Electronics Sections Dr. Azza Semiconductor P-N Junction Diode , Zener Diode BJT Transistor MosFet Transistor Some Applications Using Transistor Slide 1

Electronics Sections Dr. Amen Nasar Op-Amp Some Applications Using Op-Amp Slide 2

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 3

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 4

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 This picture shows the shared electrons Slide 6

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… 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 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… 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 14

Intrinsic Conduction If more heat is applies the process continues… More current… Less resistance… The silicon is acting as a thermistor Its resistance decreases with temperature Slide 15

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 16

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 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 18

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 LDR Symbol Slide 19

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 20

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 21

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 22

Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal Slide 23

N-type Silicon This type of silicon is called n-type 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 24

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 25

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 26

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 27

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 28

P-type Silicon This type of silicon is called p-type 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 29

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 30

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 31

The p-n Junction 0.6 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 0.6 V Slide 32

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 33

The Reverse Biased P-N Junction Thus, the depletion layer ( INSULATOR ) is widened and no current flows through the 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 Slide 34

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 35

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 36

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. 0.6 V Slide 37

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 arrow shows the direction in which it conducts current The p-n junction is called a DIODE and is represented by the symbol… Slide 38

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 39

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 40

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 41

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 42

The pn Junction Diode Schematic diagram Circuit symbol ID + VD – p-type n-type net acceptor concentration NA net donor concentration ND + VD – cross-sectional area AD Physical structure: (an example) ID + VD – metal SiO2 SiO2 p-type Si For simplicity, assume that the doping profile changes abruptly at the junction. n-type Si metal

Charge Density Distribution Charge is stored in the depletion region. acceptor ions donor ions – + – + p n – + – + – + quasi-neutral p region depletion region quasi-neutral n region charge density (C/cm3) distance

Two Governing Laws Gauss’s Law describes the relationship of charge (density) and electric field. Poisson’s Equation describes the relationship between electric field distribution and electric potential

Depletion Approximation 1 Gauss’s Law p n

Depletion Approximation 2 E0(x) n -xpo xno x s no d po a x qN E e - = ) ( Poisson’s Equation f0(x) qN qN d x 2 + a x 2 2 e no 2 e po s s n=1017 p=105 P=1018 n=104 -xpo xno x

Depletion Approximation 3

Depletion Approx. – with VD<0 reverse bias E0(x) n -xpo xno -xp xn x s no d po a x qN E e - = ) ( Higher barrier and few holes in n-type lead to little current! p=105 f0(x) qN -xp xn qN fbi-qVD Built-in potential fbi= d x 2 + a x 2 2 e no 2 e po n=1017 s s P=1018 fbi n=104 -xpo xno x

Depletion Approx. – with VD>0 forward bias E0(x) n -xpo xno -xp xn x s no d po a x qN E e - = ) ( Poisson’s Equation Lower barrier and large hole (electron) density at the right places lead to large current! f0(x) qN qN Built-in potential fbi= d x 2 + a x 2 2 e no 2 e po P=1018 s s n=1017 p=105 fbi n=104 fbi-qVD -xpo -xp xn xno x

Forward Bias As VD increases, the potential barrier to carrier diffusion across the junction decreases*, and current increases exponentially. The carriers that diffuse across the junction become minority carriers in the quasi-neutral regions; they then recombine with majority carriers, “dying out” with distance. VD > 0 – + – + p n – + – + – + ID (Amperes) VD (Volts) * Hence, the width of the depletion region decreases.

Reverse Bias As |VD| increases, the potential barrier to carrier diffusion across the junction increases*; thus, no carriers diffuse across the junction. A very small amount of reverse current (ID < 0) does flow, due to minority carriers diffusing from the quasi-neutral regions into the depletion region and drifting across the junction. VD < 0 – + – + p n – + – + – + ID (Amperes) VD (Volts) * Hence, the width of the depletion region increases.

Reference 1.Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan @ http://physics.slss.ie/forum 2. EE40 Lecture 32 Prof. Chang-Hasnain Slide 53

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