Semiconductors
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Semiconductors
Instructor: Akram Ahmed Habib Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy!
Insulators
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 Slide 2
Conductors
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 Slide 3
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 4
The Silicon, Si, Atom Silicon has a valency of 4 i.e. 4 electrons in its outer shell
This picture shows the shared electrons
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 5
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 6
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 7
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 8
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 9
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 10
Intrinsic Conduction Take a piece of silicon… And apply a potential difference across it… This sets up an electric field throughout the silicon – seen here as dashed lines When heat is applied an electron is released and…
Slide 11
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 12
Intrinsic Conduction Now, let’s apply some more heat… Another electron breaks free… And moves in the electric field. We now have a greater current than before… And the silicon has less resistance…
Slide 13
Intrinsic Conduction If more heat is applies the process continues… More heat… More current… Less resistance… The silicon is acting as a thermistor Its resistance decreases with temperature Slide 14
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
Thermistor Symbol Slide 15
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 16
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 17
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 18
The Phosphorus Atom Phosphorus is number 15 in the periodic table
It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell Slide 19
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 20
Doping – Making n-type Silicon Let’s remove another silicon atom… and replace it with a phosphorus atom As more electrons are available for conduction we have increased the conductivity of the material Phosphorus is called the dopant
If we now apply a potential difference across the silicon… Slide 21
Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal Slide 22
N-type Silicon
From now on n-type will be shown like this.
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 23
The Boron Atom Boron is number 5 in the periodic table
It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell Slide 24
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 25
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 Boron is the dopant in this case
If we now apply a potential difference across the silicon… Slide 26
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 27
P-type Silicon
From now on p-type will be shown like this.
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 28
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 29
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 30
The p-n Junction
0.7 V
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 0.7 V Slide 31
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
The junction is said to be reverse – biased Slide 32
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 33
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
The junction is said to be forward – biased Slide 34
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 35
The Forward Biased P-N Junction At the junction electrons fill holes Both disappear as they are no longer free for conduction They are replenished by the external cell and current flows
This continues as long as the external voltage is greater than the junction voltage i.e. 0.7 V Slide 36
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 this…. The p-n junction is called a DIODE and is represented by the symbol…
The arrow shows the direction in which it conducts current Slide 37
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 38
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 39
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
Symbol Slide 40
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 0.7 V approx. i.e. the junction voltage This curve is known as the characteristic curve of the diode
Slide 41
For Attending This session
Slide 42
Instructor: Akram Ahmed Habib Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan For non-commercial purposes only….. Enjoy!
Insulators
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 Slide 2
Conductors
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 Slide 3
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 4
The Silicon, Si, Atom Silicon has a valency of 4 i.e. 4 electrons in its outer shell
This picture shows the shared electrons
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 5
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 6
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 7
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 8
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 9
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 10
Intrinsic Conduction Take a piece of silicon… And apply a potential difference across it… This sets up an electric field throughout the silicon – seen here as dashed lines When heat is applied an electron is released and…
Slide 11
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 12
Intrinsic Conduction Now, let’s apply some more heat… Another electron breaks free… And moves in the electric field. We now have a greater current than before… And the silicon has less resistance…
Slide 13
Intrinsic Conduction If more heat is applies the process continues… More heat… More current… Less resistance… The silicon is acting as a thermistor Its resistance decreases with temperature Slide 14
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
Thermistor Symbol Slide 15
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 16
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 17
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 18
The Phosphorus Atom Phosphorus is number 15 in the periodic table
It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell Slide 19
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 20
Doping – Making n-type Silicon Let’s remove another silicon atom… and replace it with a phosphorus atom As more electrons are available for conduction we have increased the conductivity of the material Phosphorus is called the dopant
If we now apply a potential difference across the silicon… Slide 21
Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal Slide 22
N-type Silicon
From now on n-type will be shown like this.
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 23
The Boron Atom Boron is number 5 in the periodic table
It has 5 protons and 5 electrons – 3 of these electrons are in its outer shell Slide 24
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 25
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 Boron is the dopant in this case
If we now apply a potential difference across the silicon… Slide 26
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 27
P-type Silicon
From now on p-type will be shown like this.
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 28
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 29
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 30
The p-n Junction
0.7 V
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 0.7 V Slide 31
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
The junction is said to be reverse – biased Slide 32
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 33
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
The junction is said to be forward – biased Slide 34
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 35
The Forward Biased P-N Junction At the junction electrons fill holes Both disappear as they are no longer free for conduction They are replenished by the external cell and current flows
This continues as long as the external voltage is greater than the junction voltage i.e. 0.7 V Slide 36
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 this…. The p-n junction is called a DIODE and is represented by the symbol…
The arrow shows the direction in which it conducts current Slide 37
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 38
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 39
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
Symbol Slide 40
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 0.7 V approx. i.e. the junction voltage This curve is known as the characteristic curve of the diode
Slide 41
For Attending This session
Slide 42
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