biased

Supplied Current flows with natural (hole) diffusion current

Reversed Biased Supplied Current fights against natural

diffusion (hole) current and diode orientation

Forward-Bias Example

Charge Diffusion aided by Supply Current Current is allowed through easily

“p”

(positive chargesDominate)

-- --

- --

- -

++ ++

+ + ++ +

+ + +

“n”

(negative charges dominate)

P-N Junction

(Depletion Region / Offset voltage = 0.7V)

Diode Electric Field

Supplied Current

Diffusion (hole) Current

Reverse-Bias Example

Charges cannot diffuse unless supplied current flows towards “n”

“p”

(positive chargesDominate)

-- --

- --

- -

++ ++

+ + ++ +

+ + +

“n”

(negative charges dominate)

(Depletion Region)

Diode Electric Field

Supplied Current

Diffusion (hole) Cuurent

Diodes States

Forward biased (on)- Current flows

Real: Need about 0.7 V to initiate electron-hole combination process.

Reversed biased (off)- Diode blocks current

Ideal- Current flow = 0

Real : Iflow= 10-6 Amps

Bipolar Junction Transistors (BJT)

Three Layers in a BJT Collector Base (very thin)

has fewer doping atoms Emitter

Two Types of BJT’s PNP (figure on left)

operates with outgoing base current NPN (figure on right)

operates with incoming base current

p

P+

n

emitter

collector

base

n

n+

p

emitter

collector

i i

BJT Schematic Representation

p

P+

n

emitter

collector

base

i

n

n+

p

emitter

collector

i

iB

Corresponds to:

Corresponds to:

BJT Operation Characteristics

IC vs. VCE graph allows us to determine operating region.

Works for any IB or VCE

VBE tops out around ~0.7V

BJT Operation Regions

Operation Operation RegionRegion

IB or VCE

Char. BC and BE BC and BE JunctionsJunctions

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

Cutoff NPN BJT

n

p

n

V2

V1

+++

C

B

E

Emitter current

Collector current

Base current

Reverse biased

Reverse Biased

Saturated NPN BJT

n

p

n

V2

V1

+ +

- - - -

C

B

E

Emitter current

Collector current

Base current Forward biased

Forward biased--

Active Linear NPN BJT

n

p

n

V2

V1

+ +

- - -

- - -

- - -

C

B

E

Emitter current

Collector current

Base current

Forward biased

Reverse Biased

Possible Uses for BJT’s

Can act as Signal Current Switch (Cutoff Mode)

Can act as Current Amplifier (Active Region)

Where: Beta = intrinsic amp property (20 - 200)

Bc II

FIELD-EFFECT TRANSISTORS

In 1925, the fundamental principle of FET transistors was establish by Lilienfield.

In 1955, the first successful FET was made. Types of Transistors

MOSFET (metal-oxide-semiconductor field-effect

transistors) JEFT (Junction Field-effect transistors)

( BACKGROUND )

MOSFET

Four types: n-channel enhancement mode

Most common since it is cheapest to manufacture

p-channel enhancement mode n-channel depletion mode p-channel depletion mode

(Types)

Depletion typen-channel p-channel

Enhancement typen-channel p-channel

MOSFET (n-channel Enhancement-Mode)

Device Structure Three terminals

Gate, Drain, and Source Analogous respectively to the base, collector, and

emitter.

Substrate electrically connected to the source.

MOSFET(n-channel Enhancement-Mode)

Device Structure Substrate, source connected to ground

The drain-body n+p junction is reverse-biased. The body-source pn+ junction is reverse-biased.

Enhancement MOSFET acts as an open circuit with no gate voltage.

n-channel Enhancement Mode

Cutoff region VGS < VT.

(Regions of operation)

IDS

VGS

VT

Characteristic Curve

Cutoff region


Ohmic region VDS < 0.25 (VGS-VT),

VGS>VT

Voltage controlled resistor.


IDS

VGSVT Characteristic Curve


Saturation region VDS ≥ VGS-VT, VGS >

VT

Constant-current source.


IDSS

Ohmic SaturationIDS

VDS

VGS

VGS VTH

Characteristic curves

Breakdown region VDS > VB



Comparison

p-type charge carrier. Direction of drain current is

opposite. VDS and VGS are negative. n-channel, p-channel behave the

same way.

(n-channel and p-channel)

Depletion MOSFET

Addition of an n-type region between the oxide layer and p-type substrate.

Thus, depletion MOSFETs are normally on.

VT, threshold voltage, is negative.

Unlike enhancement MOSFET, depletion MOSFET :

Allows positive and negative gate voltages.

Can be in the saturation region for VGS= 0

JFET

JFET n-channel p-channel

D

G

S

D

G

S

n-channel p-channel

JFET (Physical and circuit representations)

JFET

Cutoff region VGS < -VP, -VP is the threshold voltage. VDS = 0

(Regions of Operations)

JEFT

Ohmic region VDS < 0.25(VGS + VP), VGS > -VP.

Resistance controlled by VGS

VP

IDS

VDS

Transfer characteristic in saturation region

(| VDS |>|VP|)

IDSS


JFET

Saturation region

VDS ≥ VGS +VP, VGS > -VP.

Constant- current source.


IDSS

Ohmic region

Saturation region

IDS

-VP

VDS

VGS = 0V

VGS

VGS = VP

Idealized output characteristic

JFET

Breakdown regions. VDS > VB.


JFET (Physical representation of the regions)

Illustration of depletion layer growth and pinch-off voltage

Use the I-V characteristic curves of BJT and MOSFET

Use the regions of operation of these transistors

BJT Cutoff Region Active Linear Region Saturation Region

MOSFET Cutoff Region Ohmic or Triode Region Saturation (Active Region)

Transistors as Amplifiers and Switches

Switch operationAmplifier operation

Switch operation

Amplifier operation

I-V Characteristic Curves

Operating Point for BJT•For each, IB there is a corresponding I-V curve. •Selecting IB and VCE, we can find the operating point, or Q point.

•Applying KVL around the base-emitter and collector circuits, we obtain : IB = IBB

VCE = Vcc – ICRC

IC = Vcc VCE

RC RC

I-V Characteristic Curves

IC = Vcc VCE

RC RC

QLoad-line curve

Transistors as Amplifiers

•BJT – common emitter mode•In Linear Active Region•Significant current GainExamplelet Gain, = 80 VB = 2V VE = 1.3V

Find IC and VC

VBE = VB – VE = 0.7VIB = VBB – VB 4 - 2 RB 40,000 = 50 AIC = x IB = 80 x 50 A = 4mA

VC = Vcc – IC x RC

= 12 – (4x10-3)(1x103) = 8 V

VCE = VC – VE = 8 – 1.3 = 6.7 V

Transistors as Amplifiers

=

Transistors as Switches

Basis of digital logic circuits Used in microprocessors Input to transistor gate can be analog or digital Common names are

TTL – Transistor Transitor Logic CMOS – Complementary Metal Oxide Semiconductor

Transistors as Switches – BJT Inverter

Use of the cutoff and saturation regions in the I-V curves. VCE = Vcc - (IC)(RC) Vout = VCE

Transistors as Switches – BJT Inverter

•Vin Low •Cutoff region•No current flows•Vout = VCE = Vcc

•Vout = High

•Vin High •Saturation region•VCE small•Vout = small

•Vout = Low

Transistors as Switches- MOSFET

•Advantages over BJT logic gates•Normally Off. Does not require much current from input signal•Easy Fabrication – Economical for large scale production•CMOS – consumes very little power. Used in pocket calculators and wrist watches

•Disadvantages over BJT logic gates•Cannot provide as much current as BJT•Switching speed is not as fast

Transistors as Switches- MOSFET Inverter

•Vin Low •Cutoff region•No Voltage drop across RD•Vout = VDD

•Vout = High

•Vin High •Ohmic region•VDS small•Vout = small

•Vout = Low

Transistors as Switches- CMOS Inverter

•Employs a p-channel, Qp, and an n-channel, Qn MOSFET•Vin = Low

•Qn = off•Qp = on

•Vout = High

•Vin = High•Qn = on•Qp = off

•Vout = Low

References

•Rizzoni - Principles and Applications of Electrical Engineering, 2nd Edition•www.HowStuffWorks.com•www.williamson-labs.com

BJT

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