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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.

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

1 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

2 Help To 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.

3 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

4 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

5 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

6 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

7 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

8 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

9 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

10 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

11 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

12 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

13 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

14 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…

15 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

16 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

17 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

18 The Thermistor in Action Slide 18

19 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 Symbol Slide 19

20 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 Symbol Slide 20

21 The LDR in Action. Slide 21

22 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

23 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

24 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

25 Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal Slide 25

26 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

27 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

28 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

29 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

30 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

31 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.

32 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 neutral Slide 32

33 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

34 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

35 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

36 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 the p-n junction Slide 36

37 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

38 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

39 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

40 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….

41 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

42 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

43 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

44 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

45 The End Slide 45© Declan O’Keeffe


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