semiconductors devices notes, Study notes of Physics

Download semiconductors devices notes and more Study notes Physics in PDF only on Docsity!

16. Semiconductor Devices 12th - Physics 12th Science Syllabus 1, Introduction 2. p-n Junction Diode as a Rectifier 2.1 Half Wave Rectifier 2.2 Full Wave Rectifier 2.3 Ripple Factor 2.4 Filter circuits 3. Special Purpose Junction Diodes 3.1 Zener Diode 3.2 Photo Diode 3.3 Solar Cell or Photovoltaic Cell 4. Bipolar Junction Transistor (BJT) 4.1 The Common Emitter (CE) Configuration 4.2 The Common Emitter (CE) characteristic 4.3 Transistor as an Amplifier 5. Logic Gates 5.1 NOT Gate 5.2 OR Gate 5.3 AND Gate 5.4 NAND Gate 5.5 NOR Gate 5.6 Exclusive OR/ X-OR Gate Theory Notes Q. How diode p-n Junction ] Ans. ; diode is used as a rectifies. | We know that when the diode i closed switch and current flows 1 is reverse biased, it behaves as an in the diode circuit. This switching used as a rectifier. i it behaves as a is forward biased, i a n the diode circuit. When the diode ch and no current flows en swit 1 oP {a diode allows it to be action 0 TT Define the terms ~ 1) Rectification g i i 1) Rectification 2) Rectifier The conversion of AC voltage into DC voltage is called as meena, 2) Rectifier AC voltage is called rectifier. An electronic circuit which reetiies A device which converts bidirectional voltage or current into a unidi- rectional voltage or current. Q. Draw the Block diagram Ans, % of. simple rectifier circuit . transformer + with respect to output wave AC mains diode fil voliage | yy forms. ar oe rectifier Mer regulator| =! 7 : AS Ap ws = =, (a) (b) (©) (d) Fig. Block diagram simple rectifier circuit with respective output wave form. Describe the wave forms | Q. Describe the wave forms | Ans. of simple rectifier circuit with | In the simple rectifier circuit the description of wave forms is as the help of block diagram. follows. 1) The primary of the transformer is connected to the AC mains supply and secondary of transformer is connected to a rectifier circuit. The AC voltage shown as a sinusoidal wave (a) from the secondary of the transformer is converted into a DC voltage by a diode rectifier, which is shown as pulsating wave(b) in above block diagram. 3) The output of the rectifier contains some AC component. This AC component is the DC output of a rectifier is called ripple and is shown at the output of rectifier (wavefrom b). 4) The ripple is removed by using filter circuit and the output of filter circuit is almost a pure DC (Waveform c} (It can still contains | some ripple). 5) The voltage regulator circuit is connected after filter circuit | restricts the output voltage to the desired value (wavefrom d) | and output voltage at this stage is a pure DC. 2 Q. Draw the neat labeled | Ans. p-n junction circuit diagram of a half A diode (D) wave rectifier and Explain ¢ He | E- B its working . What is the frequency of ripple in its | Fig. Circuit diagram of a half wave rectifier output? | Q. What are the advantages of full wave rectifier? Q. Write a short note on ripple factor. 1) A simple full wave rectifier circuit using two diode is shown in above fig. : 2) The A.C. voltage source is connected to the primary Winding of the transformer. 3) The terminals of secondary winding of transformer are connecteg to the positive terminals (Anodes) of junction diodes D, ang D;, The negative terminals (cathodes) of the diodes are connecteg to one common point. A load resistance Rk, is connected across common point and center tap of the transformer. ; 4) During positive half cycle of A.C. input, the voltage at point 4 is at higher potential with respect to that of point B. therefore the diode D, is forward bias. It conducts (works as a closed Switch so that the current flows through the circuit along path APQRC as shown in fig. . 5) During negative half cycle of A.C. input, the voltage at Point B ig at higher potential with respect to that of point A. Therefore the diode D, is forward bias. It conducts (works as a closed switch so that the current flows through the circuit along path BPQRG as shown in fig. at 6) Thus current flows through the load resistance R, is in the same direction during both the cycles and the DC output voltage obtained across the R, in the form of continuous pulses, 7) The wave form for input and output voltages are shown in fig. The first wave from shows the input AC, the second wave form shows the output of the diode D, conducts, the third wave form shows the output of diode D: conducts and the fourth wave form shows the total output wave form of the full wave rectifier. (The maximum efficiency of full wave rectifier is twice that of the half wave rectifier.) Ans. 1) Rectification takes place in both the cycles of the AC input. 2) Efficiency of a full wave rectifier is higher than that of a half wave rectifier. 3) The ripple in a full wave rectifier is less than that in a half wave rectifier. Ans. 1) The output of a rectifier consists of a small fraction of an AC component along with DC called the ripple. 2) This ripple is undesirable and is responsible for the fluctuations in the rectifier output. 3) Fig shows the ripple in the output of a rectifier, 4) The effectiveness of a rectifier depends upon the magnitude of the ripple component in its output. 5) A smaller ripple means that the rectifier is more effective. 6) The ratio of root mean square (rms) value of the AC component to the value of the DC component in the rectifier output is known as the ripple factor. MAN 4, BIT TT oe 3 | oc 3 time —> Fig. Ripple in the output of a DC output rms value of AC component Ripple Factor = Value of DC component 1) The output of a rectifier is in the form of pulses. 2) The output of any rectifier is also unidirectional but its value is not steady. 3) It keeps fluctuating due to the ripple component present in it, 4) Filter circuit removes the ripple from the output of a rectifier, 5) After filter circuit steady output is obtained, Hence filter is needed in a power supply. Q. What is filter circuit? | Filter Circuit With the help of neat labeled | A filter circuit is a circuit which removes the AC component or the circuit diagram explain the | ripple from a rectifier output and allows only the DC component, working of a capacitor filter. | | AC rectifier Fig. (ii) Output wave form ofter filtration 1) Fig(i) shows the pulsating DC voltage of a rectifier output is applied across the capacitor. 2) As the voltage across the capacitor rises, capacitor ges charged to point A (shown in fig (ii)) and supplies current to the load resistance R,. 3) At the end of the quarter cycle, the capacitor gets charged to the peak voltage shown as Vp (shown in fig(ii)) of rectifier output voltage. 4) Now the rectifier voltage begins to decrease, so that the capacitor starts discharging through the load resistance and the voltage across it begins to drop. Voltage across the load decreases only slightly, up to the point B (shown in fig(ii)), because the next voltage peak recharges the capacitor immediately. 5) This process is repeated again and again, and the output voltage waveform takes the form as shown if fig(ii). In this way capacitor filter works in a circuit. Q. Explain why capacitor Ans. filter circuit is widely used in | 1) Capacitor filter is used after the rectifier circuit to remove the battery eliminators. ripple from the output of rectifier. 2) Capacitor filter circuit has low cost, small size and light weight. 3) Battery eliminators gives small load currents. 4) For above conditions capacitor filters are widely used in battery eliminators. Q. Explain how a Zener diode maintains constant voltage across a load. Q. Explain the working of a Zener Regulator with the help of neat circuit diagram. | 4) Zener voltage remains constant even when the current through the Zener diode varies over a wide range. Ans. 1) A zener diode is a voltage regulator which maintains a constant voltage across a load regardless of variations in the applied input voltage and variations in the load current. R, mA NW) — s Vv, mA mA = 3 gt é = =z, RM g = ze e- i 5 2 Fig. Voltage Regulator Using a Zener Diode According to variation in applied input voltage 1) If the input voltage increases, the current through RS and Zener diode also increase. This result in an increase in the voltage across RS but voltage across Zener diode does not change because the series resistance Rs absorbs the output fluctuations and maintains a constant voltage across the load resistance. 2) When the input voltage increases, the voltage across the series resistance RS also increases, This causes an increase in the current through the Zener diode and current through the load resistance remains constant. Hence the output voltage remains constant irrespective of the change in the input voltage According to variation in Load resistance When the input voltage is constant but load resistance R, decreases, the load current increases. This extra current can not come from the source because the drop across RS will no change as Zener is within its regulating range. This additional load current is due to | a decrease in the Zener current IZ. Hence voltage across the load remains constant as there is change in load resistance. Ans. R, mA AWWW} 3 rm mA mA 3 s £ s ce . 2 ,& =, R |v, Sb - & 2 z 5 A Circuit Construction 1) A zener diode of break down voltage V. is connected in reverse bias to an input voltage source ; as shown in fig. 2) The resistor RS connected in series with the Zener diode limits the current flow through the diode. 3) The load resistance R, is connected in parallel with the Zener diode, so that the voltage across R; is always the same as the Zener Voltage (Vi = V2). Q. What are the limitations of the Zener regulator circuit? Q. How is a Zener diode different than an ordinary diode? tor = Working of Zehet Rear eat Current, Jam) =Maximum Zen., 1) Let Tamn = =Zener Voltage. . _ 2) Garrent and Vee ele Rs, when current is minimum the, s Ve i ek input voltage is given by =\.t+\; .'. Lower inp 3) We know that, we * ¥ iv © Votan = Lams + Vesseesenern(l) Similarly, the voltage across Ks, when current is maximum js given by Vs = (Tamu Rs), | " o Vaanign) = Frimaar R's + We sneeevsee (2) i er rating (P;) of a Zener diode is, Pz = Taney Vy. 3 With th help of specification of the Zener diode, the value of series resistance is determined for Zener regu eho eR | 6) If the input voltage (V) decreases, the sarren a gh Rs & the | Zener diode also decreases. Therefore ‘,, the i Bee Doross RS also decreases without any change in the vol tage Vz (voltage across Zener diode), means any change in the input voltage their will not be change in voltage across the Zener aes 7) Thus a Zener diode gives constant output voltage (Vi) across load resistance (R,). Ans, 1) The current through a Zener diode should never excess the Ing. value, as the current exceeds this value, the Zener diode gets damaged due to heating, , _ 2) The voltage stabilizer is effective when there is a minimum Zener current. oe, 3) The Zener diode must be always operated within its breakdown region when there is a load connected in the circuit. 4) V, must be greater than V;. Ans. 1) Zener diode is heavily doped, p-n junction diode therefore its depletion layer very thin. 2) Moderately doped ordinary diode gets damaged when reverse voltage is increased beyond its breakdown value, 3) Zener diode is operated in reverse bias mode due to its heavily doping by the manufacture. 4) Ordinary diode is majorly used for rectification while the Zener diode is used for voltage regulation. Q. What are the applications | Ans. of Zener diode? 1) Zener diode is used as a vi 2) Fixed reference volta; 3) Peak clipper or limit. 4) Protector against mi oltage regulator, ge provider in transistor biasing circuit. €r In a wave shaping circuit. eter damage from accidental fluctuation. Q. What is photodiode? Phototdiode Explain. Photodiode is a special type of a P-n junction diode which converts light energy into electrical energy. x& anode cathode Q. Represent graphically, the EV characteristics of a photo- diode for different intensities of illumination Q. Explain graphically the saturation current of photodiode. | Q. What is dark resistance of| a photodiode? Q. What are the advantages of photodiode? 5) If the diode is forward biased it conducts a lot of c H current and becomes difficult to detect the small amount of excess cell produced by the photoelectric effect 6) When the diode is reversed biased no ordi : inary cu the detection of the photo current becomes oe fows and 7) Hence a photodiode should be operated in reverse bias mode Ans. reverie bias forward bias voltage voltage 40302-1005 15 dark current | ww -100 200 gé a6 -300 £3 ~400 | TA Fig. The -V characteristic of a photodiode Ans. V,=constat 1,=saturation current I, =device current 1,=dark current V ateverse voltage 1,(HA) light intensity(mW/em) Fig. Relation between the reverse current of a photodiode and the intensity of illumination 1) The total current passing through a photodiode is the sum of the photocurrent and the dark current. | 2) The sensitivity of the device can be increased by minimizing the dark current. 3) In the graph, reverse current increase initially with increase in the intensity of illumination. It reaches a constant value after certain voltage is reached. This constant value is called the satu- ration current of the photodiode. Dark resistance : The ratio of the maximum reverse voltage and its dark current is called as Dark resistance (Rd) of a photodiode. kc resist (Rs) = Maximum reverse current Dark resistance (Riu) =~ ark current Ans. 1) Quick response when exposed to light. 2) Linear response. The reverse current is linearly proportional to intensity of incident light. 3) High speed of operations. 4) Light weight and compact size. ; 5) Wide spectral response. E.g. photodiodes made from Si respond to radiation of wavelengths from 190 nm (UV) to 1100 nm (IR). 6) Relatively low cost. oe Q. What are the disadvan- tages of ‘photodiode? Q. What are the applications of photodiode? Q. What is solar cell. Explain the construction and working of Solar Cell? Ans. ‘ imi th 1) Its properties are temperature dependent, similar to many other semiconductor devices. oo. 1 2) Low reverse current for low illumination levels. Ans. s A 1) Photodiodes are widely used in domestic as well as industrial applications. 2) It is used in counters and switches. | 3) Detection of visible and invisible radiations. 4) Circuits in which fast switching and high spe required, 5) Fiber optic communication systems. 6) Optocouplers, used to provide an ele electronic circuits. t of light 7) Photo sensors / detectors, for accurate measuremen 8. intensity. 8) Safety electronics like fire and smoke detectors. See ed operations are ctric isolation between two Solar cell ; a Solar cell is a photovoltaic cell & solar energy converter. It is a p into electrical energy. junction device, which converts solar energy 1 sunlight antireflecting / contact 1— front contact LT emitter (n-side) €+ base (p-side) |-rare contact Fig. Schematic structure of a solar ceil. Construction of Solar Cell 1) It is a p-n junction device whose n-side of the junction facing towards the solar radiation while p-side is relatively thick and is at the back of the solar cell. 2) Both p-side and n-side are coated with a conducting material and the n-side is coated with antireflection coating which allows visible light to pass through it while the main function of this coating is to reflect the infrared radiations (Heat) and protects the solar cell from heat. 3) The coating works as the electrical contact of the solar cell. The contact on the n-side is called front contact and the contact on the p-side is called rear contact. 4) n-side of the solar cell is thin so that the light incident on it can easily reaching to the depletion region where the electro-hole pairs are gencrated. Working of Solar Cell ‘rom sunlight coe / / / —_ I TT] mregion Ae | all @0 Xe) z R region p-region L...92.99.... F reat contact Fig. Separation of carriers in a solar cell 1 Q. What is a LED? Explain the construction and working of a LED? | LED LED is a light emitting diode which emits light when large forward current passes through it. OR LED . : Light emitting diode is forward biased p-n junction diode, which emits light when recombination of electrons & holes takes place at the junction. JM, Fig. Circuit symbol of LED Construction of LED emitted light —~ , plastic lens \] 7 LED chip P a K A reflecting epoxt lens cavity wire bond: anode cathode + - Fig. Schematic structure of LED 1) The n-type region is heavily doped than the p-type of the p-n junction. The LED p-n junction is encased in a dome-shaped transparent case so that light is emitted uniformly in all directions and internal reflections are minimized. Metal electrodes attached on either side of the p-n junction, which are used as contacts for the external electrical connec- tions. The larger leg of a LED is the positive electrode (anode). 4) The electron-hole recombination takes place on the surface for maximum light output. 5) LEDs are fabricated by using compound semiconductors made with elements such as gallium, phosphorus and arsenic. 2 3) Working of LED recombination of electrons and holes L i T eftee electrons Be) Home) depletion mhe region lr Fig. Emission of light from LED 1) When the LED is forward bias, electrons from the semiconduc- tors conduction band recombine with holes from the valence band releasing sufficient energy to produce photons which emit a monochromatic light (Light of single wavelength/single colour). 2) The reasonable number of these photons can leave the junction and emit coloured light due to thin layer. 3) The amount of output light is directly proportional to the forward current; higher the forward current, higher the light output. 13 Q. On which factors does the wavelength / colour of light emitted by a LED depend? Q. Draw I-v characteristics curve of an LED, Q. Explain, why high resis- tance is connected in series with LED? Q. What are the advantages of LED? Q. What are the disadvan- tages of LED? Ans. The wavelength / col proportions of semico ight emitted by LED depends on th so cor used in the fabrications of the Lzp. le Example 1) When LED is manu (AlGaAs) it emits i 4 2) When LED is manulactu Ton saloitt E i its either red or yellow colour. _ 3) Wun Leo is manufactured by bo Rome ee gallium . sphi IGaP) it emits either red or Ir. 4) Fae Lan ts at etured by using Zinc selenide (ZnSe) it emits either blue colour. factured by using aluminium gallium arsenide radiation. ; pared by using gallium arsenic phosphate = Ans, r - A The 1-V characteristic of LED is similar to the forward characteris. tics of an ordinary diode. forwors current 1(mA) 50 40) 30 20 10 ovw7T 2 3 4 5 Y Fig. Light Emitting Diode (LED) I-V Characteristic Curves showing different colours available Ans. 1) The current rating of LED is of a few tens of milli-ampere. 2) Therefore, it is necessary to connect high resistance in series with LED so that the voltage drop across LED is large. Ans. 1) Energy efficient: More light output for lesser electrical power. LEDs are now capable of producing 135 lumens / watt. 2) Long Lifetime: 50000 hours or more if properly manufactured. 3) Rugged : LEDs are also called Solid State Lights (SSL) as they are made of solid material with no filament or tube or bulb to break. 4) Almost no warm up nanoseconds. 5) Excellent colour rendering : Colours Produced by LED do not fade out making them perfect for displays and retail applications. 6) Environment friendly LEDs do not contain mercury or other hazardous substances, 7) Controllable : Brightness and can be controlled. period. LEDs start emitting light in colour of light emitted by LEDs Ans. 1) Hazardous blue light quality, 2) Temperature dependence, 3) Voltage sensitive, 4) High initial cost. 14 Q. Explain the struct ure of transistor, Q. State the biasing condi- tions for a typical transistor. Q. Explain the working of a n-p-n transistor with the help of circuit diagram. y p-n-p eons a n-type semiconductor (base) layer separates n p-n-p trai i itter and collector), - semiconductor (emi 1 Lin the dees Suibel of pon-p transistor the arrow points towards the he emitter. . ; nis obtained oy growing a thin layer of ne eer ironiniaa in between two relatively thick layers of p-type ' “tthe transistor has the three layers and which are the emitter (E), the Base (B) and the collector (C). Cc B -O E — n-p-n p-n-p Fig. Two-diode Analogy of a BJT 2) The transistor can have two junction diodes connected back to back as shown in above fig. ; ; 3) Emitter: It is a thick heavily doped layer. This supplies a large number of majority carriers for the current flow through the transistor. 4) Base: It is thin, lightly doped central layer. ; : 5) Collector: It is on the other side of the base. It is also thick and moderately doped layer. Its area is larger than that of the emitter and base. This layer collects a major portion of the majority carriers supplied by the emitter. The collector also helps dissipa- tion of any small amount of heat generated. Ans. 1) There are two p-n junctions in a transistor, the emitter-base (E-B) junction and the collector-base (C-B) junction. 2) The biasing of transistor can be in different ways. 3) In most common method of biasing a transistor, the emitter base junction is forward biased and the collector base junction is a reversed bias. Ans, 1) Electrons are the m transistor. 2) As emitter-base (E-B) junction through this junction is large. 3) As collector-base (C-B) is junction is reversed bias, the current flowing through this junction is also large. many electrons ajority carriers in the emitter of a n-p-n is forward bias, the current flowing many electrons 7 Fig. Biasing of n-p-n transistor 4) At the instant the forward bias is applied to the E-B junction, electrons in the emitter region (n-type) have not entered the base region (p-type). 5) When the biasing voltage VBE is greater than the (0.6-0.7V, for silicon transistor region and form the emitter cu barrier potential ) many electrons enter the base trent /;, 16 Q. What are the possible configurations of transistor in qa circuit? Explain each in brief. Fig. Majority carriers - in emitter 6) These electrons can either fiow through the ba Fig. Injection of majority carriers into base | se circuit and an also flow through the e collector current Ic). t is only 5% constitute the base current (/s) or they c collector circuit & contribute towards th ~ base being thin and lightly doped, base curren Ol fe. a > opin > Fig. Electron flow through a transistor . . 8) The electrons Injected from the emitter into the base diffuse into the collector-base depletion region due to the thin base region. 9) When electron enters the collector-base depletion region, they are pushed into the collector region by the electric field at the collector-base depletion region and hence the collector current (Jc)flow through the external circuit. 10) The collector current Jc is about 95% of Jr. 11) Therefore we can write as J: = Js + Jc. As the small so we can write as/c ~ Ir. base current is very Ans. The possible configurations of transistor in a circuit are (a) Common Emitter, CE (b) Common Base, CB and (c) Common Collector, CC. (a) Common Emitter, CE The emitter of transistor is common to both the input & the output. collector collector base base emitter emitter mp-n pa-p Fig. Common emitter configuration. (b) Common Base, CB The base of transistor is common to both the input & the output. emitter collector — emitter collector np-n np-n Fig. Common base configuration (c) Common Collector, cc The collector of transistor is common to both the input & the output. collector collector base base emitter emitter n-p-n p-n-p Fig. Common collector configuration w a typical circuit ged 10 study the common an ‘ater (CE) characteristics of a pransistor. g. Dra Q. Explain input and output characteristics of a transistor. Ans. I Ll Va Fig. Circuit to study Common Emitter (CE) characteristic _ j) 2) 3) 4) 5) 1) The variation of base current /, with ba 2) The current is very small till the barrier po 3) When the voltage Vie, js more than the bi 4) The input dynamic resistance / Input characteristics of a transistor I,wA wf 80 60. 40. 20. V,=10V Vealv 0.2 0.4 0.6 0.8 1 Fig. The Input characteristics se-emitter voltage, Vee is called input characteristics of a transistor. tential is overcome. arrier potential, the characteristics is similar to that of a forward biased diode. of a transistor is defined as the age and the resulting ratio of the change in the base-emitter volt lector-base voltage. change in the base current at a constant co! = AVsr "="Ae Output characteristics of a transistor base current (I,) 2 4 6 8 10 I 12 13 collector to emitter voltage (V_,) in volts variation of collector current Ic with variation in the collec- The Itage is called the output characteristics of a tor-emitter vo. transistor. While studying the output characteristics of transistor, the base current J; is kept constant. The collector current Jc is independent of Vcc as long as the collec- tor-emitter junction is reverse biased. The collector current Jc is large for large values of the base current J, when Vcr is constant. The output dynamic resistance / of a transistor is defined as the ratio of the change in the collector-emitter voltage Ver & the change in the collector current /- for constant base current /». = AMe o Ak Q. Explain construction & Amplifier working of a transistor as Amplifier is a device which is used for increasing the amplitude of an amplifier with the help of | the alternating signals like voltage, current or power. circuit diagram. Construction of Circuit “ 1) A small sinusoidal input signal is superimposed on the DC bias . 2) The base current J, and the collector current Jc will have these sinusoidal variation superimposed on them due to which output voltage V, also to change sinusoidally. 3) A capacitor is connected in the outpul component. 4) A load resistance R, is connected in the collector circuit and output is obtained across the load resistance. tt circuit to block the DC Fig. Typical transistor amplifier circuit Working of the an amplifier . 1) When input voltage is not applied, then by applying Kirchhoff’s law to the output loop, we can write as Veo = Vee FcR i vieseeee(1) 2) Similarly for the input loop we have, Vos = Vee + InRe... eZ) 3) When some AC input is applied, v, is not zero and we can write Vap + Vv, = Vee + IpRe t+ Ala(Ra + Mi) soe (3) 4) The change in Vs can be related to the input resistance and the change in the base current /,. Hence we can write as, v= Ale(Ratn) = Alen as Ry is small. 5) AC current gain (ic) is given by — te Buc= 7, 6) The AC current gain (Bac) is almost same as the DC current gain (Boc) for normal operating voltages. 7) The change is the base current /, causes the change in the collector current Jc. This changes the voltage drop across the load resistance because Vr is constant. Hence we can write as, AVec = AVee + Rile = 0 vw AVee =~ Ric 8) The change in the output voltage AVcz is the output voltage Vo, hence we can write as, Vo = AVee = BacRi Als Q. State and explain the (a) AC current gain(B.c) following terms. The ratio of change in collector current to change in the base current is AC gain of the amplifier. Ake _ le "Ala To 20