Tuesday, June 27, 2023

H C Verma solutions, SEMICONDUCTORS AND SEMICONDUCTOR DEVICES, Chapter-45, QUESTIONS FOR SHORT ANSWER, Concepts of Physics, Part-II

QUESTIONS FOR SHORT ANSWER


       1. How many 1s energy states are present in one mole of sodium vapor? Are they all filled in normal conditions? How many 3s energy states are present in one mole of sodium vapor? Are they all filled in normal conditions?        

ANSWER: The atomic number of sodium is 11 and there are 11 electrons in a sodium atom. The electronic configuration of electrons is - 
1s²2s²2p⁶3s¹. 
    So there are two 1s energy states in a sodium atom wholely filled. One mole of sodium vapor will contain a 6.022x10²³ number of sodium atoms (Avogadro's number). Hence there are 2x6.022x10²³ =12.044x10²³ 1s energy states present in one mole of sodium vapor. They are all filled in normal conditions. 
    
      A sodium atom has two 3s energy states, though half-filled. Thus there will be 12.044x10²³ 3s energy states present in one mole of sodium vapor. But they are partially filled in normal conditions.       







       2. There are energy bands in a solid. Do we have really continuous energy variation in a band or do we have very closely spaced but still discrete energy levels?       

ANSWER: There is no continuous energy variation in a band in a solid. It has very closely spaced but still discrete energy levels. These energy levels have a very small energy gap between two consecutive levels.          






       3. The conduction band of a solid is partially filled at 0 K. Will it be a conductor, a semiconductor, or an insulator?      

ANSWER: Since the conduction band of the solid is partially filled, the electrons in this band will find empty states close to it. Thus it will be a conductor.           






       4. In semiconductors, thermal collisions are responsible for taking a valence electron to the conduction band. Why does the number of conduction electrons not go on increasing with time as thermal collisions continuously take place?        

ANSWER: The energy gap between the valance band and the conduction band is low in semiconductors. Thermal collisions are responsible for taking a valence electron to the conduction band. But this jump of an electron leaves a vacancy in its previous position in the valence band which is commonly known as a hole. Thus a hole-electron pair is formed. The hole is sometimes filled with another covalent bond, which results in the moving of holes. But often other electrons from a hole-electron pair jump down to fill a hole losing some of their energies. This recombination makes the electron-hole pair disappear. So the forming and disappearing of electron-hole pair is a continuous process and makes a balance. That is why the number of conduction electrons does not go on increasing with time as thermal collisions continuously take place.            






       5. When an electron goes from the valance band to the conduction band in silicon, its energy is increased by 1.1 eV. The average energy exchanged in a thermal collision is of the order of kT which is only 0.026 eV at room temperature. How is a thermal collision able to take some of the electrons from the valance band to the conduction band?      

ANSWER: At room temperature, some of the electrons occupying the highest energy level in the valence band already have sufficient energy that is near the required 1.1 eV energy. So when 0.026 eV of thermal collision energy is transferred to them, those electrons that get 1.1 eV or more are able to shift to the conduction band.          






       6. What is the resistance of an intrinsic semiconductor at 0 K?      

ANSWER: At 0 K, none of the valance electrons can get the required 1.1 eV of energy to jump to the conduction band. So there are no carriers in the conduction band of an intrinsic semiconductor. Thus its conductivity is zero which means it has infinite resistance.  






       7. We have valence electrons and conduction electrons in a semiconductor. Do we also have "valance holes" and "conduction holes"?     

ANSWER: When a valance electron leaves its place to jump to the conduction band, the vacant place on the valance band is called a hole. When an electron from the conduction band jumps down to the vacant place or hole the hole disappears, its place in the conduction band is not a vacant place or hole. So there is no concept of "valance holes" and "conduction holes", holes are always in the valance band.          






       8. When a p-type impurity is doped in a semiconductor, a large number of holes are created. This does not make the semiconductor charged. But when holes diffuse from the p-side to the n-side in a p-n junction, the n-side gets positively charged. Explain.      

ANSWER: In a p-type semiconductor, the majority of charge carriers are holes but it is electronically neutral as a whole because the created holes are due to the formation of covalent bonds in the trivalent impurity with the help of silicon valence electrons. So it is not charged. Similarly, an n-type semiconductor is not charged though the majority of charge carriers are electrons. When holes diffuse from the p-side to the n-side in a p-n junction, in other words, the electrons diffuse from the n-side to the p-side thus making the n-side positively charged.           






       9. The drift current in a reversed biased p-n junction increases in magnitude if the temperature of the junction is increased. Explain on the basis of the creation of hole-electron pairs.      

ANSWER: When a hole-electron pair is created in the depletion region, the electron goes to the n-side and the hole to the p-side due to the electric field in the depletion region. This creates a current from the n-side to the p-side and is called the drift current. The reverse bias increases the electric field and the increased temperature allows the formation of more hole-electron pairs. Thus more holes are pushed to the p-side and more electrons to the n-side thus the drift current increases.         






       10. An ideal diode should pass a current freely in one direction and should stop it completely in the opposite direction. Which is closer to the ideal - a vacuum diode or a p-n junction diode?      

ANSWER: A p-n junction diode allows a very small current in the opposite direction i.e. in the reverse bias. But in a vacuum diode current in the opposite direction is not possible due to its construction. Here electrons are supplied by the cathode and collected by the anode when in the forward connection resulting in a current from the anode to the cathode. But in the opposite connection, there is no electron-supplying mechanism in the anode so no current in the opposite direction. Thus a vacuum diode is closer to the ideal.          






       11. Consider an amplifier circuit using a transistor. The output power is several times greater than the input power. Where does the extra power come from?      

ANSWER: The extra power comes from the power supply. An amplifier controls the output signal by matching it with the input signal but with greater amplitudes, in other words, an amplifier amplifies the signal with the help of power supplied to it.   

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Links to the Chapters





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".

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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)

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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


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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"

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