Slide 1 Semiconductors Copyright © Declan O’Keeffe Ard Scoil na nDéise, Dungarvan 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

Semiconductor Industry in 2003 The semiconductor business: $166B. – transistors produced during the year. US semiconductor industry: $80B. $13B reinvested in research, $10B in equipment jobs in US alone.

GaAs ZnS (Zinc Blende) Structure 4 Ga atoms at (0,0,0)+ FCC translations 4 As atoms at (¼,¼,¼)+FCC translations Bonding: covalent, partially ionic Silicon Diamond Cubic Structure 4 atoms at (0,0,0)+ FCC translations 4 atoms at (¼,¼,¼)+FCC translations Bonding: covalent Typical Semiconductors

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 8 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 9

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 10

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 11

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 12

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 13

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 14

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 15

Intrinsic Conduction Now, let’s apply some more heat… Slide 16 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 17 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 18 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 19

The Thermistor in Action Slide 20

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 21

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 22

The LDR in Action. Slide 23

The Phosphorus Atom Slide 24 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 25

Doping – Making n-type Silicon Let’s remove another silicon atom… and replace it with a phosphorus atom Slide 26 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 27

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 28

The Boron Atom Slide 29 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 30

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 31 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 32

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

Typical Donor and Acceptor Dopants for Si For Silicon: Donors (n type): – P, As, Sb Acceptors (p type): – B, Al, Ga, In

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 35

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 36

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 37

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 38 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 the p-n junction Slide 39

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 40 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 41

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 42

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 43 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 44

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 45

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 46 Symbol

47 LED cont. Diode Fundamentals Light Emitting Diodes (LEDs) are solid- state semiconductor devices that convert electrical energy directly into light. LED "cold" generation of light leads to high efficacy because most of the energy radiates within the visible spectrum. Because LEDs are the solid-state devices, they can be extremely small and durable; they also provide longer lamp life than other sources. LEDs are made of various semiconducting compounds, depending on the desired colour output. Infrared and red LEDs generally use a gallium, aluminum, and arsenide compound. Orange and yellow LEDs most often use gallium, aluminum, and either indium or phosphorus. Green and blue LEDs typically use either silicon and carbon, or gallium and nitrogen.

48 LEDs cont. Diode Fundamentals Light Emitting Diodes (LEDs )

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 49

Introduction to Transistors A transistor is a device with three separate layers of semiconductor material stacked together – The layers are made of n–type or p–type material in the order pnp or npn – The layers change abruptly to form the pn or np junctions – A terminal is attached to each layer (The Art of Electronics, Horowitz and Hill, 2 nd Ed.) (Introductory Electronics, Simpson, 2 nd Ed.)

Modern Transistors

53 Three terminal device Three semiconductor regions, above is “pnp” E: Emitter, B: Base, C: Collector Voltage between two terminals to control current Use as Amplifier or Switch BJT (Bipolar Junction Transistor)

54 Current flow in an npn transistor biased to operate in the active mode Forward bias of Emitter-Base Junction: current flows to emitter, electrons move towards base, holes to emitter Reverse bias of Base-Collector Junction: I C independent of V CB NPN Transistor

55 Current in PNP mainly due to holes injected from emitter to base PNP Transistor

Transistor types MOS - Metal Oxide Semiconductor FET - Field Effect Transistor BJT - Bipolar Junction Transistor

Moore’s Law It’s an observation made by Gordon E. Moore, in which he predicted that the number of transistors, inside an Integrated Circuit, could be doubled every 24 months. At the density that also minimized the cost of a transistor.

Transistor problems Power density increased Device variability Reliability Complexity Leakage Power dissipation limits device density Transistor will operate near ultimate limits of size and quality – eventually, no transistor can be fundamentally better

The Future of transistors Molecular electronics Carbon nanotubes transistors Nanowire transistors Quantum computing CMOS devices will add functionality to CMOS non-volatile memory, opto- electronics, sensing…. CMOS technology will address new markets macroelectronics, bio-medical devices, … Biology may provide inspiration for new technologies bottom-up assembly, human intelligence "Photo: National Research Council of Canada.“ fundamental/nrc-nint_moleculartransistor_e.html

Pictorial History of Transistors

The End Slide 62© Declan O’Keeffe