Kerala Plus Two Physics Notes Chapter 14 Semiconductor Electronics: Materials, Devices and Simple Circuits<\/h2>\n
Introduction<\/span> 1. A Comparison of Vacuum Tubes and Transistors:<\/p>\n Classification Of Metals, Conductors, And Semiconductors<\/span> (i) Metals: (ii) Semiconductor: (iii) Insulators: They have high resistivity (or low conductivity). 2. Band Theory: Conduction Band, Valence Band, and Energy Gap: These energy levels are very closely spaced Hence it appears as continuous variation of energy. This collection of energy levels are called energy band.<\/p>\n The energy band which includes energy levels of valence electrons is called valence band. The energy band above valence band which includes energy levels of conduction electrons is called conduction band.<\/p>\n The gap between the top of valence band and bottom of conduction band is called energy band gap (Energy gap, Eg<\/sub>). 3. Classification on the basis of Energy bands Conductors: OR<\/p>\n Conduction band and valence band are overlapped so that Eg<\/sub> = 0ev Insulators: Semiconductors: Intrinsic Semiconductor<\/span> For intrinsic semiconductor: Explanation: * The total current in intrinsic semi conductor is the sum of free electron current Ie<\/sub> and hole current Ih<\/sub>. Explanation: Extrinsic Semiconductor<\/span> 2. Doping and Dopants: 3. n-type semiconductor: At room temperature this electron become free to move. Thus each pentavalent atom donate one extra electron for conduction and hence it is called donor impurity. 4. p-type semiconductor: The neighboring Si atom needs an electron to fill the vacancy and hence one electron in outer orbit of nearby Si atom move to this vacancy leaving a hole in its own site. Thus hole can move through the lattice.<\/p>\n Each trivalent atom creates a hole and it act as acceptor. Hence it is called acceptor impurity. The semiconductor doped with trivalent impurity has more number of holes than free electrons. Here holes are the majority carriers and electrons are the minority carriers. Hence it is called p-type semiconductor. (II) In thermal equilibrium electron and hole concentration in a semiconductor is given by ne<\/sub>nh<\/sub> = n2<\/sup>r<\/sub>.<\/p>\n 5. Energy band structure of Extrinsic semiconductors P-type semiconductor: p-n Junction<\/span> 1. p-n junction formation: (i) Diffusion: (ii) Drifting – Formation of Depletion region: Similarly when holes diffuse from p to n, it leaves behind negatively charged immobile ions on p side. As holes continue to diffuse from p to n, negative space charge region is developed on p side.<\/p>\n The positive space-charge region on n-side and negative space-charge region on p-side, is known as depletion region. This region contain only immobile ions.<\/p>\n 2. Barrier Potential: Semiconductor Diodes<\/span> 1. p-n junction diode under forward bias: similarly the holes of p-side get repelled by positive terminal of battery and cross depletion region, reach n-side. The total forward current is sum of hole current and current due to electron.<\/p>\n 2. p-n junction diode under reverse bias: Breakdown Voltage (VBr<\/sub>): V-I characteristics: To measure reverse current microammeter is used. A voltmeter is connected across diode to measure voltage. The current is measured for different values of volt and a graph (V-I) is plotted. In reverse bias, current is very small. It remains almost constant up to break down voltage (called reverse saturation current). Afterthis voltage reverse current increases sharply. Application Of Junction Diode<\/span> 1. Diode as half wave rectifier: Working: 2. Full wave rectifier: Working: During the negative half cycle of the a.c signal at secondary, the diode D1<\/sub> is reverse biased and D2<\/sub> is forward biased. So that current flows through D2<\/sub> and RL<\/sub>.<\/p>\n Thus during both the half-cycles, the current flows through RL<\/sub> in the same direction. Thus we get a +ve voltage across RL<\/sub> for +ve and -ve input. This process is called full-wave rectification.<\/p>\n Special Purpose p-n Junction Diodes<\/span> Explanation for large reverse current: 1. (a) Zener diode as avoltage regulator Principle: The Zener diode is connected to a fluctuating voltage supply through a resistor Rz<\/sub>. The output is taken across RL<\/sub>.<\/p>\n Working: Thus the voltage across Rz<\/sub> increases, by keeping the voltage drop across zenerdiode as a constant value. (This voltage drop across Rz<\/sub> is proportional to the input voltage).<\/p>\n 2. Optoelectronic junction devices: The direction of the electric field is such that electrons reach n-side and holes reach p-side. Electrons collected on n-side and holes collected on p- side produce an emf. When an external load is connected, the current flows through the load. (ii) Light-emitting diode [LED]: Working: Uses: Advantages of LED over conventional incandescent lamps:<\/p>\n 3. Solar cell: Working: Junction Transistor<\/span> 1. Emitter: 2. Collector: 3. Base: Transistor action: Working: At the base, electron-hole combination takes place. As the base is lightly doped and very thin, only a few electrons combine with holes to constitute the base current, IB<\/sub>.<\/p>\n The remaining electrons are attracted towards collector because the collector is kept at reverse bias. Due to this electron flow, a collector current IC<\/sub> is produced.<\/p>\n In this way, the emitter current is divided into base current and collector current. 2. Basic transistor circuit configurations and transistor characteristics: a. Common base configuration: b. Common emitter configuration: c. Common collector configuration: Relation between \u03b1 and \u03b2: Output Characteristics (CE. Configuration): Line OA is called saturation line .The region right of the saturation line is the active region. Transistor is operated as amplifier in this region. The region below IB<\/sub> = 0 is the cut off region.<\/p>\n The output resistance is the ratio of a small change in collector voltage to the change in collector current at constant base current. 3. Transistor as a device The variation of output voltage with input voltage: a. Cut off region: b. Active region: c. saturation region: Working of transistor as switch: If Vi<\/sub> is high enough to drive the transistor into saturation, then Vo<\/sub> is low (very near to zero). In this stage the transistor driven into saturation it is said to be switched on. 4. Transistoras an Amplifier (CE-Configuration): RB<\/sub> is the resistor connected to base in order to reduce the base current. Rc<\/sub> is the resistor which is connected in between Vcc<\/sub> and collector terminal. We take the voltage across Rc<\/sub> and Vcc<\/sub> with the help of a capacitor C. We maintain voltages VBB<\/sub> and Vcc<\/sub> such that the transistor is always on the active region.<\/p>\n Working: Case 2:
\nBefore the discovery of transistor, vacuum tube or valves were considered as the building blocks of electronic circuit.<\/p>\n\n\n
\n Vacuum Tubes\/valves<\/td>\n Transistors<\/td>\n<\/tr>\n \n 1. External heating is required. (Electrons are supplied by heated cathode)<\/td>\n No external heating is required.<\/td>\n<\/tr>\n \n 2. Large evacuated space (vacuum) is required be\u00ad tween cathode and anode<\/td>\n Evacuated space is not required<\/td>\n<\/tr>\n \n 3. The electrons from heated cathode flows through vacuum.<\/td>\n The charge carriers flows within solid itself.<\/td>\n<\/tr>\n \n 4. Bulky (large in size)<\/td>\n Small in size<\/td>\n<\/tr>\n \n 5. Consume high power<\/td>\n Low power consumption<\/td>\n<\/tr>\n \n 6. Operate at high voltage<\/td>\n Operate at low voltage<\/td>\n<\/tr>\n \n 7. Limited life arid low reliability.<\/td>\n Long life and high reliability.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n
\n1. On the basis of conductivity:
\nOn the basis of relative values of electrical conductivity (\u03c3) and resistivity \u03c1 = \\(\\frac{1}{\\sigma}\\) solids are classified as<\/p>\n
\nThey possess very low resistivity (or high conductivity).
\n\u03c1 \u2192 10-2<\/sup> – 10-8<\/sup> \u2126 m
\n\u03c3 \u2192 102<\/sup> – 108<\/sup> S m-1<\/sup><\/p>\n
\nThey have resistivity or conductivity intermediate to metals and insulators.
\n\u03c1 \u2192 10-5<\/sup> – 106<\/sup> \u2126 m
\n\u03c3 \u2192 105<\/sup> – 10-6<\/sup> S m-1<\/sup><\/p>\n
\n\u03c1 \u2192 1011<\/sup> – 1019<\/sup> \u2126 m
\n\u03c3 \u2192 10-11<\/sup> – 10-19<\/sup> S m-1<\/sup><\/p>\n
\nIn an isolated atom, electrons will have definite energy level. When atoms combine to form solid, the energy levels of outer electrons overlap. Hence outer energy levels split in to many energy levels.<\/p>\n
\nEnergy level diagram of different bands:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-1.jpg\")
\nThe band gap energy of Ge and Si are 0.3ev and 0.7ev respectively.<\/p>\n
\nConduction band is partially filled and valence band is partially empty.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-2.jpg\")
\nDue to overlapping, electrons are partially filled in conduction band. These partially filled electrons are responsible for current conduction.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-3.jpg\")
\nConduction band is empty. Valence band may fully or partially filled. There is a wide energy gap between valence band and conduction band (Eg<\/sub> > 3ev).<\/p>\n
\nConduction band may be empty or lightly filled. Valence band is fully filled. The energy gap is very small (< 3ev)
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-4.jpg\")
\nAt room temperature some electrons in valence band get enough energy to cross the energy gap and move into conduction. Hence semiconductors show intermediate conductivity.<\/p>\n
\nA semiconductor in its pure form is called intrinsic semiconductor.<\/p>\n
\n* The number of free electrons is equal to number of holes.
\nie. ne<\/sub> = nh<\/sub> = ni<\/sub>
\nne<\/sub>, nh<\/sub> and ni<\/sub> are the free electron concentration, hole concentration and intrinsic carrier concentration.<\/p>\n
\nEach Si atom is covalently bonded to nearest four neighboring atoms. When temperature increase, some of covalent bond brakes and electrons become free leaving a vacancy (hole). Thus each free electron creates hole in the lattice. Hence number of free electrons equals number of holes.<\/p>\n
\nI = Ie<\/sub> + Ih<\/sub><\/p>\n
\nWhen an electric field is applied, free electrons move towards positive potential and give rise to electron current, le. The holes move towards negative potential and give rise to hole current. Thus total current is contributed by both free electrons and holes.<\/p>\n
\n1. Extrinsic semiconductor or impurity semiconductor:
\nThe addition of suitable impurity improves the conductivity of intrinsic semiconductors. Such semiconductors are called extrinsic semiconductor. They are of two types n-type and p-type semiconductors.<\/p>\n
\nThe deliberate addition of suitable impurity to semiconductors to improve its conductivity is called doping.
\nThe impurity atoms are called dopants. There are two types of dopants;<\/p>\n\n
\nWhen a pentavalent impurity is added to Si crystal, four electrons of impurity atom make bond with neighboring four Si atoms. The fifth electron remains weakly bound to its parent atom.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-5.jpg\")
\nThus in doped semiconductor the number of conduction electrons will be large compared to number of holes. Hence electrons are the majority carriers and holes the minority carriers. Hence semiconductors doped with pentavalent impurity is called n-type semiconductor.
\nNote:
\nIn n-type semiconductors
\nne<\/sub> >> nh<\/sub>
\nBut as a whole n-type semiconductor is neutral (ie. electrons is equal and opposite to ionized (donor) core in lattice).<\/p>\n
\nWhen a trivalent impurity is added to Si crystal, three electrons of impurity atom make covalent bond with neighboring three Si atom. The fourth bond with neighboring Si atom lacks one electron. Thus a vacancy or a hole is created in fourth bond.<\/p>\n
\nNote: I
\n(I) In p-type semi conductor
\nnh<\/sub> >> ne<\/sub>
\nBut as a whole p-type semiconductor is electrically neutral. (The charge of additional holes is equal and opposite to acceptor ions).<\/p>\n
\nn-type semiconductor:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-6.jpg\")
\nIn n-type semiconductors, the donor energy level (ED<\/sub>) is slightly below conduction band.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-7.jpg\")
\nIn p-type semiconductors, the acceptor energy level (EA<\/sub>) lies slightly above valence bond.<\/p>\n
\nA p-n junction is basic building block of semiconductor devices like diode, transistor, etc.<\/p>\n
\nWhen pentavalent impurity is added to a part of p-type Si semiconductor wafer, we get both p region and n region in a single wafer.
\nThe formation of p-n junction includes two processes.<\/p>\n
\nIn n type semiconductor, concentration of electrons is more than that of holes. In p-region, the hole concentration is more than electron concentration. Because of this concentration gradient, electrons diffuse from n side to p-side and holes diffuse from p-side to n-side during the formation of p-n junction. This produces diffusion current.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-8.jpg\")
\nWhen electrons diffuses from n to p, it leaves behind positively charged immobile donor ions on n-side. As electrons continue to diffuse from n to p, a layer of positive charge is developed on n- side.<\/p>\n
\nThe n-region losses electrons and p-region gains electrons. Because of this a potential is developed across the junction. This potential is called barrier potential.<\/p>\n
\nA semiconductor diode is a p-n junction provided with metallic contact at both ends to apply external voltage.
\nThe symbol of p-n junction diode is given below.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-9.jpg\")
\nThe arrow shows conventional direction of current.<\/p>\n
\nWhen p-side of p-n junction diode is connected to positive terminal of the battery and n-side to the negative terminal it is said to be in forward biased.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-10.jpg\")
\nDue to the applied voltage, electrons of n-side get repelled by negative terminal of battery. Hence they cross depletion region and reach at p-side.<\/p>\n
\nWhen p-side of p-n junction diode is connected to negative terminal of battery and n-side to the positive terminal, it is said to be in reversed biased.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-11.jpg\")
\nIn reverse bias the electrons of n-side and holes on p-side cannot cross the junction. But the minority carriers – holes on n-side and electrons on p-side drift across the junction and produce current. The reverse current is of the order micro ampere.
\nNote: Junction width increases in reverse bias.<\/p>\n
\nThe reverse current remains independent of bias voltage up to a critical reverse bias voltage called reverse break down voltage. At breakdown voltage, reverse current increases sharply.<\/p>\n
\nTo study variation of current with voltage for p-n junction diode, it is connected to a battery through a rheostat. Rheostat is used to vary the biasing voltage. A milliammeter is connected in series with diode to study forward current.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-12.jpg\")
\nIn forward bias, current first increases very slowly up to a certain value of bias voltage. After this voltage, diode current increases rapidly. This voltage is called Knee voltage or cut-in voltage or threshold voltage. (0.2v for Ge and 0.7v for Si). The diode offers low resistance in forward bias.<\/p>\n
\nNote:
\n(i) In forward bias, resistance is low compared to reverse bias.
\n(ii) The dynamic resistance of diode is defined as ratio of change in voltage to change in current.
\nrd<\/sub> = \\(\\frac{\\Delta v}{\\Delta l}\\)<\/p>\n
\nDiode as a rectifier:
\nThe process of converting AC into DC is known as Rectification. A p-n junction diode conducts current when it is forward biased, and does not conduct when it is reverse biased. This feature of the junction diode enables it to be used as rectifier.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-13.jpg\")
\nCircuit details:
\nA half wave rectifier consists of transformer, a diode and a load resistor RL<\/sub>. The primary coil of transformer is connected to a.c input and secondary is connected to RL<\/sub> through diode.<\/p>\n
\nDuring the positive half cycle of the input a.c at secondary, the diode is forward biased and hence it conducts through RL<\/sub>. During negative half cycle of a.c at secondary, diode is reverse biased and does not conduct. Thus, we get +ve half cycle at the output. Hence the a.c input is converted into d.c output.<\/p>\n
\nCircuit details:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-14.jpg\")
\nFull wave rectifier consists of transformer, two diodes, and a load resistance RL<\/sub>. Input a.c signal is applied across the primary of the transformer. Secondary of the transformer is connected to D1<\/sub> and D2<\/sub>. The output is taken across RL<\/sub>.<\/p>\n
\nDuring the +ve half cycle of the a.c signal at secondary, the diode D1<\/sub> is forward biased and D2<\/sub> is reverse biased. So that current flows through D1<\/sub> and RL<\/sub>.<\/p>\n
\n1. Zener diode:
\nZener diode is designed to operate under reverse bias in the breakdown region. It is used as a voltage regulator. The symbol for Zener diode is shown in figure.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-15.jpg\")
\nZener diode is heavily dopped. Hence depletion region is very thin.
\nI-V characteristics of zener diode:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-16.jpg\")
\nThe l-V characteristics of a Zener diode is shown in above figure. At break down voltage, current increases rapidly. After breakdown, zener voltage remains constant. This property of the Zenerdiode is used for regulating supply voltages.<\/p>\n
\nReverse current is due to the flow of electrons (minority carriers) from p to n and holes from n to p. When the reverse bias voltage increases and becomes V = V2<\/sub> high electric field is developed. This high electric field can pull valence electrons from the atoms. These electrons account for high current.<\/p>\n
\nIn reverse breakdown region, the voltage across the diode remains constant.
\nCircuit details:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-17.jpg\")
<\/p>\n
\nWhenever the supply voltage increases beyond the breakdown voltage, the current through zener increases (and also through Rz<\/sub>).<\/p>\n
\n(i) Photodiode:
\nThe photodiode can be used as a photodetector to detect optical signals.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-18.jpg\")
\nIt is operated under reverse bias. When the photodiode is illuminated with light (photons) electron-hole pairs are generated. Due to electric field of the junction, electrons and holes are separated before they recombine.<\/p>\n
\nThe I-V characteristics of a photodiode:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-19.jpg\")
<\/p>\n
\nLED is heavily doped pn junction diode working under forward bias .Gallium Arsenide is used for making infrared LEDs.<\/p>\n
\nWhen the junction diode is forward biased, electrons and holes flow in opposite directions across junction. Some of the electrons and holes combine at junction and energy is produced in the form of light.<\/p>\n
\nLEDs are used in remote controls, burglar alarm systems, optical communication, etc.<\/p>\n\n
\nSolar cell is junction diode used to convert solar energy into electrical energy.
\nCircuit details:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-20.jpg\")
\nIts p-region is thin and transparent and is called emitter. The n-region is thick and is called base. Output is taken across RL<\/sub>.<\/p>\n
\nWhen light falls on this layer, electrons from the n-region cross to the p-region and holes in the p-region cross in to the n-region. Thus a voltage is developed across RL<\/sub>. Solar cells are used to charge storage batteries during daytime.
\nThe I-V characteristics of a solar cell:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-21.jpg\")
\nThe I-V characteristics of solar cell is drawn in the fourth quadrant of the coordinate axes. This is because a solar cell does not draw current but supplies the same to the load.<\/p>\n
\n1. Transistor: structure and action
\nTransistor is a three-layered doped semiconductor device. There are two types of transistors:<\/p>\n\n
![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-22.jpg\")
\nSymbols:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-23.jpg\")
\nNaming the transistor terminals:
\nA transistor has 3 terminals:<\/p>\n\n
\nThe section, which supplies charge carriers, is called emitter. Emitter is heavily doped. The emitter should be forward biased.<\/p>\n
\nThe section which collects the charge carriers, is called collector. Collector is moderately doped. The collector should be reverse biased.<\/p>\n
\nMiddle section between emitter and ‘ collector is called base. Base is lightly doped.<\/p>\n
\nCircuit details:
\nEmitter is maintained at forward bias and collector is maintained at reverse bias. VEB<\/sub> is the emitter base voltage and VCB<\/sub> is the collector base voltage.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-24.jpg\")
<\/p>\n
\nEmitter is kept at forward bias so that the electrons are ejected into base. Thus an emitter current IE<\/sub> is produced.<\/p>\n
\nMathematically this can be written as
\nIE<\/sub> = IB<\/sub> + IC<\/sub>
\nIB <\/sub>> is small, so IE<\/sub> = IC<\/sub><\/p>\n
\nTransistor can be used in three modes:<\/p>\n\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-25.jpg\")
\nIn Common base configuration .base is common to both input and output
\nCurrent amplification = \\(\\frac{\\text { output current }}{\\text { input current }}\\)
\nCurrent amplification, \u03b1 = \\(\\frac{l_{C}}{l_{E}}\\)<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-26.jpg\")
\nCurrent amplification = \\(\\frac{\\text { output current }}{\\text { input current }}\\)
\nCurrent amplification, \u03b2 = \\(\\frac{l_{C}}{l_{B}}\\)<\/p>\n
\nCurrent amplification \u03b3 = \\(\\frac{l_{E}}{l_{B}}\\)<\/p>\n
\ni.e. \u03b2 = \\(\\frac{\\alpha}{I-\\alpha}\\)
\nCommon Emitter Configuration:
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-27.jpg\")
\nInput Characteristics (CE configuration):
\nThe graph connecting base current with base-emitter voltage (at constant VCE<\/sub>) is the input characteristics of the transistor.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-28.jpg\")
\nTo study the input characteristics, the collector to emitter voltage (VCE<\/sub>) is kept at constant. The base current IB<\/sub> against VBE<\/sub> is plotted in a graph.
\nThe ratio \u2206 VBE<\/sub>\/\u2206 IB<\/sub> at constant VCE is called the input resistance.
\ni.e,,Input resistance \\(r_{1}=\\frac{\\Delta V_{B E}}{\\Delta I_{B}}\\)<\/p>\n
\nThe output characteristics is a graph connecting the collector current lc with collector-emitter voltage (VCE<\/sub>) at a constant base current (IB<\/sub>).
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-29.jpg\")
\nThis is obtained by measuring the collector current IB<\/sub> at different collector voltage by keeping the base current fixed.<\/p>\n
\nOutput resistance \\(\\mathrm{r}_{0}=\\frac{\\Delta \\mathrm{V}_{\\mathrm{CE}}}{\\Delta \\mathrm{I}_{\\mathrm{C}}}\\)<\/p>\n
\nTransistor as a switch:
\nA circuit diagram for transistor switch is given below.
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-30.jpg\")
\nApplying Kirchoff’s voltage rule to the input side of this circuit, we get
\nVBB<\/sub> = IB<\/sub>RB<\/sub> + VBE<\/sub>
\nand applying Kirchoff’s voltage rule to the output side of this circuit, we get
\nVCE<\/sub> = VCC<\/sub> – IC<\/sub>RC<\/sub>.
\nWe shall treat VBB<\/sub> as the dc input voltage Vi<\/sub> and VCE<\/sub> as the dc output voltage Vo<\/sub>.
\nSo, we have
\nVi<\/sub> = IB<\/sub>RB<\/sub> + VBE<\/sub> ____(1) and
\nVo<\/sub> = VCC<\/sub> – IC<\/sub>Ri<\/sub>______(2)<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-31.jpg\")
\nThe variation of output voltage with input voltage is shown in the above graph. This graph contain three regions<\/p>\n\n
\nIn the case of Si transistor, if input voltage Vi<\/sub> is less than 0.6V, the transistor will be in cut off state and out put current (Ic<\/sub>) will be zero.
\nSubstituting Ic<\/sub> = 0 in the eq (2) we get out put voltage Vo<\/sub> = VCC<\/sub><\/p>\n
\nWhen Vi<\/sub> becomes greater than 0.6 V the transistor is in active state with some current Ic<\/sub>. The eq(2) shows that, the output Vo<\/sub> decrease as the term Ic<\/sub>Rc<\/sub> increases. With increase of Vi<\/sub>, Ic<\/sub> increases almost linearly and so Vo<\/sub> decreases linearly till its value becomes less than about 1.0 V.
\nNote:
\nAmplifier is working in the active region.<\/p>\n
\nWhen the output voltage becomes 1.0V, the change becomes non linear and transistor goes into saturation state. With further increase in Vi<\/sub> the output voltage is found to decrease towards zero (though it may never become zero).<\/p>\n
\nWhen Vi<\/sub> is low (unable to give forward-bias to the transistor) we get high output (ie. Vo<\/sub> = Vcc<\/sub>). In this stage the transistor doesn\u2019t conduct. Hence transistor is said to be switched off.<\/p>\n
\nNote:
\nThe switching circuits are designed in such a way that the transistor does not remain in active state.<\/p>\n
\n![\"Plus](\"https:\/\/www.aplustopper.com\/wp-content\/uploads\/2019\/02\/Plus-Two-Physics-Notes-Chapter-14-Semiconductor-Electronics-Materials-Devices-and-Simple-Circuits-32.jpg\")
\nThe working of an amplifier can be explained using the circuit given above. It is an n-p-n transistor connected in common emitter configuration. VBB<\/sub> is the biasing voltage used in the input side and Vcc<\/sub> is the reverse bias voltage used in the output side.<\/p>\n
\nCase 1:
\nWhen there is no input signal (ie. Vi<\/sub> = 0,)
\nThe input voltage can be written as
\nVBB<\/sub> = VBE<\/sub> + IB<\/sub>RB<\/sub>
\nThis base voltage produces a base current IB<\/sub> which in turn produces a dc collector current IC<\/sub>. The output voltage can be written as
\nVCE<\/sub> = VCC<\/sub> – IC<\/sub>RC<\/sub>
\nThis dc output voltage is unable to produce an output signal due to the presence of a capacitor. Because, the capacitor prevents the flow of dc current through it.<\/p>\n
\nWhen there is an input ac signal, (ie. Vi<\/sub> \u2260 0):
\nwhen we apply an AC signal as input, we get an AC base current denoted by iB<\/sub>. Hence input AC voltage can be written as
\nVi<\/sub> = iB<\/sub>r ______(1)
\nwhere \u2018r\u2019 is the effective input resistance.
\nThis AC input current produces an AC output current (ic<\/sub>) which can flow through a capacitor. Hence the output voltage can be written as