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Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please to the SLSS physics website forum @ http://physics.slss.ie/forum

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Page 1: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Slide 1

Semiconductors

Copyright © Declan O’Keeffe

Ard Scoil na nDéise,

Dungarvan

For non-commercial purposes only….. Enjoy!

Comments/suggestions please to the SLSS physics website forum

@ http://physics.slss.ie/forum

Page 2: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

HelpHelpTo view contents/show animation on each slide use

back/forward ‘arrow keys’ on keyboard

mouse click (mouse click must be outside any interactive flash animation area present on a slide)

To view the ‘flash’ content in this presentation you need to have Shockwave & Flash player installed on your system. If you can see a flashing bulb (top right hand corner) then you

may proceed….if not click and and to download & install.

Page 3: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Insulators

Slide 3

Insulators have tightly bound electrons in their outer shell

These electrons require a very large amount of energy to free them for conduction

Let’s apply a potential difference across the insulator above…

The force on each electron is not enough to free it from its orbit and the insulator does not conduct

Insulators are said to have a high resistivity / resistance

Page 4: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Conductors

Slide 4

Conductors have loosely bound electrons in their outer shell

These electrons require a small amount of energy to free them for conduction

Let’s apply a potential difference across the conductor above…

The force on each electron is enough to free it from its orbit and it can jump from atom to atom – the conductor conducts

Conductors are said to have a low resistivity / resistance

Page 5: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Semiconductors

Semiconductors have a resistivity/resistance between that of conductors and insulators

Their electrons are not free to move but a little energy will free them for conduction

The two most common semiconductors are silicon and germanium

Slide 5

Page 6: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Silicon, Si, Atom

Silicon has a valency of 4 i.e. 4 electrons in its outer shell

Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms

These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms

Slide 6

This picture shows the shared electrons

Page 7: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Silicon – the crystal lattice

If we extend this arrangement throughout a piece of silicon…

We have the crystal lattice of silicon

This is how silicon looks when it is cold

It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator

Slide 7

Page 8: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Electron Movement in Silicon

However, if we apply a little heat to the silicon….

An electron may gain enough energy to break free of its bond…

It is then available for conduction and is free to travel throughout the material Slide 8

Page 9: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Hole Movement in Silicon

Let’s take a closer look at what the electron has left behind

There is a gap in the bond – what we call a hole

Let’s give it a little more character…

Slide 9

Page 10: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Hole Movement in Silicon

This hole can also move…

An electron – in a nearby bond – may jump into this hole…

Effectively causing the hole to move…

Like this…

Slide 10

Page 11: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Heating Silicon

We have seen that, in silicon, heat releases electrons from their bonds…

This creates electron-hole pairs which are then available for conduction

Slide 11

Page 12: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Intrinsic Conduction

Take a piece of silicon…

This sets up an electric field throughout the silicon – seen here as dashed lines

When heat is applied an electron is released and…

And apply a potential difference across it…

Slide 12

Page 13: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Intrinsic Conduction

The electron feels a force and moves in the electric field

It is attracted to the positive electrode and re-emitted by the negative electrode

Slide 13

Page 14: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Intrinsic Conduction

Now, let’s apply some more heat…

Slide 14

Another electron breaks free…

And moves in the electric field.

We now have a greater current than before…

And the silicon has less resistance…

Page 15: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Intrinsic Conduction

If more heat is applies the process continues…

Slide 15

More heat…

More current…

Less resistance…

The silicon is acting as a thermistor

Its resistance decreases with temperature

Page 16: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Thermistor

The thermistor is a heat sensitive resistor

When cold it behaves as an insulator i.e. it has a very high resistance

When heated, electron hole pairs are released and are then available for conduction as has been described – thus its resistance is reduced

Slide 16

Thermistor Symbol

Page 17: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Thermistor

Thermistors are used to measure temperature

They are used to turn devices on, or off, as temperature changes

They are also used in fire-warning or frost-warning circuits Thermistor

Symbol

Slide 17

Page 18: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Thermistor in Action

Slide 18

Page 19: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Light Dependent Resistor (LDR)

The LDR is very similar to the thermistor – but uses light energy instead of heat energy

When dark its resistance is high As light falls on it, the energy

releases electron-hole pairs They are then free for conduction Thus, its resistance is reduced LDR

SymbolSlide 19

Page 20: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Light Dependent Resistor (LDR)

LDR’s are used as light meters

LDR’s are also used to control automatic lighting

LDR’s are used where light is needed to control a circuit – e.g. Light operated burgler alarm LDR

SymbolSlide 20

Page 21: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The LDR in Action.

Slide 21

Page 22: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Phosphorus Atom

Slide 22

Phosphorus is number 15 in the periodic table

It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell

Page 23: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Doping – Making n-type Silicon

Relying on heat or light for conduction does not make for reliable electronics

Suppose we remove a silicon atom from the crystal lattice…

and replace it with a phosphorus atom

We now have an electron that is not bonded – it is thus free for conduction

Slide 23

Page 24: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Doping – Making n-type Silicon

Let’s remove another silicon atom…

and replace it with a phosphorus atom

Slide 24

As more electrons are available for conduction we have increased the conductivity of the material

If we now apply a potential difference across the silicon…

Phosphorus is called the dopant

Page 25: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Extrinsic Conduction – n-type Silicon

A current will flow

Note:

The negative electrons move towards the positive terminal

Slide 25

Page 26: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

From now on n-type will be shown like this.

N-type Silicon

This type of silicon is called n-type This is because the majority charge carriers are

negative electrons A small number of minority charge carriers – holes –

will exist due to electrons-hole pairs being created in the silicon atoms due to heat

The silicon is still electrically neutral as the number of protons is equal to the number of electrons

Slide 26

Page 27: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Boron Atom

Slide 27

Boron is number 5 in the periodic table

It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell

Page 28: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Doping – Making p-type Silicon

As before, we remove a silicon atom from the crystal lattice…

This time we replace it with a boron atom

Notice we have a hole in a bond – this hole is thus free for conduction

Slide 28

Page 29: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Doping – Making p-type Silicon

Let’s remove another silicon atom…

and replace it with another boron atom

As more holes are available for conduction we have increased the conductivity of the material

Slide 29

If we now apply a potential difference across the silicon…

Boron is the dopant in this case

Page 30: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

Extrinsic Conduction – p-type silicon

A current will flow – this time carried by positive holes

Note:

The positive holes move towards the negative terminal

Slide 30

Page 31: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

P-type Silicon

This type of silicon is called p-type This is because the majority charge carriers are positive

holes A small number of minority charge carriers – electrons –

will exist due to electrons-hole pairs being created in the silicon atoms due to heat

The silicon is still electrically neutral as the number of protons is equal to the number of electrons

Slide 31

From now on p-type will be shown like this.

Page 32: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The p-n Junction

Suppose we join a piece of p-type silicon to a piece of n-type silicon

We get what is called a p-n junction

Remember – both pieces are electrically neutralSlide 32

Page 33: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The p-n Junction

When initially joined electrons from the n-type migrate into the p-type – less electron density there

When an electron fills a hole – both the electron and hole disappear as the gap in the bond is filled

This leaves a region with no free charge carriers – the depletion layer – this layer acts as an insulator

Slide 33

Page 34: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The p-n Junction

As the p-type has gained electrons – it is left with an overall negative charge…

As the n-type has lost electrons – it is left with an overall positive charge…

Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V

0.6 V

Slide 34

Page 35: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Reverse Biased P-N Junction

Take a p-n junction

Apply a voltage across it with the

p-type negative

n-type positive

Close the switch

The voltage sets up an electric field throughout the junction

Slide 35

The junction is said to be reverse – biased

Page 36: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Reverse Biased P-N Junction

Negative electrons in the n-type feel an attractive force which pulls them away from the depletion layer

Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer

Thus, the depletion layer ( INSULATOR ) is widened and no current flows through thep-n junction Slide 36

Page 37: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Forward Biased P-N Junction

Take a p-n junction

Apply a voltage across it with the

p-type postitive

n-type negative

Close the switch

The voltage sets up an electric field throughout the junction

Slide 37

The junction is said to be forward – biased

Page 38: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Forward Biased P-N Junction

Negative electrons in the n-type feel a repulsive force which pushes them into the depletion layer

Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer

Therefore, the depletion layer is eliminated and a current flows through the p-n junction

Slide 38

Page 39: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Forward Biased P-N Junction

At the junction electrons fill holes

They are replenished by the external cell and current flows

Both disappear as they are no longer free for conduction

This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V

Slide 39

Page 40: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Forward Biased P-N Junction

If we apply a higher voltage…

The electrons feel a greater force and move faster

The current will be greater and will look like

Slide 40

The p-n junction is called a DIODE and is represented by the symbol…

The arrow shows the direction in which it conducts current

this….

Page 41: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Semiconductor Diode

The semiconductor diode is a p-n junction

In reverse bias it does not conduct In forward bias it conducts as long

as the external voltage is greater than the junction voltage

A diode should always have a protective resistor in series as it can be damaged by a large current

Slide 41

Page 42: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Semiconductor Diode

The silver line drawn on one side of the diode represents the line in its symbol

This side should be connected to the negative terminal for the diode to be forward biased

Diodes are used to change alternating current to direct current

Diodes are also used to prevent damage in a circuit by connecting a battery or power supply the wrong way around

Slide 42

Page 43: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Light Emitting Diode (LED)

Some diodes emit light as they conduct These are called LED’s and come in various colours LED’s have one leg longer than the other The longer leg should be connected to the positive terminal for the

LED to be forward biased LED’s are often used as power indicators on radios, TV’s and other

electronic devices

Slide 43

Symbol

Page 44: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The Characteristic Curve of a Diode

Diodes do not obey Ohm’s Law A graph of CURRENT vs

VOLTAGE for a diode will not be a straight line through the origin

The curve will look like this one Note how the current increases

dramatically once the voltage reaches a value of 0.6 V approx. i.e. the junction voltage

This curve is known as the characteristic curve of the diode

Slide 44

Page 45: Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy! Comments/suggestions please

The End

Slide 45© Declan O’Keeffe