This post presents a useful set of Semiconductor class 12 Important Questions & answers. These important questions are selected from the section Electronic Devices and from the chapter Semiconductor Electronics of Class 12 Physics syllabus of major boards like CBSE, ISC, and other state boards. The answers are also written after the questions to help the readers and students.
Semiconductor class 12 Important Questions and answers – for CBSE, ISC, State Boards
Q. In semiconductors, what is meant by “doping”? How does it affect the conductivity of a semiconductor?
Answer: It is a process of adding impurities to an intrinsic semiconductor in a controlled manner.
This increases the conductivity of the semiconductor significantly and the semiconductor becomes an extrinsic semiconductor.
Q. What are n-type and p-type semiconductors? What are the majority and minority charge- carriers in each of them?
Answer: These are doped (extrinsic) semiconductors. In the n-type semiconductor, the dopant is pentavalent (like arsenic). And, in the p-type semiconductor, the dopant is trivalent (like indium).
In the n-type semiconductor electrons are the majority charge carriers while holes are the minority charge carriers. But, in the p-type semiconductor holes are the majority charge carriers while electrons are the minority charge carriers.
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Q. What are intrinsic and extrinsic semiconductors?
Answer: A pure semiconductor (Pure Si and pure Ge) is an intrinsic semiconductor while one doped with some impurity (called dopant) is an extrinsic semiconductor.
Q. Why are n-type and p-type semiconductors so-called?
Answer: This is so because conduction in n-type semiconductors is due to negative charge- carriers (electrons) while in p-type semiconductors conduction is due to positive charge- carriers (holes).
Q. Define the term Fermi level in the context of the energy band diagram.
Answer: The highest energy level in the valence band at absolute zero is called the Fermi level.
Q. What is the main difference between the energy band diagrams of n-type and p-type semiconductors?
Answer: In an n-type semiconductor, the donor levels lie just below the conduction band while in a p-type semiconductor, the acceptor levels lie just above the valence band.
Q. State any one difference between the energy band diagram of conductors and that of insulators.
Answer: For a conductor, the valence band and the conduction band overlap; while for an insulator, there is a large energy gap (=10 eV) between the two.
Q. Define the term dopant.
Answer: In the process of doping the impurity atoms added, to a pure intrinsic semiconductor, are called dopants.
Q. What are donor impurities? Name Any Three.
Answer: These are pentavalent impurities that ‘donate’ conducting electrons to a pure semiconductor crystal and convert it into an n-type extrinsic semiconductor. Antimony, arsenic, and phosphorus are examples of donor impurities.
Q. What are acceptor impurities? Name Any Three.
Answer: These are trivalent impurities that create holes in a pure semiconductor crystal and convert it into p-type extrinsic semiconductors. The holes ‘accept’ electrons.
Gallium, indium, and aluminium are examples of donor impurities.
Q. What is a valence band?
Answer: The energy band formed from those energy levels of the atoms which contain the valence electrons is called the valence band.
Q. What is a conduction band?
Answer: The lowest unfilled energy band formed just above the valence band is called the conduction band.
Q. Why does the electrical conductivity of a pure semiconductor increase on heating?
Answer: Near absolute zero, a pure semiconductor is an insulator because the valence band is full and there are no free electrons in the conduction band. As the temperature rises, more and more of the electrons in the valence band gain energy to cross the energy gap and enter the conduction band. Hence, the conductivity increases.
Q. Why is Germanium preferred over silicon for making semiconductor devices?
Answer: The energy gap for Germanium is only about 0.7 eV while for silicon it is about 1.1eV.
Q. What is the change in the resistance of the semiconductor with the increase in temperature?
Answer: The resistance of the semiconductor decreases with an increase in temperature.
Q. Name the majority of charge carriers in the n-type semiconductor.
Answer: In n-type semiconductors majority of charge carriers are electrons.
Q. Name the majority of charge carriers in p-type semiconductors.
Answer: In p-type semi-conductors majority of charge carriers are holes.
Q. Are the mobilities of electrons and holes equal in a semiconductor?
Answer: No, electron mobility is higher than hole mobility.
Q. What are holes in semiconductors?
Answer: A hole indicates a vacancy left by an electron. It is a seat of a positive charge of magnitude equal to the charge of an electron.
Q. How does a semiconductor behave at absolute zero temperature?
Answer: At absolute zero temperature a semiconductor becomes an ideal insulator.
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