slide 1 semiconductors copyright © declan o’keeffe ard scoil na ndéise, dungarvan for...
TRANSCRIPT
Slide 1
Semiconductors
Copyright © Declan O’Keeffe
Ard Scoil na nDéise,
Dungarvan
For non-commercial purposes only….. Enjoy!
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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
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
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
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
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
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
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
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
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
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
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
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…
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
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
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
The Thermistor in Action
Slide 18
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
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
The LDR in Action.
Slide 21
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
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
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
Extrinsic Conduction – n-type Silicon
A current will flow
Note:
The negative electrons move towards the positive terminal
Slide 25
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
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
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
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
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
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.
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
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
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
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
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
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
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
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
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….
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
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
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
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
The End
Slide 45© Declan O’Keeffe