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NEET : Physics [ 304 ] www.allendigital.in  Digital 6 SEMICONDUCTORS 1. On increasing the temperature the specific resistance of a semiconductor :– (1) increases (2) decreases (3) does not change (4) first decreases and then increases 2. Platinum and silicon are cooled after heating up to 25° C then :– (1) resistance of platinum will increase and that of silicon decreases. (2) resistance of silicon will increase and that of platinum decreases. (3) resistance of both will decrease. (4) resistance of both increases. 3. Electric conduction in a semiconductor takes place due to :– (1) electrons only (2) holes only (3) both electrons and holes (4) neither electrons nor holes 4. The atomic bonding is same for which of the following pairs :– (1) Ag and Si (2) Ge and Si (2) Ne and Ge (4) NaCl and Ge 5. Which of the following energy band diagram shows the n-type semiconductor :– (1) Conduction Band Valence Band Eg=1eV (2) Conduction Band Valence Band Impurity level 1 eV (3) Conduction Band Valence Band Impurity level 1 eV (4) Band Valence Band 1 eV Impurity level Conduction 6. Let np and ne be the numbers of holes and conduction electrons in an extrinsic semiconductor. (1) np > ne (2) np = ne (3) np < ne (4) np  ne 7. A p-type semiconductor is :– (1) positively charged (2) negatively charged (3) uncharged (4) uncharged at 0K but charged at higher temperatures 8. Which statement is correct for p-type semiconductor (1) the number of electrons in conduction band is more than the number of holes in valence band at room temperature (2) the number of holes in valence band is more than the number of electrons in conduction band at room temperature (3) there are no holes and electrons at room temperature (4) number of holes and electrons is equal in valence and conduction band 9. When an impurity is doped into an intrinsic semiconductor, the conductivity of the semiconductor : (1) increases (2) decreases (3) remains the same (4) become zero 10. When we convert pure semiconductor into N type the number of hole :– (1) Increases (2) Decreases (3) Remains constant (4) None 11. A semiconductor is damaged by a strong current, because :– (1) lack of free electrons (2) decrease in electrons (3) excess of electrons (4) none of these 12. If ne and nh are the number of electrons and holes in a semiconductor heavily doped with phosphorus, then :– (1) ne > > nh (2) ne < < nh (3) ne < nh (4) ne = nh Exercise - I
Semiconductor Electronics  Digital www.allendigital.in [ 305 ] 13. Two wires P and Q made up of different materials have same resistance at room temperature. When heated, resistance of P increases and that of Q decreases. We conclude that:– (1) P and Q both are conductors but because of being made of different materials it happens so. (2) P is n-type semiconductor and Q is p-type semiconductor. (3) P is semiconductor and Q is conductor. (4) P is conductor and Q is semiconductor. 14. When the conductivity of a semiconductor is only due to breaking of covalent bonds, the semiconductor is called :– (1) intrinsic (2) extrinsic (3) p-type (4) n-types 15. In an intrinsic semiconductor, number of electrons and holes at room temperature are :- (1) equal (2) zero (3) unequal (4) infinity 16. A semiconductor wire is connected in an electric circuit in series and temperature of semiconductor increased then the current in the circuit :– (1) decreases (2) constant (3) increases (4) will not flow 17. In germanium crystal, the forbidden energy gap in joule is :– (1) 1.6 × 10–19 (2) zero (3) 1.12 × 10–19 (4) 1.76 × 10–19 18. In semiconductor, at room temperature :– (1) valence band are partially empty and conduction band are partially filled (2) valence band are fully filled and conduction band are partially empty (3) valence band are fully filled (4) conduction band are fully empty 19. The probability of electrons to be found in the conduction band of an intrinsic semiconductor at a finite temperature :– (1) decreases exponentially with increasing band gap. (2) increases exponentially with increasing band gap. (3) decrease with increasing temperature. (4) is independent of the temperature and the band gap. 20. In a p-type semiconductor, there are mainly :- (1) free electrons (2) holes (3) both (1) and (2) (4) none of these 21. A conducting wire of Copper and Germanium are cooled from room temperature to temperature 80K, then their resistance will :– (1) increase (2) decrease (3) copper's increase and Germanium's decrease (4) copper's decrease and Germanium's increase 22. Choose the false statement from the following :- (1) the resistivity of a semiconductor increases with increase in temperature. (2) substances with energy gap of the order of 10 eV are insulators. (3) in conductors the valence and conduction bands may overlap. (4) the conductivity of a semiconductor increases with increases in temperature. 23. Carbon, Silicon and Germanium atoms have four valence electrons each. Their valence and conduction bonds are separated by energy band gaps represented by (Eg)C , (Eg)Si and (Eg)Ge respectively. Which one of the following relationships is true in their case :– (1) (Eg)C < (Eg)Ge (2) (Eg)C > (Eg)Si (3) (Eg)C = (Eg)Si (4) (Eg)C < (Eg)Si 24. In semiconducting material the mobilities of electrons and holes are e and h respectively. Which of the following is true :– (1) e > h (2) e < h (3) e = h (4) e < 0; h > 0 25. Impurity energy level of n-type semiconductor lies in:- (1) just above valence band (2) just below conduction band (3) between valence and conduction band (4) none of these 26. What is the energy gap in Si semiconductor? (1) 4.4 eV (2) 0.3 eV (3) 0.7 eV (4) 1.1 eV
NEET : Physics [ 306 ] www.allendigital.in  Digital PN JUNCTION & BIASING OF DIODE 27. Region which have no free electrons and holes in a p-n junction is :– (1) p - region (2) n - region (3) junction (4) depletion region 28. In p-n junction at the near at junction there are :- (1) positive Ions (2) negative Ions (3) positive and negative Ions (4) electrons and holes 29. Depletion layer in p-n junction region is caused by (1) drift holes (2) diffusion of free carriers (3) migration of impurity ions (4) drift of electrons 30. In a P–N Junction diode not connected to any circuit (1) potential is the same everywhere. (2) the P - type side is at a higher potential than the N - type side. (3) there is an electric field at the junction directed from the N-type side to the P-type side. (4) there is an electric field at the junction directed from the P-type side to the N-type side. 31. The minority current in a p-n junction is :– (1) from the n-side to the p-side (2) from the p-side to the n-side (3) from the n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse biased. (4) from the p-side to the n-side if the junction is forward-biased and in the opposite direction if it is reverse biased 32. The majority current in a p-n junction is :– (1) from the n-side to the p-side (2) from the p-side to the n-side (3) from the n-side to the p-side if the junction is forward-biased and in the opposite direction if it is reverse biased (4) from the p-side to the n-side if the junction is forward-biased and in the opposite direction if it is reverse biased 33. Diffusion current in a p-n junction is greater than the drift current in magnitude :– (1) if the junction is forward-biased (2) if the junction is reverse-biased (3) if the junction is unbiased (4) in no case 34. In a ............... biased P-N junction, the net flow holes is from N-region to the P-region :– (1) F.B. (2) R. B. (3) Unbiased (4) both 1 & 2 35. A 2 V battery forward biases a diode however there is a drop of 0.5 V across the diode which is independent of current. Also a current greater then 10 mA produces large joule loss and damages diode. If diode is to be operated at 5 mA, the series resistance to be put is :– 2V R (1) 3 k (2) 300 k (3) 300  (4) 200 k 36. Which of the following diode is reverse biased:- (1) –5V (2) 10V 5V (3) –10V (4) –12V –5V 37. Find VAB :– 10 A B 30 V 10 10 (1) 10 V (2) 20 V (3) 30 V (4) none 38. Assuming that the junction diode is ideal then the current through the diode is:- 3V 100 1V (1) 200 mA (2) 20 mA (3) 2 mA (4) zero 39. The resistance of a reverse biased pn junction diode is about :– (1) 1 ohm (2) 102 ohm (3) 103 ohm (4) 106 ohm
Semiconductor Electronics  Digital www.allendigital.in [ 307 ] 40. Current I in the circuit will be :– 20 20 30 5V I (1) 5 40 A (2) 5 50 A (3) 5 10 A (4) 5 20 A 41. In a p-n junction the depletion layer of thickness 10–6 m has potential across it is 0.1 V. The average electric field is (V/m) :– (1) 107 (2) 10–6 (3) 105 (4) 10–5 42. In a unbias p-n junction :- (1) high potential is at n side and low potential is at p side. (2) high potential is at p side and low potential is at n side. (3) p and n both are at same potential. (4) undetermined. 43. On increasing the reverse bias to a large value in p-n junction diode then value of current (1) remains fixed (2) increases slowly (3) decrease slowly (4) suddenly increase 44. Reverse bias applied to a junction diode :– (1) lowers the potential barrier. (2) raises the potential barrier. (3) increases the majority carrier current. (4) increases the minority carrier current. 45. Correct statement for diode is :– (1) in full wave rectifier both diodes work alternatively. (2) in full wave rectifier both diodes work simultaneously. (3) efficiency of full wave rectifier and half wave rectifier is same. (4) full wave rectifier in bidirectional. 46. When a junction diode is reverse biased, the flow of current across the junction is mainly due to :– (1) diffusion of charges (2) depends upon the nature of material (3) drift of charges (4) both drift and diffusion of charges 47. The width of depletion region in a p-n junction diode (1) increases when reverse bias is applied. (2) increases when a forward bias is applied. (3) decreases when a reverse bias is applied. (4) remains the same irrespective of the bias voltage. SPECIAL TYPES OF DIODES 48. For the given circuit shown in fig, to act as full wave rectifier :– a.c. input should be connected across ................. and ..................... the d.c. output would appear across ................ and ..................... A C B D (1) A, C, B, D (2) B, D, A, C (3) A, B, C, D (4) C, A, D, B 49. Forbidden energy gap of Ge is 0.75 eV, maximum wave length of incident radiation of photon for producing electron - hole pair in germanium semiconductor is :– (1) 4200 Å (2) 16500 Å (3) 4700 Å (4) 4000 Å 50. In the circuit given the current through the zener diode is :- 15 V VZ=10V R2 R1 500  1500  (1) 10 mA (2) 6.67 mA (3) 5 mA (4) 3.33 mA 51. A full wave rectifier circuit along with the input and output voltage is shown in the figure then output due to diode D2 is :– D1 D2 RL A B C D E F G + Input Output (1) A, C (2) B, D (3) B, C (4) A, D