"Class 12 Subject- PhysicsChapter - Semiconductor Electronics,Materials, Devices and Sample CircuitsIt is the branch of science which deals with the electron flow through a vacuum, gas orsemiconductor.Classification of substances on the basis of conduction of electricity.SolidWe know that, each substance is composed of atoms. Substances are mainly classified intothree categories namely solids, liquids and gases.In each solid atoms are at a definite positions and the average distance between them isconstant.Depending upon the internal arrangement of atoms, solids are further divided into two groups.1. Crystalline SolidsThe solid in which the atoms are arranged in a regular order are called the crystalline solids. Inother words, we can say that in a crystalline solid. There is periodicity and regularity of itscomponent atoms in all the directions. For example sodium chloride (common salt), diamond,Sugar, silver etc are the crystalline solids.Their atoms are arranged in a definite geometrical shape.They have a definite melting point.They are anisotropic, i.e., their physical properties such as thermal Conductivity refractiveindex etc, are different in different directions.They are the real solids.2. Amorphous SolidsThe Solids in which the atoms do not have a definite arrangement are called the amorphoussolids. They are also called the glassy solids. For example glass, rubber, plastic, power, etcare the amorphous solids.They do not have a definite arrangement of its atoms, i e., they do not have a characteristicgeometrical shape. They do not have a definite melting point.They are isotropic. i.e., their physical properties such as conductivity of heat refractiveindex etc, are same in all the directions.They are not the real solids.Monocrystal and PolycrystallineMonocrystal is a crystal in which the ordered arrangements of the atoms or molecules extendthroughout the piece of solid, irrespective of its size.Polycrystal is a crystalline solid in which each piece of the solid has a number of monocrystalswith developed faces joined together.The polycrystal ceramic made from PbO, ZnO and TiO are used in gas lighters and telephonereceivers.Liquid CrystalsSome organic crystalline solid when heated acquires fluidity but retain theiranisotropic properties. They are called liquid crystals.Some liquid crystals like cyanobiphenyl can change the plane of polarization of light andsuch Liquid Crystal Displays (LCD) are used in watches and micro calculators.Crystal LatticeA crystal is made up of a three- dimensional array of points such that each point is surroundedby the height bouring Points in an identical way. Such an array of points is known as bravaislattice or space lattice.Unit cell is the smallest unit of the crystal lattice, repetition of which in three dimensions givesrise to crystal lattice.The length of three sides of a unit cell are called Primitives or lattice constant represented by a,b, c. The angle between three crystallographic axis are called interfacial angles represented bya, ß and ?. The primitives and interfacial angles constitute the lattice parameters of a unit cell.[The cubic crystal may be of the form, simple cubic (sc) lattice, the body centered cubic(bee) lattice, the face centered Cubic (fcc) lattice.]The coordination number is defined as the number of nearest neighbors around any latticepoint (or atom) in the crystal lattice.(a) For sc, coordination number is 6. (b) For bee, coordination number is 8.(c) For fcc, coordination number is 12.(d) For sc, atomic radius is a / 2.(e) For bcc, atomic is a v3 / 4.(f) For fcc. atomic radius is a / 2v2.Classification of solids on the basis of conductivity(i) Conductor Conductors are those substances through which electricity can pass easily, e.g.,all metals are conductors.(ii) Insulator Insulators are those substances through which electricity cannot pass, e.g., wood.rubber, mica etc.(iii) Semiconductor Semiconductors are those substances whose conductivity lies betweenconductors and insulators. e.g., germanium, silicon, carbon etc.Energy Bands of Solids1. Energy BandIn a crystal due to interatomic interaction valence electrons of one atom are shared by morethan one atom in the crystal. Now splitting of energy levels takes place. The collection of theseclosely spaced energy levels is called an energy band.2. Valence BandThis energy band contains valence electrons. This band may be partially or completely filledwith electrons but never be empty. The electrons in this band are not capable of gaining energyfrom external electric field to take part in conduction of current.3. Conduction BandThis band contains conduction electrons. This band is either empty or Partially filled withelectrons.Electrons present in this band take part in the conduction of current.4. Forbidden BandThis band is completely empty. The minimum energy required to shift an electron from valenceband to conduction band is called band gap (E ).g Thermionic Emission Thermionic emission occurs when a metal is heated to a high temperature, the free electronsin the metal gain kinetic energy sufficient to escape through the surface of the metal.Thermionic DiodeThe thermionic diode is a two electrode (cathode and plate) device based on thermionicemission.A diode allows unidirectional flow of electrons, i.e., only when the plate is positive withrespect to cathode. Hence, it is also called a valve.The triode value consists of three electrodes, e.g., cathode. plate and grid enclosed in anevacuated glass bulb.Grid influences the space charge and controls the flow of plate current.[When the grid is given a negative potential with respect to cathode. It repels the electronsescaping from the cathode and Increases the effect of space charge, at sufficiently negativegrid potential is known as cut-off grid bias.If the grid is given a positive potential with respect to cathode, it attracts the electrons anddecreases the effect of space charge. The increasing the plate current. In this case a current flowinto the circuit, thus grid modifies the function of valve.]Grid is always kept at small negative potential with respect to cathode.Triode can be used as an amplifier, oscillator modulator and demodulator.An oscillator is an electronic device which generates AC voltage froID DC power. It isbasically a positive feedback amplifier with infinite voltage gain.Types of Semiconductor(i) Intrinsic Semiconductor A semiconductor in its pure state is called intrinsicsemiconductor.(ii) Extrinsic Semiconductor A semiconductor doped with suitable impurity to increaseits impurity, is called extrinsic semiconductor.On the basis of doped impurity extrinsic semiconductors are of two types(i) n-type Semiconductor Extrinsic semiconductor doped with pentavalent impurity like As,Sb, Bi, etc in which negatively charged electrons works as charge carrier, is called n-typesemiconductor. Every pentavalent impurity atom donate one electron in the crystal, therefore it is called adoner atom(ii) p -type Semiconductor Extrinsic semiconductor doped with trivalent impurity like Al, B,etc, in which positively charged holes works as charge carriers, is called p-type semiconductor.Every trivalent impurity atom have a tendency to accept one electron, therefore it is called anacceptor atom.2 In a doped semiconductor n n = n where n and n are the number density of electronse h i e h and holes and n is number density of intrinsic carriers, i.e., electrons or holes.i In n-type semiconductor, n > > n e hIn p -type semiconductor, n > > n h eElectrical conductivity of extrinsic semiconductor is given bys = 1 / ? = e (n µ + n µ )e e h h where ? is resistivity, µ and µ are mobility of electrons and holes respectively.e h Note Energy gap for Ge is 0.72 eV and for Si it is 1.1 eV.p-n JunctionAn arrangement consisting a p -type semiconductor brought into a close contact with n-typesemiconductor, is called a p -n junction.The current in a p-n junction is given byeV/k BT k = I (e – 1 )B o where I is reverse saturation current, V is potential difference across the diode, and k iso B the Boltzmann constant. Terms Related to p-n Junction(i) Depletion Layer At p-n. Junction a region is created where there are no charge carriers.6 This region is called depletion layer. The width of this region is of the order of 10 m.(ii) Potential Barrier The potential difference across the depletion layer is called potentialbarrier.Barrier potential for Ge is 0.3 V and for Si is 0.7 V.(iii) Forward Biasing In this biasing, the p -side is connected to positive terminal and n-side tonegative terminal of a battery.In this biasing, forward current flows due to majority charge carriers.The width of depletion layer decreases.(iv) Reverse Biasing In this biasing, the p-side is connected to negative terminal and n-side topositive terminal of a battery.In this biasing, reverse current flow due to minority charge carriers.The width of depletion layer increases.A p-n junction diode can be utilized as a rectifier.Zener diode, photo-diode, light-emitting diode, etc are specially designed p-n. junction diodes.p-n Junction DiodeThe current through p-n junction flow only from p toward n and not from n toward p.The maximum voltage that a junction diode can bear without break is called zener voltage andthe junction diodes possessing this voltage are known as zener diode. Resistance of diode R = V / IRectifierA device which converts alternating current or voltage into direct current or voltage IS knownas rectifier. The process of converting AC into DC IS caned rectification.Half-Wave RectifierA half-wave rectifier converts the half cycle of applied AC signal into DC signal. Ordinarytransformer may be used here.Full-Wave RectifierA full-wave rectifier converts the whole cycle of applied AC signal into DC signal. Centretop, transformer is used here.[Half-wave rectifier converts only one-half of AC Into DC while full wave rectifier rectifiesboth halves of AC inputTransistorA transistor is an arrangement obtained by growing a thin layer of one type of semiconductorbetween two thick layers of other similar type semiconductor.Types of Transistors? The left side semiconductor is called emitter, the right side semiconductor is calledcollector and the thin middle layer is called base.? Emitter is highly doped and base is feebly doped.? A transistor can be utilized as an amplifier and oscillator but not a rectifier? Maximum amplification is obtained in common-emitter configuration. Transistor as an AmplifierAn amplifier is a device which is used for increasing the amplitude of variation of alternatingvoltage 01' current or power.The amplifier thus produces an enlarged version of the input signal.The general concept of amplification is represented in figure. There are two input terminalsfor the signal to be amplified and two output terminals for connecting the load; and’ a meansof supplying power to the amplifier.1. In Common Base Amplifier,AC current gain (a ) = ?I / ?I AC c ewhere ?I is change in collector current and ?I change in emitter current.c e AC voltage gain (A ) = Output voltage / Input voltageV = a * Resistance gain = a * R / R AC AC o iwhere R is output resistance of the circuit and R is input resistance of the circuit.o i AC power gain = Change in output power / Change in input power= AC voltage gain * AC current gain2 = a * resistance gainAC"