TransistorsTransistors

After the World War 2 (1948) the After the World War 2 (1948) the TransistorTransistor was invented in the Bell was invented in the Bell Laboratories in the USA (by Bardeen, Laboratories in the USA (by Bardeen,

Brattain and Shockley)Brattain and Shockley)

The First Transistor

The First Transistor

Colour

Picture

Hearing AidsHearing Aids were the first were the first commercial application of the new commercial application of the new

solid state invention (1952) solid state invention (1952)

The First Transistorized The First Transistorized Hearing AidHearing Aid

No I don’t Know how it was used

In 1954 the In 1954 the first transistor radio first transistor radio called the “Regency TR-1” was called the “Regency TR-1” was produced with the help from Texas produced with the help from Texas Instruments Instruments

It sold for about $49 which would be It sold for about $49 which would be equal to over $300 dollars todayequal to over $300 dollars today

The First Transistor RadioThe First Transistor Radio

Your Grandparents probably had one to listen to the new music that was just starting called

Rock and Roll

So why are transistors so important.So why are transistors so important.

• They can be thought of as an They can be thought of as an electronic switchelectronic switch

• The development of the transistor The development of the transistor was the key component to the was the key component to the development of the modern development of the modern computer.computer.

• Without the development of the Without the development of the transistor your home computer would transistor your home computer would look like..look like..

No there wasn’t a smaller “Laptop version” available

What is a transistor?What is a transistor?

• In basic terms a transistor is a semi- In basic terms a transistor is a semi- conductor device made up of three conductor device made up of three separate partsseparate parts

• These parts are called the Emitter, These parts are called the Emitter, the Base and the Collectorthe Base and the Collector

These parts are put together like a These parts are put together like a sandwich with the Base being sandwich with the Base being between the Emitter and the between the Emitter and the Collector Collector

Emitter

Base

Collector

Origin of the namesOrigin of the names

• the the EmitterEmitter 'emits' the electrons 'emits' the electrons which pass through the device which pass through the device

• the the CollectorCollector 'collects' them again 'collects' them again once they've passed through the once they've passed through the Base Base

• ...and the ...and the BaseBase?... ?...

TransistorsTransistors essentially consist of a pair of PN essentially consist of a pair of PN JunctionJunction Diodes that are joined back-to-back. Diodes that are joined back-to-back. This forms a sort of a sandwich where one kind This forms a sort of a sandwich where one kind of semiconductor is placed in between two of semiconductor is placed in between two others. There are therefore others. There are therefore twotwo kinds of kinds of sandwich, the sandwich, the NPN and and PNP varieties.varieties.

Schematic SymbolsSchematic Symbolscollector

base

emitter

collector

base

emitter

NPN transistor PNP transistor

Notice the difference in the arrow head on the Emitter it always points towards the part (either the Base or the Emitter) that must be more negative for the transistor to turn on.

Transistors have 2 basic uses Transistors have 2 basic uses todaytoday

• To amplify an input signalTo amplify an input signal

• To act as an electronic switchTo act as an electronic switch

• When a transistor is used as a switch it When a transistor is used as a switch it uses the base voltage to turn the uses the base voltage to turn the transistor on or offtransistor on or off

• By doing this a transistor can use a very By doing this a transistor can use a very low current and voltage to control a much low current and voltage to control a much higher currenthigher current

• When a transistor is turned on essentially When a transistor is turned on essentially the electrons flow straight from the the electrons flow straight from the Emitter to the Collector Emitter to the Collector

In the next few slides we will look at In the next few slides we will look at how an NPN transistor is used to how an NPN transistor is used to simulate a switchsimulate a switch

Transistor as a Switch

•Transistors can eitherconductconduct or not conductnot conduct current.

•ie, transistors can either be onon or offoff.

Transistor

Switch

How Transistors Work

•Switching is controlled by the voltage between the Base and the Emitter.

Collector

Emitter

Base

•When VBE < 0.7V the transistor switches off andno current flows between the Collector and the

Emitter.

•When VBE ≥ 0.7V the transistor switches on andcurrent flows between the Collector and the

Emitter.

• When a Transistor is “turned off” . Which in When a Transistor is “turned off” . Which in the case of the NPN transistor is when the the case of the NPN transistor is when the difference between the base voltage and the difference between the base voltage and the Emitter voltage is less than 0.7 Volts there is Emitter voltage is less than 0.7 Volts there is no connection between the Emitter and no connection between the Emitter and Collector (open circuit) Collector (open circuit)

• If you used a meter to measure the voltage If you used a meter to measure the voltage difference between the Emitter and the difference between the Emitter and the Collector you would read all the voltage Collector you would read all the voltage availible availible

• When a Transistor is “turned on” . Which in When a Transistor is “turned on” . Which in the case of the NPN transistor is when the the case of the NPN transistor is when the difference between the Base voltage and the difference between the Base voltage and the Emitter voltage is more than 0.7 Volts there Emitter voltage is more than 0.7 Volts there is a connection between the Emitter and is a connection between the Emitter and Collector (closed circuit) Collector (closed circuit)

• If you used an meter to measure the voltage If you used an meter to measure the voltage difference between the Emitter and the difference between the Emitter and the Collector you would read approximiately 0.2 Collector you would read approximiately 0.2 Volts Volts

Base ThicknessBase Thickness

The thickness of the Base has to be just right.The thickness of the Base has to be just right.– Too thin, and the Base would essentially vanish. Too thin, and the Base would essentially vanish.

The Emitter and Collector would then form a The Emitter and Collector would then form a continuous piece of semiconductor, so current continuous piece of semiconductor, so current would flow between them whatever the base would flow between them whatever the base potential.potential.

– Too thick, and electrons entering the Base from Too thick, and electrons entering the Base from the Emitter wouldn't notice the Collector as it the Emitter wouldn't notice the Collector as it would be too far away. So then, the current would would be too far away. So then, the current would all be between the Emitter and the Base, and all be between the Emitter and the Base, and there'd be no Emitter-Collector current.there'd be no Emitter-Collector current.

• So what is the correct thickness of So what is the correct thickness of the base. This depends on the the base. This depends on the transistor and the amount of current transistor and the amount of current it controls but on most transistors the it controls but on most transistors the base is only a micron to an atom in base is only a micron to an atom in thickness.thickness.

Why does it have to be so thin you askWhy does it have to be so thin you ask

Pentium D processors Pentium D processors 376 million transistors 376 million transistors