ELECTRONIC DEVICES AND CIRCUITS

Size: px

Start display at page:

Download "ELECTRONIC DEVICES AND CIRCUITS"

Theodore Douglas
5 years ago
Views:

1 ELECTRONIC DEVICES AND CIRCUITS 1. As compared to a full wave rectifier using 2 diodes, the four diode bridge rectifier has the dominant advantage of (a) Higher current carrying (b) lower peak inverse requirement (c) lower ripple factor (d) higher efficiency 2. Compared to field effect photo transistors, bipolar photo transistors are (a) More sensitive & faster (b) lesss sensitive & slower (c) More sensitive & slower (d) Less sensitive & faster 3. The 6V zener diode shown in the figure, has zero zener resistance and a knee current of 5 ma. The minimum value of R so that the voltage across it does not fall below 6V is (a) 0 ma, 180 ma (b) 5 ma, 110 ma (c) 10 ma, 55 ma (d) 60 ma, 180 ma 4. The voltages at V 1 and V 2 of the arrangement shown in the figure will be respectively, (a) 6 V and 5.4 V (b) 5.4 V and 6 V (c) 3 V and 5.4 V (d) 6 V and 3 V 5. A heavily doped n- type semiconductor has the following data: Hole-electron ratio : 0:4 Doping concentration : 4.2 x 10 8 atoms/m 3 Intrinsic concentration : 1.5 x 10 4 atoms/m3

2 The ratio of conductance of the n -type semiconductor to that of the intrinsic semiconductor of same material and ate same temperature is given by (a) (b) 2000 (c) (d) The bandgap of Silicon at room temperature is (a) 1.3 ev (b) 0.7 ev (c) 1.1 ev (d) 1.4 ev 7. A Silicon sample A is doped with 1018 atoms/cm3 of boron. Another sample B of identical dimension is doped with 1018 atoms/cm3 phosphorus. The ratio of electron to hole mobility is 3. The ratio of conductivity of the sample A to B is (a) 3 (b) 3 1 (c) 3 2 (d) 2 8. The impurity commonly used for realizing the base region of a silicon npn transistor is (a) Gallium (b) Indium (c) Boron (d) Phosphorus 9. If for a silicon npn transistor, the base-to-emitter voltage V BE is 0.7 V and the collectorto-base voltage V CB is 0.2 V, then the transistor is operating in the (a) normal active mode (b) saturation mode (c) inverse active mode (d) cutoff mode 10. Consider the following statements S1 and S2. S1 : The b of a bipolar transistor reduces if the base width is increased. S2 : The b of a bipolar transistor increases if the dopoing concentration in the base is increased. Which remarks of the following is correct? (a) S1 is FALSE and S2 is TRUE (b) Both S1 and S2 are TRUE (c) Both S1 and S2 are FALSE (d) S1 is TRUE and S2 is FALSE 11. Assuming V CEsat = 0.2 V and b = 50, the minimum base current I B required to drive the transistor in the figure to saturation is (a) 56 ma (b) 140 ma (c) 60 ma (d) 3 ma 12. In an abrupt p n - junction, the doping concentrations on the p - side and n-side are N A 9x10 16 /cm3 respectively. The p n junction is reverse biased and the total depletion width is 3 µm. The depletion width on the p -side is (a) 2.7 µm (b) 0.3 µm (c) 2.25 µm (d) 0.75 µm

3 13. The resistivity of a uniformly doped n -type silicon sample is 0 5 Ω-cm. If the electron mobility (µ n ) is 1250 cm 2 /V-sec and the charge of an electron is 16 x Coulomb, the donor impurity concentration (N D ) in the sample is (a) 2 x /cm 3 (b)1x10 16 /cm 3 (c) 2.5 x /cm 3 (d) 5 x /cm n-type silicon is obtained by doping silicon with (a) Germanium (b) Aluminium (c) Boron (d) Phosphorus 15. The Bandgap of silicon at 300 K is (a) 1.36 ev (b) 1.10 ev (c) 0.80 ev (d) 0.67 ev 16. The intrinsic carrier concentration of silicon sample at 300 K is 1 5 x /m3. If after doping, the number of majority carriers is 5 x /m3, the minority carrier density is (a) 4.50 x /m 3 (b) x 10 4 /m 3 (c) 5.00 x /m 3 (d) 3.00x 10-5 /m Choose proper substitutes for X and Y to make the following statement correct Tunnel diode and Avalanche photo diode are operated in X bias ad Y bias respectively (a) X: reverse, Y: reverse (b) X: reverse, Y: forward (c) X: forward, Y: reverse (d) X: forward, Y: forward 18. For an n - channel enhancement type MOSFET, if the source is connected at a higher potential than that of the bulk (i.e. VSB > 0), the threshold voltage V T of the MOSFET will (a) remain unchanged (b) decrease (c) change polarity (d) increase 19. An n -type silicon bar 0.1 cm long and 100 µm 2 i cross-sectional area has a majority carrier concentration of 5 x /m 2 and the carrier mobility is 0.13 m 2 /V-s at 300 K. If the charge of an electron is 1.5 x coulomb, then the resistance of the bar is (a) 10 6 Ohm (b) 10 4 Ohm (c) 10-1 Ohm (d) 10-4 Ohm 20. The electron concentration in a sample of uniformly doped n-type silicon at 300 K varies linearly from /cm 3 at x = 0 to 6 x / cm 3 at x = 2µm. Assume a situation that electrons are supplied to keep this concentration gradient constant with time. If electronic charge is 1.6 x coulomb and the diffusion constant D n = 35 cm 2 /s, the current density in the silicon, if no electric field is present, is (a) zero (b) -112 A/cm 2 (C) A/cm 2 (d) A/cm Match items in Group 1 with items in Group 2, most suitably. Group 1Group 2 P. LED 1. Heavy dopingq. Avalanche photo diode 2. Coherent radiation R. Tunnel diode

4 3. Spontaneous emission S. LASER 4. Current gain (a) P - 1, Q - 2, R - 4, S - 3 (b) P - 2, Q - 3, R - 1, S - 4 (c) P - 3 Q - 4, R - 1, S 2 (d) P - 2, Q - 1, R - 4, S At 300 K, for a diode current of 1 ma, a certain germanium diode requires a forward bias of V, whereas a certain silicon diode requires a forward bias of V. Under the conditions state above, the closest approximation of the ratio of reverse saturation current in germanium diode to that in silicon diode is (a) 1 (b) 5 (c) 4 x 10 3 (d) 8 x The action of JFET in its equivalent circuit can best be represented as a (a) Current controlled current source (b) Current controlled voltage source (c) Voltage controlled voltage source ND (d) Voltage controlled current source 24. MOSFET can be used as a (a) current controlled capacitor (b) voltage controlled capacitor (c) current controlled inductor (d) voltage controlled inductor 25. The effective channel length of MOSFET in saturation decreases with increase in (a) gate voltage (b) drain voltage (c) source voltage (d) body voltage 26. The early effect in a bipolar junction transistor is caused by (a) fast turn-on (b) fast turn-off (c) large collector-base reverse bias (d) large emitter-base forward bias 27. An n-channel JEFT has I DSS = 2 ma and Vp =- 4 V. Its transconductance gm (in milliohm) for an applied gate-to-source voltage V GS of -2 V is (a) 0.25 (b) 0.5 (c) 0.75 (d) An npn transistor (with C = 0.3 pf) has a unity-gain cutoff frequency f T of 400 MHz at a dc bias current Ic = 1 ma. The value of its C µ in (pf ) is approximately (V T =26mV ) (a) 15 (b) 30 (c) 50 (d) 96

5 29. The electron and hole concentrations in a intrinsic semiconductor are ni and pi respectively. When doped with a p-type material, these change to n and p, respectively, Then (a) n+p = n i +p i (b) np i = n i p (c)n+n i = p i +p i (d) np = n i p i 30. The f T of a BJT is related to its g m, C π and C µ as follows (a)f T = Cπ+Cµ gm (b) f T = 2π(Cπ+Cµ) (c) f T = (d) f T = gm gm Cπ+Cµ gm 2π(Cπ+Cµ) 31. The static characteristic of an adequately forward biased p-n junction is a straight line, if the plot is of (a) log I vs V (b) log I vs V (c) I vs logv (d) I vs V 32. A long specimen of p-type semiconductor material (a) is positively charged (b) is electrically neutral (c) has an electric field directed along its length (d) acts as a dipole 33. Two identical FETs, each characterized by the parameters g m and r d are connected in parallel. The composite FET is then characterized by the parameters (a) gm rd and 2r d (b) gm rd and rd 2 (c) 2g m and (d) 2g m and 2r d 34. The units of kt q are (a) V (b) V -1 (c) J (d) J/ K 35. For a MOS capacitor fabricated on a p-type semiconductor, strong inversion occurs when (A) surface potential is equal to Fermi potential (B) surface potential is zero (C) surface potential is negative and equal to Fermi potential in magnitude (D) surface potential is positive and equal to twice the Fermi potential 36. The intrinsic carrier density at 300 K is 1.5 x /cm 3, in silicon. For n-type silicon doped to 2.25 x / atoms cm 3, the equilibrium electron and hole densities are (a) n = 1.5 x /cm 3, p = 1.5 x /cm 3 (b) n = 1.5 x /cm 3, p = 2.25 x /cm 3 (c) n = 2.25 x /cm 3, p = 1.0 x /cm 3

6 (d) n = 1.5 x /cm 3, p = 1.5 x /cm If a transistor is operating with both of its junctions forward biased, but with the collector base forward bias greater than the emitter base forward bias, then it is operating in the (a) forward active mode (b) reverse saturation mode (c) reverse active mode (d) forward saturation mode 38. The common-emitter short-circuit current gain b of a transistor (a) is a monotonically increasing function of the collector current IC (b) is a monotonically decreasing function of IC (c) increase with IC, for low IC, reaches a maximum and then decreases with further increase in IC (d) is not a function of IC 39. In a bipolar transistor at room temperature, if the emitter current is doubled the voltage across its base-emitter junction (a) doubles (b) halves (c) increases by about 20 mv (d) decreases by about 20 mv 40. In a MOSFET operating in the saturation region, the channel length modulation effect causes (a) an increase in the gate-source capacitance (b) a decrease in the transconductance (c) a decrease in the unity-gain cutoff frequency (d) a decrease in the output resistance 41. Drift current in the semiconductors depends upon (a) only the electric field (b) only the carrier concentration gradient (c) both the electric field and the carrier concentration (d) both the electric field and the carrier concentration gradient 42. A silicon PN junction is forward biased with a constant current at room temperature. When the temperature is increased by 10ºC, the forward bias voltage across the PN junction (a) Increases by 60 mv (b) Decreases by 60 mv (c) Increases by 25 mv (d) Decreases by 25 mv

7 43. The channel resistance when VGS =- 3 V is (a) 360 Ω (b) 917Ω (c) 1000 Ω (d) 3000Ω 44. At room temperature, a possible value for the mobility of electrons in the inversion layer of a silicon n-channel MOSFET is (a) 450 cm 2 / V-s (b) 1350 cm 2 / V-s (c) 1800 cm 2 / V-s (d) 3600 cm 2 / V-s 45. In an n-type silicon crystal at room temperature, which of the following can have a concentration of 4 x cm -3? (a) Silicon atoms (b) Holes (c) Dopant atoms (d) Valence electrons 46. The ratio of the mobility to the diffusion coefficient in a semiconductor has the units(a) V -1 (b) cm.v 1 (c) V.cm -1 (d) V. s 47. Consider the following two statements about the internal conditions in a n -channel MOSFET operating in the active region. S1 : The inversion charge decreases from source to drain S2 : The channel potential increases from source to drain. Which of the following is correct? (a) Only S2 is true (b) Both S1 and S2 are false (c) Both S1 and S2 are true, but S2 is not a reason for S1 (d) Both S1 and S2 are true, and S2 is a reason for S1 48. The built-in potential of the junction (A) is 0.70 V (B) is 0.76 V (C) is 0.82 V (D) Cannot be estimated from the data given 49. Which of the following is NOT associated with a p n - junction? (A) Junction Capacitance (B) Charge Storage Capacitance (C) Depletion Capacitance (D) Channel Length Modulations 50. The electron and hole concentrations in an intrinsic semiconductor are n i per cm 3 at 300 K. Now, if acceptor impurities are introduced with a concentration of N A per cm 3 (where N A >> n i, the electron concentration per cm3 at 300 K will be) (a) n i (b) n i + N A (c) N A - n i (d) ni2 NA

8 51. In a p + n junction diode under reverse biased the magnitude of electric field is maximum at (a) the edge of the depletion region on the p-side (b) the edge of the depletion region on the n-side (c) the p + n junction (d) the centre of the depletion region on the n-side 52. The concentration of minority carriers in an extrinsic semiconductor under equilibrium is (a) Directly proportional to doping concentration (b) Inversely proportional to the doping concentration (c) Directly proportional to the intrinsic concentration (d) Inversely proportional to the intrinsic concentration 53. Under low level injection assumption, the injected minority carrier current for an extrinsic semiconductor is essentially the (a) Diffusion current (b) Drift current (c) Recombination current (d) Induced current 54. The phenomenon known as Early Effect in a bipolar transistor refers to a reduction of the effective base-width caused by (a) Electron - hole recombination at the base (b) The reverse biasing of the base - collector junction (c) The forward biasing of emitter-base junction (d) The early removal of stored base charge during saturation-tocut off switching 55. A Silicon PN junction at a temperature of 20c C has a reverse saturation current of 10 pico - Ameres (pa). The reserve saturation current at 40cC for the same bias is approximately (a) 30 pa (b) 40 pa (c) 50 pa (d) 60 pa 56. The primary reason for the widespread use of Silicon in semiconductor device technology is (a) abundance of Silicon on the surface of the Earth (b) larger bandgap of Silicon in comparison to Germanium. (c) favorable properties of Silicon - dioxide (SiO2 ) (d) lower melting point 57. A Silicon PN junction diode under reverse bias has depletion region of width 10 µm. The relative permittivity of Silicon, ɛ r =.11 7 and the permittivity of free space ɛ 0 =8.85x10-12 F/m. The depletion capacitance of the diode per square meter is (a) 100 µf (b) 10 µf (c) 1 µf (d) 20 µf 58. Find the correct match between Group 1 and Group 2 Group 1 Group

9 E - Varactor diode F - PIN diode G - Zener diode H - Schottky diode (a) E - 4, F - 2, G - 1, H 3 (b) E - 3, F - 4, G - 1, H 3 (c) E - 2, F - 4, G - 1, H 2 (d) E - 1, F - 3, G - 2, H 4 1. Voltage reference 2. High frequency switch 3. Tuned circuits 4. Current controlled attenuator 59. In a forward biased pn junction diode, the sequence of events that best describes the mechanism of current flow is (a) injection, and subsequent diffusion and recombination of minority carriers (b) injection, and subsequent drift and generation of minority carriers (c) extraction, and subsequent diffusion and generation of minority carriers (d) extraction, and subsequent drift and recombination of minority carriers 60. If for a silicon npn transistor, the base-to-emitter voltage(v BE ) is 0.7 V and the collectorto- base voltage (V CB ) IS 0.2V, then the transistor is operating in the (a) normal active mode (b) saturation mode (c) inverse active mode (d) cutoff mode 61. A zener diode, when used in voltage syabilization circuits, is biased in (a) reverse bias region below the (b) reverse breakdown region (c) breakdown voltage (d) forward bias constant current mode 62. If a donor impurity is added to the semiconductor, then at a given temperature, the Fermi level (a) In a semiconductors, electron and holes move in an electric field in the same direction (b) Electric field density is exactly equal to the sum of electric field intensity and polarization (c) Ampere s circuital law states that the line integral of H about any closed path is exactly equal to the direct current enclosed by that path (d) None of the above 63. If a donor type impurity is added to the semiconductor, then at a given temperature, the Fermi level (a) Moves towards the center of the energy gap (b) Moves towards the valence band (c) Moves towards the conduction band

10 (d) Doesn t change 64. A uniform plane wave in air impinges at 45 0 angles on a lossless dielectric material with dielectric constant ɛ r. The transmitted wave propagation in a 30 0 C direction with respect to the normal. The value of is ɛ r is (a) 1.5 (b) 2 (c) 1.5 (d) Compared to electric field effect photo transistor, bipolar photo transistors are (a) More sensitive and faster (b) Less sensitive and slower (c) More sensitive and slower (d) Less sensitive and faster 66. The temperature coefficient of resistance for a thermister is (a) low and negative (b) low and positive (c) high and negative (d) high and positive 67. The best definition of a super conductor is (a) It is a material showing perfect conductivity and Meissner effect below a critical temperature (b) It is a conductor having zero resistance (c) It is a perfect conductor with a highest diamagnetic susceptibility (d) It is a perfect conductor but becomes resistive when the current density through it exceeds a critical value 68. Pure metal generally have (a) High conductivity& low temperature coefficient (b) High conductivity& High temperature coefficient (c) Low conductivity& zero temperature coefficient (d) Low conductivity& High temperature coefficient 69. The mean free path for electron drift increases with (a)purity (b) Strain Hardening (C) Elastic modules (d) None 70. Which of the following gives piezo electric effect (a) µ metal (b) PVDF (C) Saphire (d) Ferrite 71. For an n-channel MOSFET, if condention parameter (k n ) is ma/v 2,gate to source voltage V QS IS 2V TN = 0.75V. The current will be (a) ma (b) ma (c) ma (d) ma 72. If α=0.98, Ico=6 µa & I R =100 µa for a transistor, then the value of I c will be (a) 2.3 ma (b) 3.1 ma (c) 4.6 ma (d) 5.2 ma

73. In a p-n diode, with the increase reverse current (a) increases (b) decreases (c) remain constant (d) uncertain 74. The dynamic resistance of diode varies as (a) 1/I 2 (b) 1/I (c) I (d) I 2 75.

11 73. In a p-n diode, with the increase reverse current (a) increases (b) decreases (c) remain constant (d) uncertain 74. The dynamic resistance of diode varies as (a) 1/I 2 (b) 1/I (c) I (d) I Photo diode operate at (a) Forward bias (b) Breakdown region (c) Reverse bias (d) Saturation region 76. Which statement is correct for schottkey diode? (a) Current voltage characteristics is totally different than that of a p-n junction diode (b) The current is controlled by the diffusion of minority charge carriers (c) The current results from the flow of minority carriers (d) The storage time t s is almost zero. 77. A BJT is having common emitter current gain 100. Considering 10 V supply and V BE =0.7V what will be the value of R C and R B to set the quiescent point at I C =10 ma and V CE = 8V? (a) R C = 200 Ω, R B =93 kω (b) R C = 2kΩ, R B =100 kω (c) R C = 83 kω, R B =100 Ω (d) R C = 20Ω, R B =93 kω 78. Which device is suitable for higher order RF frequency multiplier? (a) P-I-N diode (b) P-N Junction diode (c) BJT (d) Step recovery diode 79. Which of the following diodes is most suitable for detection of microwave signal? (a) P-I-N diode (b) Schottky barrier diode (c) Varactor diode (d) P-N Junction diode 80. Common emitter DC current gain of the transistor is 100. The current through the 10V zener diode (assuming V EE of the transistor is 0.7 V) is:

4 ev (d) 4.0 ev 83. Common base current gain of a p-n-p bipolar transistor is 0.99. The common emitter current gain of the transistor is (a) 101 (b) 0.01 (c) 99 (d) 1.0 84.

12 (a) 10.3 ma (b) 19.3mA (c) 20mA (d) 40mA 81. The effective resistance faced by the voltage source is: (a) 4Ω (b)12ω (c) 3 Ω (d)16 Ω 82. The electrical conductivity of a semiconductor increases when a radiation of wavelength shorter than 1000 nm is incident on it. The band gap of the semiconductor is (a) 2.4 ev (b) 1.2 ev (c) 3.4 ev (d) 4.0 ev 83. Common base current gain of a p-n-p bipolar transistor is The common emitter current gain of the transistor is (a) 101 (b) 0.01 (c) 99 (d) A sample of Si is doped with donar atoms/cm 3. Considering electron mobility in the doped Si 700 cm 2/ V-sec, the approximate resistivity of the doped Si is: (a) 1 Ω-cm (b) 10 Ω-cm (c) 0.1 Ω-cm (d) 100 Ω-cm 85. Electron mobility of the following intrinsic elements in descending order is (a) GaAs, Ge,Si (b) GaAs, Si, Ge (c)si, Ge, GaAs (d) Ge, Si, GaAs 86. Which of the following meters is based on the principle of Hall effect? (a) Ammeter (b) Gaussmeter (c) Voltmeter (d) All of the above 87. The orbital period of a satellite in circular orbit of radius R from the centre of the earth, is proportional to: (a) R 1/2 (b) R 3/2 (c) R 2 (d) R 88. Which of the following has the highest skin depth? (a) Aluminium (b) Gold (c) Silver (d) copper

13 89. The zener diode shown in following figure is temperature compensated and current gain β of transistor is very high. If current through 100 ohm resistor is 55mA at 25 0 C, what is approximate current through it at 65 0 C? (a) 55mA (b) 54mA (c) 56 ma (d) 100 ma 90. Depletion mosfet operates in (a) Depletion model only (b) enhancement mode only (c) both depletion and enhancement mode (d) none of the above 91. A 24 V, 600 mw, Zener diode is to be used for providing a 24 V stabilized supply to a variable load. Assume that for proper Zener action, a minimum of 10 ma must flow through the Zener. If the input voltage is 32 V, what would be the value of R and the maximum load current (a) 320, 10 ma (b) 400, 15 ma (c) 400, 10 ma (d) 320, 15 ma 92. Oscillator requires (a) No feedback (b) Negative feedback (c) Positive feedback (d) Either positive or negative feedback 93. In a capacitor, the electric charge is stored in (a) Dielectric (b) metal plates (c) dielectric as well as metal plates (d) neither dielectric nor metal plates 94. A PN junction in series with a 100 ohm resistor is forward biased so that a current of 100 ma flows. If voltage across the combination is instantaneously reversed to 10 V at

14 time t = 0, the reverse current that flows through the junction at t = 0 is approximately given by (a) 0 ma (b) 200 ma (c) 50 ma (d) 100 ma 95. Ripple factor for a half wave rectifier is (a) 1.65 (b) 1.45 (c) 1 (d) Assume the intrinsic standoff ratio for the UJT is 0.8 for the relaxation oscillator circuit given below. The frequency of oscillation of this circuit is approximately equal to (a) 1.43 khz (b) 620 Hz (c) 7 khz (d) 1.6 khz 97. A Power MOSFET is a (a) Current controlled device (b) Frequency controlled device (c) Voltage controlled device (d) None of the above 98. A 5V reference is drawn from the circuit shown in the figure. Zener diode of 40 mw and 5V with firing current of 5mA is used. The value of is (a) 50 ohms (b) 500 ohms (c) 75 ohms (d) 470 ohms 99. A zener diode in the circuit shown in the figure below, has a knee current of 5 ma, and a maximum allowed power dissipation of 330 mw. What are the minimum and maximum load currents that can be drawn safely from the circuit, keeping the output voltage V o at

15 (a) 0 ma, 180 ma (b) 5 ma, 110 ma(c) 10 ma, 55 ma (d) 60 ma, 180 ma 100. The conductivity of the intrinsic germanium at 300 K is.... When, n i at 300 K= /cmand µ n and µ p in germanium are 3800 and 1800 cm 2 /Vs respectively, (a) S/cm (b) S/cm (c) 2.24 S/cm (d) S/cm 101. Which of the following parameters of a Silicon Schottky Diode is higher than that of a corresponding PN junction diode? (a) Forward voltage drop (b) Reverse recovery current (c) Reverse recovery time (d) Reverse leakage current 102. What is the output voltage across the 900 ohm load in the circuit given below? (a) 10 V (b) V (c) 20 V (d) 9.47 V

Department of Electrical Engineering IIT Madras

Department of Electrical Engineering IIT Madras Sample Questions on Semiconductor Devices EE3 applicants who are interested to pursue their research in microelectronics devices area (fabrication and/or