Friday, July 14, 2023

H C Verma solutions, SEMICONDUCTORS AND SEMICONDUCTOR DEVICES, Chapter-45, OBJECTIVE - II, Concepts of Physics, Part-II

OBJECTIVE - II


       1. In a semiconductor,
(a) there are no free electrons at 0 K 
(b) there are no free electrons at any temperature
(c) the number of free electrons increases with the temperature
(d) the number of free electrons is less than that in a conductor.    

ANSWER: (a), (c), (d).      

EXPLANATION: As the temperature decreases the average energy exchanged in a collision also decreases. So fewer valance electrons cross the gap and the number of electron-hole pairs decreases. The number of such pairs is proportional to a factor T3/2 e-ΔE/2kT, where ΔE is the band gap. Clearly, at T =0 K, the number of pairs is zero. So no free electrons at 0 K. Also, the number of free electrons increases with an increase in temperature. Option (a) and (c) are correct but option (b) is incorrect.
      The number of free electrons in a semiconductor is less than that in a conductor. For example, while the number of conduction electrons per m³ in copper is of the order of 10²⁸, it is of the order of 10¹⁵ in extrinsic n-type semiconductor silicon doped with phosphorus. Option (d) is correct.    






       2. In a p-n junction with open ends,  
(a) there is no systematic motion of charge carriers
(b) holes and conduction electrons systematically go from the p-side to the n-side and from the n-side to the p-side respectively 
(c) there is no net charge transfer between the two sides
(d) there is a constant electric field near the junction.  

ANSWER: (b), (c), (d).      

EXPLANATION: Due to the difference in concentration of holes and conduction electrons between the two sides of a p-n junction, they diffuse to the other side. But this diffusion creates a constant electric field in a region near the junction with direction from n-side to p-side. So further diffusion is difficult while any electron-hole pair created in this region drifts to the opposite direction of the diffusion. Thus a balance is reached where the diffusion and drift of charged carriers are equal. So there is a systematic motion of charged carriers between the two sides but no net charge is transferred. Thus except option (a) all other options are correct. 





       3. In a p-n junction,
(a) new holes and conduction electrons are produced continuously throughout the material 
(b) new holes and conduction electrons are produced continuously throughout the material except in the depletion region.  
(c) holes and conduction electrons recombine continuously throughout the material.
(d) holes and conduction electrons recombine continuously throughout the material except in the depletion region.  

ANSWER: (a), (d).      

EXPLANATION: New hole and conduction electrons are continuously produced throughout the material in a p-n junction. They also continuously recombine to maintain their numbers constant at a certain temperature except in the depletion region. Due to the presence of an electric field in the depletion region, electrons of the produced pairs drift toward the n-side and holes to the p-side and they are unable to recombine. So only options (a) and (d) are correct.   





       4. The impurity atoms with which pure silicon may be doped to make it a p-type semiconductor are those of 
(a) phosphorus 
(b) boron
(c) antimony
(d) aluminum.    

ANSWER: (b), (d).      

EXPLANATION: A p-type semiconductor is made by doping an intrinsic semiconductor like silicon with trivalent impurity atoms. Due to this one of the covalent bonds of a silicon atom with impurity element is deficient in an electron. This results in the production of a hole there and makes the semiconductor rich in holes thus a p-type semiconductor. Out of the four options only boron and aluminum are trivalent. Hence options (b) and (d) are correct.    





       5. The electrical conductivity of pure germanium can be increased by
(a) increasing the temperature
(b) doping acceptor impurities
(c) doping donor impurities
(d) irradiating ultraviolet light on it.    

ANSWER: All.      

EXPLANATION: With all the given methods, the number of charged carriers in germanium which is an intrinsic semiconductor increases. More the number of charged carriers more is the conductivity. Hence all of the options are true.   





       6. A semiconducting device is connected in a series circuit with a battery and a resistance. A current is found to pass through the circuit. If the polarity of the battery is reversed, the current drops to almost zero. The device may be 
(a) an intrinsic semiconductor
(b) a p-type semiconductor
(c) an n-type semiconductor
(d) a p-n junction.    

ANSWER: (d).      

EXPLANATION: An intrinsic semiconductor, a p-type semiconductor, or an n-type semiconductor allows the current to flow through it equally in both directions whatever the magnitude of the current.  
          Only a p-n junction allows the current to flow in one direction that is from the p-side to the n-side. Hence the given semiconducting device may be a p-n junction. Only option (d) is correct.  





       7. A semiconductor is doped with a donor impurity.
(a) The hole concentration increases.
(b) The hole concentration decreases. 
(c) The electron concentration increases.
(d) The electron concentration decreases.     

ANSWER: (b), (c).      

EXPLANATION: By doping with a donor impurity that has five valance electrons, some of the atoms of the semiconductor are replaced by it. An impurity atom makes covalent bonds with four adjacent atoms of the semiconductor by sharing one valence electron with each of them. Thus one valence electron is unable to make a bond and is free to move. So the free electron concentration increases. Some of these electrons combine with the holes of the present electron-hole pairs making them disappear. So the hole concentration decreases. Options (b) and (c) are correct.    





       8. Let iₑ, i₍, and iᵦ represent the emitter current, the collector current, and the base current respectively in a transistor. Then
(a) i₍ is slightly smaller than iₑ
(b) i₍ is slightly greater than iₑ
(c) iᵦ is much smaller than iₑ
(d) iᵦ is much greater than iₑ.  

ANSWER: (a), (c).      

EXPLANATION: The base is very lightly doped and very thin. The emitter is heavily doped and the collector is moderately doped. Since the emitter-base junction is in forward bias and the collector-base in reverse bias, most of the majority charge carriers diffuse from emitter to collector. Only a few of these charge carriers go through the base. So the base current is very low. In fact, the base current Iᵦ is only 1% to 5% of emitter current Iₑ. Hence option (c) is correct and (d) is incorrect.
     Using Kirchhoff's law, we can write
Iₑ =Iᵦ +I₍
Clearly, I₍ is slightly smaller than Iᵦ. Hence option (a) is correct and (b) is incorrect.  





       9. In a normal operation of a transistor, 
(a) the base-emitter junction is forward-biased
(b) the base-collector junction is forward-biased
(c) the base-emitter junction is reverse-biased 
(d) the base-collector junction is reverse-biased.  

ANSWER: (a), (d).      

EXPLANATION: The base-emitter junction is always kept forward-biased. The base-collector junction is kept reverse-biased. This makes most of the majority charge carriers diffuse from the emitter to the collector. Hence only options (a) and (d) are correct.   





       10. An AND gate can be prepared by repetitive use of  
(a) NOT gate
(b) OR gate
(c) NAND gate
(d) NOR gate.   

ANSWER: (c), (d).      

EXPLANATION: NAND and NOR gates are called basic building blocks of logic circuits. Any logical gate can be constructed by repetitively using NAND or NOR gates.   
  An AND gate can not be prepared by repetitive use of a NOT or OR gate. Hence only options (c) and (d) are correct.


---------------------------------------------------

 Buy Home Furnishing

Click here for all links → kktutor.blogspot.com 

===<<<O>>>===


My Channel on YouTube  →  SimplePhysics with KK


Links to the Chapters


CHAPTER- 45- Semiconductors and Semiconductor Devices




CHAPTER- 34- Magnetic Field

CHAPTER- 29- Electric Field and Potential











CHAPTER- 28- Heat Transfer

OBJECTIVE -I







EXERCISES - Q51 to Q55


CHAPTER- 27-Specific Heat Capacities of Gases

CHAPTER- 26-Laws of Thermodynamics


CHAPTER- 25-CALORIMETRY

Questions for Short Answer

OBJECTIVE-I

OBJECTIVE-II


EXERCISES - Q-11 to Q-18


CHAPTER- 24-Kinetic Theory of Gases







CHAPTER- 23 - Heat and Temperature






CHAPTER- 17 - Light Waves




CHAPTER- 14 - Fluid Mechanics




CHAPTER- 13 - Fluid Mechanics


CHAPTER- 12 - Simple Harmonic Motion









CHAPTER- 11 - Gravitation





CHAPTER- 10 - Rotational Mechanics






CHAPTER- 9 - Center of Mass, Linear Momentum, Collision


CHAPTER- 8 - Work and Energy

Click here for → Question for Short Answers

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → Exercises (1-10)

Click here for → Exercises (11-20)

Click here for → Exercises (21-30)

Click here for → Exercises (31-42)

Click here for → Exercise(43-54)

CHAPTER- 7 - Circular Motion

Click here for → Questions for Short Answer 

Click here for → OBJECTIVE-I

Click here for → OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → EXERCISES (11-20)

Click here for → EXERCISES (21-30)

CHAPTER- 6 - Friction

Click here for → Questions for Short Answer

Click here for → OBJECTIVE-I

Click here for → Friction - OBJECTIVE-II

Click here for → EXERCISES (1-10)

Click here for → Exercises (11-20)

Click here for → EXERCISES (21-31)

For more practice on problems on friction solve these- "New Questions on Friction".

---------------------------------------------------------------------------------

CHAPTER- 5 - Newton's Laws of Motion


Click here for → QUESTIONS FOR SHORT ANSWER

Click here for→Newton's Laws of Motion,Exercises(Q.No. 13 to 27)

-------------------------------------------------------------------------------

CHAPTER- 4 - The Forces

The Forces-

"Questions for short Answers"    


Click here for "The Forces" - OBJECTIVE-I


Click here for "The Forces" - OBJECTIVE-II


Click here for "The Forces" - Exercises


--------------------------------------------------------------------------------------------------------------

CHAPTER- 3 - Kinematics - Rest and Motion

Click here for "Questions for short Answers"


Click here for "OBJECTIVE-I"


Click here for EXERCISES (Question number 1 to 10)


Click here for EXERCISES (Question number 11 to 20)


Click here for EXERCISES (Question number 21 to 30)


Click here for EXERCISES (Question number 31 to 40)


Click here for EXERCISES (Question number 41 to 52)


CHAPTER- 2 - "Physics and Mathematics"

Click here for "Questions for Short Answers"


Click here for "OBJECTIVE-II"



Posted by at
Labels: , , ,

No comments:

Post a Comment

Subscribe to: Post Comments (Atom)