NAME OF FACULTY : MR. Harekrishna Avaiya

DEPARTMENT: E.C. (PPI-1ST)

BASIC ELECTRONICS

Transistors• History• Transistor Types

• BJT: A bipolar (junction) transistor

is a three-terminal electronic device

constructed of doped semiconductor material

and may be used in amplifying or switching

applications • FET :The field-effect transistor (FET) relies on an electric field to control the shape and hence the conductivity

of a channel of one type of charge carrier

in a semiconductor material

• Power transistorsSub: BE Topic: Transistors

What is a Transistor?

A Transistor is an electronic device

composed of layers of a semiconductor

material which regulates current or

voltage flow and acts as a switch or gate

for electronic circuit.

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History of the Transistor

John Pierce –supervised the Bell Labs team which built the first transistor (1947)

First Solid State Transistor – (1951) Gordon K. Teal (left) and Morgan Sparks at Bell Laboratories, 1951

Akio Morita, who founded a new company named Sony Electronics that mass-produced tiny transistorized radios (1961)

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Applications Switching Amplification Oscillating Circuits Sensors

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Composed of N and P-type Semiconductors

• N-type Semiconductor has an excess of electrons

– Doped with impurity with more valence electrons than silicon

P-type Semiconductor has a deficit of electrons (Holes)

– Doped with impurity with less valence electrons than silicon

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P-N Junction (Basic diode):

- Bringing P and N Semiconductors in contact

P Type N Type - Creation of a Depletion Zone

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Reverse Biased => No Current

Applying –Voltage to Anode increases

Barrier Voltage & Inhibits Current Flow

• Applying Voltage to Cathode

• Barrier Voltage to Anode allows current flow

Forward Biased =>

Current

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Types Of TransistorsNPN: transistor where the majority current carriers are electrons

The majority current carriers in the PNP transistor are holes

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Transistor Operation

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Transistors

• Semiconductor device

• First Active circuit element - gain > 1

• Discuss the Bipolar Junction Transistor only

• See Simpson Chapter 5 for more detail.

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Bipolar Junction Transistors

• NPN Bipolar Junction transistor shown (PNP also possible)

• 3 terminals: Emitter, Base, Collector

• Contains 2 p-n junctions: emitter-base junction, collector-base junction

• Can be thought of as two back-to-back diodes, but operating characteristics are very different

• Base region (P-type here) must be thin for transistor action to work

Base

Emitter

Collector

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Modes of operation

• Use 2 voltage supplies to bias the two junctions (forward or reverse)• 3 basic modes: cutoff, active and saturation, correspond to three different bias conditions.

Base

Emitter

Collector

“OFF”

“ON”

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Active Mode• Collector-Base junction is reverse biased• Emitter-Base junction is forward biased• iC=βiB (β=100 - 500) Active circuit element - gain > 1 !!• How does collector current flow when collector-base junction is reverse biased?

Emitter

CollectorBase

+

-

iC

iB

iE

VCE

VBE

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Active Mode

• What's happening?• Emitter-base is forward biased; collector-base reverse biased.• Forward bias of emitter injects electrons into thin base region• Majority shoot through the base into the collector region where they encounter the voltage source on the collector and produce a current.• Electrons combine with holes in the base region and form negative ions which impede the flow• Drawing off negative charge via the base lead reduces this effect (“making the base smaller”) - so the base current controls the flow of electrons into the collector• Nobel Prize 1956; Shockley, Bardeen & Brattain

Direction of current flow

+-

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Circuit Symbols and Conventions

• BJTs are not symmetric devices• Doping and physical dimensions are different for collector and emitter

• Collector largest, connected to heat sink as most power dissipated there• Emitter region smaller, and more heavily doped to provide an abundance

of charge carriers• Base region is very thin (~50nm) to enhance probability that electrons will

cross it• PNP devices also exist - diode senses are reversed, so bias voltage polarities must also be reversed

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Emitter

CollectorBase

+

-

iC

iB

iE

VCE

VBE

i-v Characteristics

• Simplest model for low frequencies (DC condition) "Ebers-Moll". • Relates collector current IC to base-emitter voltage VBE:

• IS=Saturation Current• Similar to Diode Law• Recall IB=IC/β

VBE

IC

TBE

TBE

VVS

VVSC

eI

eII/

/ 1

• Collector current is controlled by the base-emitter voltage VBE

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BJT Amplifier

Note labelling scheme:iC=IC+ic

"Common emitter" configuration

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Thank You