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CHAPTER - 14: SEMICONDUCTOR ELECTRONICS, Electronics is the branch of applied physics which deals with the electronic devices where the, generation and flow of electrons are controlled. The branch of electronics which deals with the, controlled motion of electrons in semiconductor devices is called solid state electronics., Band theory of solids, An single isolated atom can have different quantised energy levels. However an atom in a solid is, influenced by the closely packed neighbouring atoms. As a result the electron in any orbit of an, atom can have a range of energies rather than a single energy. This is known as energy band. The, range of energies possessed by an electrons in a solid is known as energy band., , Figure (ii) represents energy level diagram for a single atom. However, when group of atoms are, considered, first orbit electrons of all atoms can have range of energies rather than a single energy, due to slightly different charge environment. Since, there are millions of first orbit electrons with, slightly different energy levels, the bunch of these energy levels forms first band. Similarly, second, orbit electrons of all atoms forms second band and so on., The first energy band in the figure represents the bunch of energy levels possessed by all first orbit, electrons. The second band represents the bunch of energy levels possessed by all second orbit, electrons and so on. Thus, each discrete energy level of an electron of an atom is converted into, energy band. This forms the basis of band theory of solids., , Outermost orbit electrons are also called valence electrons. Bunch of energy levels possessed by all, valence electrons is called valence band., HCL, , II PU Physics - Maharani’s PU College, Mysore, , 1
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In some materials, valence electrons are loosely bounded with the field of nucleus, even at ordinary, temperature these valence electrons may detach from the field of nucleus and become free electrons., As conduction of free electrons constitute electric current, they are also called conduction electrons., Bunch of energy levels possessed by all conduction electrons is called CB., The energy separation between VB and CB is called energy gap (or) forbidden energy gap (Eg)., Based on the separation between CB and VB, solids are classified into conductors, insulators and, semiconductors., Classification of solids based on band theory, The difference in the behaviour of solids as regards their electrical conductivity can be explained in, terms of energy bands. The electrons in the lower energy band are tightly bound to the nucleus and, play no role in the conduction process. The electrons in the higher energy bands i.e., CB and VB, play major role in the conduction process. The solids are classified into conductors, insulators and, semiconductors based on VB and CB and the energy gap between them., , Conductors:, In case of good conductors and at room temperature, the valence band is completely filled and the, conduction band is also completely filled and they overlap each other., Completely filled conduction band indicates that large number of free electrons are available for, electrical conductivity and overlapping of bands indicates that even a slight potential difference, across the conductor can convert valence electrons into free electrons and conduct electricity., Insulators:, In case insulators and at room temperature, the valence band is completely filled and conduction, band is completely empty. The energy gap between valence band and conduction band is very large, varies from 3 to15 eV., Empty conduction band indicates that no free electrons are available for electrical conductivity., Large energy gap indicates, very high potential difference is required to push the valence electrons to, the conduction band. For this reason the electrical conductivity in insulators is extremely small and, regarded as Nil under ordinary conditions., Semiconductors:, Semiconductors are those substances, whose electrical conductivity lies between those of conductors, and insulators. In terms of energy band, the valence band is completely filled and the conduction, band is completely empty at absolute 0K. The energy gap between conduction and valence bands is, HCL, , II PU Physics - Maharani’s PU College, Mysore, , 2
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extremely small about 1 eV (it is 1.1 eV for silicon and 0.72 eV for germanium). Due to small, energy gap, valence electrons can acquire sufficient energy to cross over conduction band and starts, to conduct even at room temperature. As the temperature increases, more number of valence, electrons becomes free electrons and hence conductivity increases., ** In semiconductors,, At zero kelvin – the CB is completely empty thus behave like a pure insulator., At Room temperature – the CB is partially filled., At high temperature – the CB is completely filled thus behave like a pure conductor., Bonds in semiconductors, The atoms of every element are hold together by the bonding action of valence electrons., A pure conductor atom has 1 valence electron in the outermost orbit., A pure insulator atom has 8 valence electrons in the outermost orbit., A pure semiconductor atom has 4 valence electrons in the outermost orbit., Germanium and silicon are pure semiconductors widely used in fabricating semiconducting devices., During bonding process, each atom in a substance may lose (or) gain valence electrons with other, atoms., In semiconductors, bonds are formed by sharing of valence electrons and such bonds are called, co-valent bonds., Germanium consists of 32 electrons and silicon consists 14 electrons, but both Ge and Si consists 4, valence electrons in the outermost orbit. They are responsible for bonding with neighboring atoms., The following figure shows the bonding structure in Ge atoms., , At absolute zero temperature, all the electrons are tightly held by the semiconductor atoms. The, inner orbit electrons are bond with nucleus whereas the valence electrons are engaged in co-valent, bonding and the bonds are very strong. Hence, at this temperature no free electrons are available for, conduction and thus behaves as insulator., When temperature of semiconductor is raised, some of the co-valent bonds will break due to thermal, agitation. Hence, valence electrons acquires energy sufficient to overcome the energy gap to become, free electrons. Under the influence of electric field these free electrons will constitute electric, current. Clearly, with increase in temperature more number of valence electrons becomes free, electrons due to breakage of co-valent bonds. Thus, the electrical resistance decreases and electrical, conductivity increases with rise in temperature, i.e., a semiconductor has –ve temperature, co-efficient. At higher temperatures, a semiconductor behaves has a pure conductor., HCL, , II PU Physics - Maharani’s PU College, Mysore, , 3
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At temperature above absolute zero, some electrons move from valence band to conduction band,, such a removal of one electron creates a “hole” in a semiconductors which acts as a +ve charge with, magnitude +e. This hole captures any electron in its immediate vicinity. The neighboring electron, which occupies the hole creates another hole. This process repeats and hence thermal energy creates, hole-electron pair., , Under external field, the free electrons moves towards +ve potential constitutes electron current, whereas corresponding holes moves towards –ve potential constitutes hole current. It should be note, that hole acts as a virtual charge and there is no physical charge on it. The current through, semiconductor is only due to free electrons., Intrinsic semiconductor, A pure semiconductor having equal number of free electrons and holes such as germanium or, silicon is known as intrinsic semiconductor., In an intrinsic semiconductor, even at room temperature, hole-electron pairs are created. When the, electric field is applied across an intrinsic semiconductor, the current conduction takes place by two, processes namely, by free electrons and by holes., Extrinsic semiconductor, The number of free electrons available for conduction in intrinsic semiconductor is considerably, small at room temperature hence, a pure semiconductor must be altered so as to increase its, conducting properties. This is achieved by adding a small amount of suitable impurity to a, semiconductor. It is then called extrinsic semiconductor. The process of adding impurities to a, semiconductor is called doping., A pure semiconductor which is doped by impurities to increase its conductivity is called extrinsic, semiconductor., The purpose of adding impurity is to increase either the number of electrons (or) holes in the, semiconductor crystal. Generally, for 108 atoms of semiconductor, one impurity atom is, added., The electron and hole concentration in a semiconductor at thermal equilibrium is given by, ne nh = nt2, Depending upon the type of impurity added, extrinsic semiconductors are classified into, n-type semiconductor and p-type semiconductor., , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 4
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n-type semiconductor, A pure semiconductor doped by pentavalent impurity is called n – type semiconductor., The following figure shows the band structure of n-type semiconductor., , When pentavalent impurity such as arsenic, phosphorous, antimony,…..etc. consisting 5 valence, electrons is suitably mixed with pure semiconductor such as Germanium (Ge) or Silicon (Si), four, among the five valence electrons of the pentavalent atom makes a co-valent bond with the four, valence electrons of Germanium (or) Silicon. Hence the fifth valence electron of impurity atom, finds no place in bonding process and thus it becomes a free electron. Therefore, for one impurity, atom added, one free electron will be available in pure semiconductor crystal. Though each impurity, atom provides one free electron, an extremely small amount of pentavalent impurity provides, millions of free electrons. Thermal energy of room temperature still generates a few hole-electron, pairs. However, the number of free electrons provided by pentavalent impurity is much more greater, than the number of holes. Thus, the electrons are majority charge carriers and holes are minority, charge carriers in n-type semiconductor., As the pentavalent impurity atoms donates free electrons to the semiconductor crystal, they are, also called donor atoms (or) donors., The impurities are added to molten Si (or) Ge during the process of crystal growth in a controlled, manner., The highest energy level in the C B is called fermi level and the corresponding energy is called, fermi energy., In n-type semiconductor n-stands for negative. This is because, they contains more free electrons, i.e., negative charges than holes., p-type semiconductor, A pure semiconductor doped by trivalent impurity is called p – type semiconductor., When trivalent impurity such as Boron (B), Gallium (Ga), Indium (In), ….etc, Consisting 3 valence, electrons is suitably added with a pure semiconductor [Ge (or) Si], three valence electrons of the, trivalent atom forms a co-valent bond with three valence electrons of the pure semiconductor., Hence, there is a vacancy in the forth co-valent bond. In other words there is a missing electron and, is called hole. Therefore for each trivalent atom, one hole is created. A small amount of trivalent, impurity added to the semi-conductor crystal provides large number of holes. However at room, temperature some conduction electrons creates many holes., HCL, , II PU Physics - Maharani’s PU College, Mysore, , 5
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The number holes is greater than the conduction electrons in p-type semiconductor. Thus, in p-type, semiconductor holes are majority charge carriers and electrons are minority charge carriers., p-stands for +ve. Since, trivalent impurity provides holes which can accept the neighboring electrons,, they are also called acceptors., pn-junction (or) pn-junction diode, When p-type semiconductor is suitably joined to n-type semiconductor, the contact surface is, called pn-junction., Construction: The simplest method of constructing pn-junction is crystal growth technique. In this method the Ge(or) Si, crystal is allowed to grow from the molten liquid in the presence of pentavalent impurity so as to produce n-type, material. Then, the crystal is again allowed to grow in the presence of trivalent impurity so as to produce p-type material, on the other side. In this way the pn-junction is formed., , Action of pn-junction, , Consider pn-junction with p-type semiconductor on the left side and n-type semiconductor on the, right side as shown in figure. Since the acceptor impurity atom is short of one electron, it becomes, –ve ion. Therefore, the p-type semiconductor contains minority –ve acceptor ions and majority +vely, charged holes. Since the donor impurity atom donates one electron, it becomes +ve ion. Therefore,, the n-type semiconductor contains minority +ve donor ions and majority free electrons., As p-type consists majority holes and n-type the majority electrons, there is a tendency for the free, electrons to diffuse the junction from n-side to p-side and holes to diffuse the junction from p-side to, n-side. This process is called diffusion. As the free electrons leaves the n-side, it makes the, pentavalent atom a immobile +ve charge. When this free electron leaves the n-side, it falls into a hole, on the p-side so as to make the trivalent atom a immobile –ve charge. Thus, immobile +ve charges, HCL, , II PU Physics - Maharani’s PU College, Mysore, , 6
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are accumulated at the border of n-type semiconductor and immobile –ve charges are accumulated at, the border of p-side of semiconductor as shown in figure. Thus, a net potential difference is, established across the pn-junction and is called potential barrier (or) junction barrier and the net, electric field is established from n-sede to p-side and is called junction field. The potential barrier is, in the order of 0.1 V to 0.3 V. The immobile -ve and +ve charges which are collected at the border, of junction opposes the further diffusion of electrons from n-side to p-side and holes from p-side to, n-side and thus the junction offers resistance for the flow of majority charge carriers and is called, junction resistance., The region at the junction that contains immobile +ve and –ve charges is called depletion region (or), depletion layer (or) barrier region. The thickness of this region is in the order of micrometer and it, depends on the doping concentration of the two types. Higher is the thickness of depletion region, higher is the junction resistance., The width of depletion region decreases with doping concentration., The barrier potential (Vo) depends on the doping concentration and it increase with doping, concentration., Because of immobility of charge carriers in depletion layer, it behaves like an insulator., Barrier potential is only due to diffusion of charge carriers without applying an external potential,, i.e., without connecting an external battery (or) cell across the pn-junction, V, Mobility () of a charge carrier is defined as the drift velocity per unit electric field. i.e., = d, E, In the absence of an external electric field i.e., when no battery is connected, the pn-junction diode, is said to be unbiased or zero biased. If an external field is applied across the diode, then it is said to, be biased. There are two types of biasing, forward bias, and reverse bias., Forward biasing (F B), When the +ve terminal of a battery is connected to the p-side and –ve terminal to the n-side of the, pn-junction diode, then it is said to be forward biased., (OR), When external voltage applied to the junction is in such a direction that it reduces the potential, barrier and hence junction resistance, thus permitting current to flow, then it is called forward, biasing., , When p-side is connected to +ve of the Battery and n-side to –ve terminal, the electric field is, established in such a way that it reduces the junction field and hence the potential barrier. Due to, decrease in potential barrier, the thickness of the depletion layer decreases. Hence, the pn-junction, offers no resistance for the flow of free electrons and holes., HCL, , II PU Physics - Maharani’s PU College, Mysore, , 7
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The –ve potential applied at n-side pushes the free electrons towards p-side and +ve potential at pside pushes the holes towards n-side. The free electrons arriving at p-side are collected by +ve, terminal of a battery. At the same time, equal number of electrons from –ve terminal of the battery, enter the right end of the crystal and combine with the holes which are already arrive there. These, electrons after entering n-side are again pushed towards p-side leaving a hole at n-side. Due to flow, of these free electrons, a steady current is established through a circuit., Thus, a pn-junction in forward bias mode offers low resistance and increases the conductivity., Reverse biasing (R B), When –ve terminal of the battery is connected to p-side of a diode and +ve terminal to the n-side,, then it is said to be reverse biased., OR, When the external voltage applied to the pn-junction is such a direction that potential barrier and, hence junction resistance increases, then it is called reverse biasing., , When p-side is connected to -ve of the Battery and n-side to +ve terminal, the electric field is, established in such a way that it increases the junction field and hence the potential barrier. Due to, increase in potential barrier, the thickness of the depletion layer increases. Hence, the pn-junction, offers high resistance for the flow of free electrons and holes., Thus no majority carriers cross the junction and hence current through the circuit is almost zero., However, in practice a very small current in the order of A flows through the circuit (reverse, current) which is due to minority charge carriers. (i.e., holes in n-side and electrons in p-side). If, reverse voltage is increased further, the kinetic energy of minority electrons may become high, enough to kick out the electrons from the semiconductor atoms. At this stage breakdown of the, junction occurs and junction resistance suddenly becomes zero. Hence, reverse current is suddenly, increases. But, this destroys the junction permanently., NOTE: The forward current through a pn-junction is due to majority charge carriers produced by, the impurity. However, the reverse current is due to the minority charge carriers produced due to, breaking of some covalent bonds., Circuit symbol of a junction diode:, , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 8
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The p-side is called the anode and the n-side is called the cathode. The arrow mark points from, p-side to n-side and it signifies the direction of conventional current i.e. hole current., Voltage-current (V – I) characteristics of a junction diode, In the forward mode of operation, as the forward voltage is increased from zero in small steps, the, forward current starts to increase in very small steps. But at a particular forward voltage called knee, voltage (Vk), the current (If) increase rapidly. Vk 0.3 V for Ge diode and 0.7 V for Si diode. The, reciprocal of the slope of the linear portion gives the forward resistance of the diode., , In the reverse mode of operation, the junction offers high resistance and practically no current flows, through the diode. But due to minority carriers, a small current in the order of A flows through the, diode and is called reverse current. This reverse current increases with increase in reverse voltage, and at a particular reverse voltage called breakdown voltage (V B), the reverse current increases, suddenly due to the breakdown of junction., The knee voltage (Vk) measures the potential barrier of the junction., Knee voltage: The forward voltage at which the current through the junction in forward bias, mode starts to increase rapidly is called knee voltage., Break down voltage: The reverse voltage at which the pn-junction in reverse bias mode breaks, down with sudden rise in reverse current is called breakdown voltage., An ideal pn-junction diode is one in which the forward resistance is zero in FB and the reverse, resistance is infinity in RB., Knee voltage increases with increase in doping concentration, whereas breakdown voltage, decreases with increase in doping concentration., Rectifier, A semiconductor diode is used for rectification as it conducts only in FB mode (unidirectional, conducting property., The process of converting alternating current (AC) into direct current (DC) is called rectification and, the device which converts AC into DC is called rectifier., Rectifiers are of two types, 1) Half wave rectifier and 2) Full wave rectifier, , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 9
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Half Wave Rectifier (HWR), If the current through a circuit flows only during one half cycle of the input AC, then the rectifier is, called Half Wave Rectifier., , The HWR circuit consists of a semiconductor diode and a load resistance R L connected in series as, shown in figure. The sinusoidal AC is applied across input and the out put voltage is drawn, across RL., During +ve half cycle of the input AC, the diode is in forward biased and therefore it conducts. The, out put voltage is drawn across RL. During the –ve half cycle, the diode is in RB and hence no, current flows through the diode, the output voltage is zero. Thus, diode rectifies only half cycles of, input AC., Full Wave Rectifier (FWR), If the current flows in rectifier circuit over a complete cycle of input AC, then it is called Full Wave, Rectifier., , The FWR circuit consists of two diodes D1 and D2 and load resistance RL and RL is centre tapped, with transformer. The AC to be rectify is fed across input and output is drawn across R L., During +ve half cycle of AC input, the point A is +ve and B is −ve, therefore the diode D1 is in FB, and it conducts. The output voltage is drawn across RL. D2 is RB in this stage., During –ve half cycle of AC input, B is +ve and A –ve, the diode D1 is in RB and D2 is in FB and it, conducts. The output is drawn across RL., Thus the current flow through the circuit over the complete cycle of input AC., HCL, , II PU Physics - Maharani’s PU College, Mysore, , 10
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Zener Diode (By C. Zener), A highly doped pn-junction diode which can works in the reverse breakdown voltage without, damaging itself is called a Zener diode., It is commonly used as a voltage regulator and its circuit symbol is, The breakdown voltage (or) zener voltage V z depends on the doping concentration. Since the zener, diode is heavily doped, the thickness of the depletion layer is very thin. So, even for small voltage,, the electric field across the junction will be very high. This electric field is called ionizing field. The, reverse current suddenly increases at the breakdown voltage because of high junction field (10 6 v / m ), as shown in VI characteristics., , Zener diode as a voltage Regulator, , Ri, Vi, , RL, , Vo, , A zener diode as a voltage regulator (or) voltage stabilizer is connected in reverse bias to the, unregulated input supply whose voltage varies over a wide range. The input resistance R i is used to, control the input current. The output regulated voltage is drawn across the load resistance., This voltage regulator works in such a way that, if the input voltage is less than the zener voltage, (V p V z ) then the output voltage is same as input voltage (Vo = Vi ) . But for any input voltage, higher than the zener voltage the output voltage is always equal to zener voltage. Thus the output, voltage remains constant for any input variation greater than zener voltage., , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 11
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Optoelectronic junction devices, Special type of semiconductor diodes where charge carriers are generated by photons are called, optoelectronic junction devices., i), Photovoltaic cell/Solar cell, ii), Light Emitting diode, iii), Photodiode, Photovoltaic Cell (or) Solar Cell, , A solar cell is a specially doped PN junction diode used to convert Solar energy into electrical, energy at zero bias mode. Solar cell is fabricated by using alloyed semiconductors having energy, gap around 1.5ev in order to create high junction field. A transparent window is provided in a diode, to receive sun light. When sunlight of energy greater than the energy gap is made to fall on a diode, through a transparent window, electron-hole pairs are created near the junction. Due to high junction, field, these electrons and holes get separated and move apart before they recombine. As a result, electrons are collected at n-side edge and holes at p-side edge. Thus, an emf is created between the, terminals of a diode and it can be drawn by using suitable external device., Uses of solar cells: They are used in street lights, in solar heater, in power supply of satellites, space, vehicles and in calculators., Light emitting diode (LED), , Circuit symbol, , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 12
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A specially and heavily doped pn – junction diode which emits spontaneous visible light in forward, bias mode is called LED., The colour of radiation depend on the energy gap of the semi conducting materials used., Similar to the normal diode, LED consists of p-type and n-type materials. The free electrons in, n-region are lie in conduction band (higher energy) and holes in p region are lie in valence band, (lower energy). When the diode is forward biased, electrons which are majority carriers at n-side are, drifted towards p-side where they are minority carriers. Similarly, holes which are majority carriers, at p-side are drifted towards n-side where they are minority carriers. Near the depletion boundary at, both sides, excess minority carriers recombine with majority carriers, i.e., electrons with holes at, p-side and holes with electrons at n-side. During the recombination of free electrons which are in, high energy CB with holes which are in low energy VB, energy equivalent to energy gap is released., In a normal diode, this energy is released as heat or as far infrared invisible light. But in LED’s this, energy is released in the form of radiation (photons). The wavelength of emitted radiation and, colour depends on the energy gap of the materials forming p-n junction. The energy gap of the, materials used for LED is such that, they emit near infrared and visible light. LED’s having energy, gap ranging from 1.8 eV to 2.8 eV emit visible light. As energy gap (Eg) of Ge and Si (which are, used in normal diode) is less than 1.8 eV, they cannot be used in LED’s. GAAs and GaP having, energy gap greater than 1.8 eV are used in LED’s., Applications of LED’s They are used, 1) In the manufacture of signal lamps, display devices and calculators., 2) As ON / OFF lights on professional and consumer equipments., 3) In remote controls for TVs and DVDs etc., 4) In the computer mouse as movement sensors., 5) In optical fibre communication (OFC)., The advantages of LEDs over incandescent lamps –, (i) LEDs operate at low voltages and consume less power., (ii) LEDs have long life, are rugged and have fast switching (on-off) capability., Photo diode (photodetector):, , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 13
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A photodiode is a specially doped PN junction diode used in reverse bias mode to measure the, intensity of light., A photodiode is fabricated with a transparent window such that a light of energy greater than the, energy gap of the semiconductor is allowed to fall on the diode. Due to absorption of this light, energy, electron-hole pairs are created within the depletion region. Due to electric field of the, junction, electron and holes get separated before they recombine. As the diode is reverse biased, the, negative potential at p-side attracts holes and positive potential at n-side attracts electrons. Now these, electrons flows through the circuit constitutes an electric current (photocurrent). The magnitude of, photo current is proportional and very sensitive to intensity of incident light. Thus, the strength of, photocurrent is a measure of intensity of light., Applications Photo diodes are used, 1), for defection of visible and invisible light, 2), to measure the intensity of radiation, 3), in camera light meters, 4), in switching circuits, 5), as PIN [Positive-intrinsic-Negative diodes] diodes in optical communication equipments., , DIGITAL ELECTRONICS AND Logic gates, A logic gate is an electronic circuit contains pn-junction diode, transistor etc, which makes logic, decisions by relating input and output signals., These gates act as switching ON (or) OFF the circuit or close (OR) open the circuit. These close, and open state of a gate may be referred as high (or) Low, True (or) false, yes (or) no, etc.,, Mathematically, the high (or) low states of a sate are expressed as 1 and 0 respectively., Boolean Algebra [By Boole], The mathematical analysis of logic operators is done by Boolean algebra which uses only two digits, 0 and 1, hence it is also called Binary algebra., Boolean algebra uses letters and symbols to represent statements and their logical connections., For example, in the logical equation A + B = C, each of three variables A, B and C can take either 0, or 1 value., Advantages: The main advantages of Boolean algebra are of simplicity, speed and accuracy., Moreover, it provides an economical and straight forward way of describing computer circuitry and, complicated switching circuits., Fundamental logic gates and circuit symbols, There are three fundamental logic gates,, 1) OR gate, 2) AND gate 3) NOT gate, , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 14
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OR gate, It has two (or) more inputs and a single output. Its operation follows OR Laws., OR Law: The output of OR gate is logic 1. When one or more inputs are logic 1 state., The simplest OR gate consists of two input signals A and B and one output signal Y., Equivalent switching circuit:, A, , B, , B, , It is seen that the bulb will glow if A is closed (or) B is closed (or) when both are closed. The bulb, will not glow when both A & B are opened., Y=A+B, , Boolean Equation:, , Truth table:, Truth table of a logic gate is a table which gives the logic output for all possible input combinations., Truth table for or gate:, Input, A, 0, 0, 1, 1, , B, 0, 1, 0, 1, , Out put, Y=A+B, 0, 1, 1, 1, , AND Gate, It has two (or) more inputs and a single output. Its operation follows AND law., AND Law: The output of an AND gate is logic ‘1’ state only when all the inputs are logic ‘1’ state., The simplest AND gate consists of two inputs and one output., , HCL, , II PU Physics - Maharani’s PU College, Mysore, , 15
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Equivalent Switching Circuit:, , It is seen that the bulb will glow only when both A and B are switched ON., Y = A• B, , Boolean Equation:, Truth Table for AND gate:, , Input, A, B, 0, 0, 0, 1, 1, 0, 1, 1, , Out put, Y=A•B, 0, 0, 0, 1, , NOT Gate: [Complementary or Inverter gate], It has single input and single output and its operation follows NOT laws., NOT Law: The output of NOT gate is logic 1 state if the input is at logic 0 state and vice-versa., Equivalent Switching Circuit:, , Clearly, the bulb will be ‘ON’ only if switch A is ‘OFF’ and Vice-Versa., , Boolean Equation:, , Y=A, , Truth Table for NOT gate:, Input, A, 0, 1, , HCL, , Output, Y=A, 1, 0, , II PU Physics - Maharani’s PU College, Mysore, , 16
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Combination of Fundamental gates:, The fundamental gates may be combined to form various important combination of gates. Two such, gates are 1) NOR gate 2) NAND gate., NOR gate, A NOT gate following OR gate is called a NOR gate., Boolean equation:, , Y=A+B, , Truth Table for NOR gate:, , Input, A, 0, 0, 1, 1, , Out put, Y=A+B, 1, 0, 0, 0, , B, 0, 1, 0, 1, , NAND gate, A NOT gate following AND gate is called NAND gate., Boolean Equation:, , Y = A•B, , Truth table for NAND gate:, Input, , Out put, , A, , B, , 0, 0, 1, 1, , 0, 1, 0, 1, , Y = A•B, 1, 1, 1, 0, , Applications of Logic Gates:, 1) OR gate and AND gate are used in alarm system., 2) A logic circuit can be built for any Boolean expression, For example: Y = (A + B) . C, , 3) NAND gates and NOR gates are also be used to perform all the logic operations of OR, AND, & NOT gate. [Hence, NAND & NOR gates are called universal gates], 4) Logic gates are used to construct simple arithmetic circuits such as adders, subtractors,, multiplier, divider, etc.,, , ** ** ** ** **, HCL, , II PU Physics - Maharani’s PU College, Mysore, , 17
