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SEMICONDUCTORS Semiconductors : Semiconductors are the substances which are insulators at zero kelvin, but starts conducting as the temperature is increased. The size of forbidden energy gap is much smaller than that for insulators. Due to smaller forbidden gap electrons can easily shift from valence band to the conduction band. Semiconductors are divided into two categories: (i) Intrinsic Semiconductors (ii) Extrinsic Semiconductors Intrinsic Semiconductors: Pure germanium and silicon crystals in their natural states are called Intrinsic semiconductors. In germanium crystals atoms are arranged on the corners of a regular tetrahedron. Each of the four with valence electron is shared a nearest atom to form covalent bond. No free electrons are available in pure semiconductors. However, some bonds are broken due to thermal agitation and electron is released. The vacant space left by electron is called hole. Only 1 bond is broken for 109 germanium atoms. So number of free electrons available for conduction are very small. As the crystal is electrically neutral so number of free electrons is equal to number of holes. If electric potential difference is applied across the semiconductors the electrons will move opposite to field and hole moves in the direction of field, thus forming current. But due to small number of electrons and holes the magnitude of current is very small. Doping: The process of addition of impurities to increase the conductivity of Silicon or Germanium crystal is called doping and the impurity atom added is called dopant. Extrinsic Semiconductors: The impurity atoms added are of two types: Pentavalent impurity atoms i.e. atoms having 5 valence electrons such as Antimony or Arsenic. Such atoms added will create excess of free electrons. This type of doped semiconductors is called n-type semiconductor. Trivalent impurity atoms i.e. atoms having 3 valence electrons, such as Indium or Gallium. Such atoms on being added to germanium crystal make the crystal deficient in electrons and holes will be produced. This type of doped semiconductors are called p-type semiconductors. N – Type Semiconductors: When an impurity atom with 5 valence electrons is added to germanium crystal, it replaces one of the germanium atoms. Four of the five valence electrons form covalent bonds with neighboring atoms and fifth electron becomes free to move in the crystal structure. Thus by adding impurity atom we are increasing the number of electrons and hence conductivity increases. Since charge carriers are negatively charged electrons they are called n-type semiconductors. In n-type semiconductor, some covalent bonds are also broken resulting in formation of electrons and hole. In N type semiconductors, electrons are thus majority carriers and holes are minority carriers. An N type semiconductor has freely moving electrons and an equal number of stationary positively charged donor atoms. The crystal as a whole is neutral. The fifth valence electron is in fact bound with the parent atom with a very small energy of the order of 0.05 eV. P – Type Semiconductors: If we add trivalent impurity to germanium crystal it will replace germanium atom in the crystal structure. The three valence electron will form three covalent bond with neighboring Ge atoms and fourth space is left vacant which is called a hole. Thus, for every trivalent impurity added a hole is created. The Ge crystal so formed is called p-type semiconductor as it contains free holes. Each hole is equivalent to positive charge. The Ge crystal also contains few electrons which are present due to breakage of covalent bonds. For each bond breakage an electron and a hole are released. Thus, p-type semiconductor contains holes in majority and electrons are present in minority. Doping: While doping following points should be noted: (i) The dopant atom should take the position of semiconductor atom. (ii) The presence of dopant should not distort the crystal. (iii) The size of dopant should be almost same as semiconductor atom (iv) Dopant atoms should not be more than 1% of the crystal atoms. Methods of Doping; Heat the crystalline semiconductor in an atmosphere having dopant atoms so that they can diffuse into semiconductor. Implant dopants by bombarding the semiconductor with their ions. Formation of P-N junction diode: It is formed by placing a P-type crystal in contact with N type crystal and subjecting it to high pressure so that it becomes single piece. The assembly so obtained is called P-N junction or junction diode or crystal diode. The surface of contact of P and N type crystal is called junction. During the formation of junction diode, holes from P region diffuse into N region and electrons from N region diffuse into P region. In both cases, when an electron meets a hole, they cancel the effect of each other and as a result a thin layer at the junction becomes free from any of charge carriers. This is called depletion layer. The thickness of depletion layer is of the order of 10—6 m. Due to movement of electrons from N type it gets positive potential and similarly P type gets negative potential. Due to this, there is potential gradient in the depletion layer, negative on P side and positive on N side. In other words, it appears as if some fictitious battery is connected across the junction with its negative pole connected to p-type and positive pole to N type. This potential difference is called potential barrier. Forward Biasing: When P type is connected to positive pole of battery and N type with negative pole it is called forward biasing. When the forward bias is more than the barrier voltage then majority carrier i.e. electrons in N type and holes in P type moves towards junction and cross it. As the forward bias voltage is increased, the flow of majority carriers increases. Because of attraction between electrons and holes, they rush towards each other and recombine at the junction. For each combination a covalent bond in P region near positive terminal of battery breaks and electron hole pair is used. Hole moves forward and electrons moves to the positive of the battery. Thus, in external circuit current passes due to flow of electrons. When the forward bias voltage is less than the barrier voltage then the charge carriers experience higher resistance and cannot cross this region. As forward bias is increased the current increases with increase in forward bias.Beyond a certain forward bias the electrons passing from the junction gain sufficient kinetic energy to expel valence electrons from atoms, resulting in large increase in current. Reverse Biasing: When P type is connected with negative terminal and N type with positive terminal of battery then it is said to be reverse biased. In this case, majority carriers move away from the junction. The depletion region width increases and it offers high resistance to charge carriers. However, a few minority charge carriers on being accelerated by reverse bias cross the junction and result in small current in reverse direction. This current is called Leakage current. When reverse voltage is 25V the excessive high temperature destroys the covalent bond strucure in germanium and reverse current rises sharply. This voltage is called Breakdown Voltage. Diode As a Rectifier: Rectifier is a device which changes A.C. to D.C. Principle: It is based on the principle that junction diode offers low resistance path when forward biased and high resistance when reversed biased. Half wave Rectifier: A single diode is used as a half wave rectifier. The a.c. to be rectified is connected across the primary coil P of step down transformer. The terminal A of the secondary coil S of transformer is connected to the junction diode and load resistance RL as shown. During first half cycle, diode gets forward biased, the conventional current flows in the direction of arrow heads. During second half cycle diode get reverse biased and hence no output is obtained across RL. A small current will although be present due to minority carriers. Thus, we shall get the discontinuous and pulsating output across the load resistance. Full Wave Rectifier: In case of full wave rectifier, both halves of input a.c. are rectified. The P region of two diodes are connected to two extreme ends of secondary. A load resistance RL is connected across the common N region and central tapping of the transformer. Output is taken across the load resistance RL. During the first half cycle of input a.c.let the upper end of secondary be at positive potential with respect to the central tapping. The upper junction diode will be forward biased and diode will be reversed biased. Current will pass through in direction as shown. Similarly, during negative half cycle the diode will be forward biased, the current passes through diodes D2 and the direction remains the same as in the first case. As the direction of current (in both the cases) in load resistance remains the same, we call it d.c. Types of Diodes: Solar Cell: A solar cell is a diode used to convert light energy into electrical energy. A p-n junction with p or n region made so thin that light energy entering is not absorbed appreciably before reaching the junction constitutes solar cell. The thin region is the emitter and other region is the base. The LED (Light Emitting Diode): An LED is made of semiconductors gallium arsenide or indium phosphate emits radiation in the visible region. In an LED that is forward biased, the combination of electrons and holes at junction releases energy that is emitted as light. Photodiode: Photodiode is so fabricated so that light is allowed to fall at the junction. If light of frequency v falls on the junction and hv is greater than the energy required to move as electron from valence to the conduction band. Zener diode: These are specially designed junction diodes which can operate in the reverse breakdown voltage region without being damaged are called Zener diode. An important application of zener diode is that it can be used as voltage regulator. The regulating action takes place because of the fact that in reverse breakdown region, a very small change in voltage produces large change in current. This causes a sufficient increase in voltage drop across the resistance to lower voltage back to normal. Similarly, when the voltage across the diode tends to decrease, the current through diode goes down out of proportion so that voltage drop across the resistor is much less and it raises voltage back to normal. Transistor: It is a three layer semiconductor device. It can be either pnp or npn. In p-n-p, n layer is sandwiched between two p layers and in n-p-n, p layer is sandwiched between two n layers. The three layers are called emitter, base and collector. The base of transistor is lightly doped and is thin in comparison with emitter and collector. The emitter supplies the charge barriers and collector collects them. Input circuit is generally forward biased and output circuit is reverse biased. Transistor Actions: N-P-N Transistor (Common Base): To understand the action of common base transistor, the base emitter junction is forward biased and collector base junction is reverse biased. The electrons being majority carriers in the emitter are repelled by the negative potential of the emitter junction towards the base. The base has holes as majority carriers and some holes and electrons recombine in the base region and the base is lightly doped. Due to this, the probability of the electron-hole combination in the base region is very small (about 5%). The remaining electrons enter the collector region due to positive potential at the collector terminal. For each electron entering the collector terminal an electron from ve terminal of emitter base battery enters the emitter junction maintaining the number of electrons in emitter junction. Thus in NPN transistor, current inside as well as outside is carried by electrons. For transistor, IE = IC + IB P-N-P Transistor (Common Base): In this case also the emitter base junction is forward biased and base collector junction is reversed biased with batteries VEB and VCB respectively. The holes being in majority in P type emitter are repelled towards the N type base region by positive potential of emitter base battery. In the base region some of the holes recombine with electrons in the N type semiconductor. Remaining holes move into the collector region due to ve terminal of the collector base battery. For each hole that reaches collector terminal an electron leaves the negative pole of the battery V CB and neutralize it. At the same time, an electron from some covalent bond in emitter enters into the positive terminal of battery VEB, creating a hole in the emitter. Thus current inside the semiconductor is carried by holes and outside the semiconductor it is carried by electrons. In this case also IE = IC + IB Concept of Amplifier: Amplifier is a device which produces enlarged version of the input signal. It is used for increasing the amplitude of variation of alternating voltage or current or power. Common Base Amplifier: The signal to be amplified is connected between emitter and base while a high load resistance is connected between collector and base. The input circuit is forward biased while the output circuit is reverse biased. The input circuit being forward biased is a low resistance circuit and output circuit being reverse biased is a high resistance circuit. When no a.c. signal is fed to the input circuit voltage drop across RL due to collector is ICRL. If VCB is the voltage of collector-base battery, then potential difference, VC between the collector and the base would be given by, VC = VCB IC RL Suppose now an a.c. signal is fed to the input circuit. As the emitter is negative w.r.t the base, positive half cycle of input wave opposes and hence decreases the forward bias. Thus, IE and IC both decreases. The value of VC increases as IC decreases i.e. the collector becomes more positive. During negative half cycle of a.c., the forward bias increases. Due to increase in forward bias, IE increases and consequently IC also increases. VC would decrease i.e. collector becomes less positive or more negative. Current Gain: It is the ratio of collector current to the emitter current, i.e. I C IB A.C. Current Gain: It is defined as the ratio of change in collector current to the change in emitter current (IE), I ac C I E is constant. Voltage Gain: It is defined as the change in output voltage to the change in input voltage. VC I C R0 VoltageGain V I I E R1 Voltage Gain ac resistance gain Power Gain: It is defined as the change in output power to the change in input power. V C I C Power Gain V I I E 2 Resistance Gain Common Emitter Amplifier: Here also the input circuit is forward biased and the output circuit is reverse biased. When no a.c. signal is applied the potential differene VC between the collector and emitter is given by, VC = VCE IC RL When an a.c. signal is fed to the input circuit, the forward bias increases during positive half cycle of the input. This results in increase in IC and consequent decrease in VC , thus during positive half cycle of the input, the collector becomes less positive. During negative half cycle of the input, forward bias decreases, therefore, the value of IE and IC also decreases and VC would increase making the collector more positive. In common emitter amplifier, thus there is 180ºout of phase amplification. Current Gain: It is defined as the ratio of collector current to the base current. I IC C Ib I E IC Dividing the numerator and denominator by IE, IC IE I IC 1 IE A.C. Current Gain: It is defined as the change in collector current to the change in base current, I ac C I b Voltage Gain: It is defined as the change in output to the change in input voltage, V C I C R 0 VoltageGain V I I b R i a .c . Resistance Gain Power Gain: It is defined as the change in output power to the change in input power. V I Power Gain C C Vi I b 2 Resistance Gain > therefore, extremely high power gain is possible in common emitter configuration. Transistor as an Oscillator: Oscillator converts direct current into alternating current and produces high frequency undamped oscillations. The base oscillatory circuit consists of an inductance and capacitance called tank circuit. Due to resistance of circuit, a part of energy is dissipated, therefore, amplitude of oscillations goes on decreasing with time and damped oscillations are produced. In order to maintain these oscillations, energy is supplied to circuit at the right moment and in the right direction using a feedback arrangement. The feedback arrangement consists of primary P and secondary with variable capacitor C of suitable range. The secondary coil of inductance L. The inductance L and capacitance C constitute tank circuit. Working: When key is inserted collector current in primary increases. The flow of electrons will be in upward direction. Induced current is produced in secondary in opposite direction. The lower plate of the capacitor will become negatively charged and upper plate positively charged. The forward bias in emitter base circuit increases. This further increases the collector current. When collector current attains maximum value, induced emf supporting the forward bias decreases. Thus collector current decreases and emf will be induced in such a way that it opposes the forward bias. Thus, collector current reaches a certain minimum value. In this way the oscillations in tank circuit will be maintained. DIGITAL ELECTRONICS In these electronics circuits, the current or voltages will have only two values, High (1) and Low (0). In digital circuits, the electrical pulses of two levels only are used as signal voltages. Logic Gates: A gate is a digital circuit which is used to perform certain specific function. The three basic logic gates are: a. OR gate b. AND gate c. NOT gate All other logic gates can be formed by combination of these three gates. Truth Table: It is table that indicate all possible combinations of input signals and their output. Boolean Algebra: This is the algebra which can be applied to logic gates based on Binary number system. OR Gate: It is a two input single output gate. The output is one if any of the two inputs or both the inputs are one. The truth table and symbol of OR gate are: A B Y 0 0 0 0 1 1 1 0 1 1 1 1 The circuit diagram for OR gate is: The diodes used are considered to be ideal diodes ie. during forward bias they offer zero resistance and during reverse bias they offer infinite resistance. Case I: When A = 0, B = 0: Here both the diodes are in off state. There is no current that flows through R, thus output voltage is Y=0. Case II: When A = 0, B = 1: In this case D1 is in off state and D2 is forward biased. The current flows through D2 and sets up a potential difference of 5V across it, so Y = 1. Case III: When A = 1, B = 0: in this case diode D1 is forward biased and D2 is reverse biased. Diode D1 conducts and Y=1. Case IV: When A=1, B=1: Here both the diodes are forward biased and hence conduct perfectly. A potential difference of 5V appears across resistance. Thus, Y = 1. AND Gate: It is also a two input single output gate. The output is one iff both the inputs are one. (a) Suppose A=0 and B=0: The potentials at A and B are forward biased and offers no resistance. The diode D1 conducts and net potential difference appears across R and Y=0. (b) Suppose A = 0 and B = 1: In this case also A is forward biased and B is in off state. The diode D1 conducts and net potential difference appears across R and Y = 0. (c) When A = 1 and B = 0: In this case also A is in off state and B is forward biased. The diode D2 conducts and Y = 0 (d) When A = 1 and B = 1: Here both diodes are in off state, hence no potential drop occurs across R and Y = 1. NOT Gate: It is a single input single output gate. The truth table and symbol is A Y A 0 1 1 0 It is realised with the help of a transistor. Consider an pnp transistor to be used as NOT gate. If A = 0, the emitter base junction is reverse biased and no current flows through it. Correspondingly current through RC is also equal to zero. The potential Y = 1. On the other hand, if A is 5V i.e. A =1, the emitter base junction is forward biased. Potential drop occurs R and Y=0 NAND Gate: It is AND gate followed by a NOT Gate. It is two input single output gate. The truth table and symbol are, Y X A B X 0 0 0 1 0 1 0 1 1 0 0 1 1 1 1 0 NOR Gate: It is OR gate followed by a NOT gate. It is a two input single output gate. The truth table and symbol are, Y X A B X 0 0 0 1 0 1 1 0 1 0 1 0 1 1 1 0 Exclusive OR (XOR) Gate: It is also two input, single output gate. The output is one iff one of the inputs is one. The truth table and symbol are; A B 0 0 0 1 1 0 1 1 Exclusive NOR Gate: It is an exclusive The truth table is, A 0 0 1 1 B 0 1 0 1 Y AB AB 0 1 1 0 OR gate followed by a NOT gate. Output is one either both the inputs are one or zero. Y AB A B 1 0 0 1