# Class 12 Physics Chapter 14 Important Questions Semiconductor Electronics: Materials, Devices and Simple Circuits

Q 1:- What is forbidden energy gap in solids? Ans:- The energy gap (Eg) between the top of valence band and bottom of conduction band of a solid is

Science is a complex and challenging subject, as it involves so many principles and concepts that are difficult to memorize. Those student who opt for science have to face many challenges and work hard to get good marks in the exam. In this lesson, students will learn about Semiconductor Electronics: Materials, Devices and Simple Circuits. The best solution of the problem is to practice as many important question of 'Physics Class 12 Chapter 14' as possible to clear the doubts.

Q 1:- What is forbidden energy gap in solids?
Ans:- The energy gap (Eg) between the top of valence band and bottom of conduction band of a solid is called its "forbidden energy gap" or "energy band gap".

Q 2:- What is the order of energy gap in a semiconductor?
Ans:- The energy gap in a semiconductor varies from 0.2 eV to 3.0 eV.

Q 3:- At what temperature would an intrinsic semiconductor behave like a perfect insulator?
Ans:- At absolute zero temperature (0 K), each semiconductor, whether intrinsic or extrinsic, has zero conductivity and behaves like a perfect insulator.

Q 4:- Name two factors on which electrical conductivity of a pure semiconductor depends?
Ans:- The electrical conductivity of a pure semiconductor depends on

(a) the energy band gap of the given semiconductor solid, and

(b) the temperature.

Q 5:- Give the ratio of the number of holes and the number of conduction electrons in an intrinsic semiconductor.
Ans:- In an intrinsic semiconductor, the number of holes is exactly equal to the number of conduction electrons and their ratio is exactly 1, i.e,

ne = nh = ni

Q 6:- What is an extrinsic semiconductor?
Ans:- A pure semiconductor doped with a small but controlled quantity of suitable impurity (either pentavalent or trivalent), so as to enhance its conductivity manifold, is called an extrinsic semiconductor.

Q 7:- What is depletion region in a p-n junction?
Ans:- A thin layer on either side of the junction, which is devoid of free charge carriers (electrons and holes) and contains only immobile ions is called the depletion region/layer.

Q 8:- What happens to the width of depletion layer of a p-n junction when it is (a) forward biased, and (b) reverse biased?
Ans:- (a)Width of depletion layer of a p-n junction decreases when the junction is forward biased.

(b) The width of depletion layer of a p-n junction increases when the junction is reverse biased.

Q 9:- Name the diode (a) for which the output voltage is a regulated voltage, and (b) that emits spontaneous radiation when forward biased.
Ans:- (a) Zener diode provides regulated output voltage,

(b) A light emitting diode (LED) emits spontaneous radiation when forward biased.

Q 10:- Write the main use of the (a) photodiode, and (b) Zener diode,
Ans:- (a) A photodiode is used for measuring light intensity,

(b) A Zener diode is used as a voltage regulator in DC power supplies.

Q 11:- Name the p-n junction diode that emits spontaneous radiation when forward biased. How do we choose the semiconductor, to be used in these diodes, if the emitted radiation is to be in the visible region?
Ans:- Light emitting diode (LED) emits spontaneous radiation when forward biased. The visible light photons have energies ranging from 1.8 eV (for red photon) to 3.0 eV (for blue photon). Therefore, for radiation to be in the visible region, we choose those semiconductors to fabricate LEDs whose energy band gap (Eg) lies within the limits 1.8 eV and 3.0 eV.

Q 12:- What do you mean by rectification?
Ans:- Rectification is the process of converting AC input voltage into unidirectional DC voltage.

Q 13:- Name the diode which directly converts solar radiation into electrical energy.
Ans:- A solar cell directly converts solar radiation into electrical energy.

Q 14:- What is a BJT? Why is it called so?
Ans:- A BJT means a bipolar junction transistor. Commonly used junction transistors are BJTs. A junction transistor is called BJT because it has two junctions joined back to back.

Q 15:- State the biasing used in a transistor.
Ans:- In a transistor, the emitter-base junction is forward biased but the base-collector junction is reverse biased.

Q 16:- Name the three currents in a transistor. How are these related to?
Ans:- There are three parts of a transistor namely emitter, base and collector. Electric currents passing through these elements of a transistor, when transistor is appropriately biased, are called the emitter current IE, base current IB and collector current IC. These currents are related as: IE = IB + IC Moreover, IB << IE or IC and IC is nearly equal (but never equal) to IE.

Q 17:- In a transistor, doping level in base is increased slightly. How will it affect (a) collector current, and (b) base current?
Ans:- If the doping level in base region of a tansistor is increased slightly, then greater fraction of charge carriers (electrons or holes) entering into base from the emitter will be neutralised due to recombination of opposite charge carriers in base region. As a result,

• (a) the collector current lc will decrease, but
• (b) base current IB will increase.

Q 18:- Draw the general shape of the transfer characteristics of a transistor in its CE configuration. Mark the three broad regions in which the characteristic curve can be divided.
Ans:- The general shape of the transfer characteristics (Vo - Vi graph) of a transistor in CE configuration has been shown in Fig.

The characteristic curve has been divided into three broad regions marked as

• (a) cut-off region,
• (b) active region, and
• (c) saturation region.

Q 19:- What is a hole? Write its important characteristics.
Ans:- A hole in a semiconducting material is a vacancy of an electron which behaves as an apparent free particle with an effective positive charge (q=+e). Important characteristics of holes are as follows:

(a) It is a convenient way of describing the actual motion of bound electrons whenever there is an empty bond in the semiconducting crystal.

(b) Conduction, due to the holes, take place when holes are actually present in the valence band of crystal.

(c) Under the action of an external electric field, holes move towards negative potential.

Q 20:- How does the forbidden energy gap (Eg) of an intrinsic semiconductor vary with increase in temperature?
Ans:- The width of forbidden energy band gap Eg of an intrinsic semiconductor does not change at all with increase in temperature. However, at higher temperatures, electrons in valence band may have higher average thermal energy. So, probability of motion of electrons from valence band to conduction band increases and in that sense, one may say that energy gap has been effectively reduced.

Q 21:- Give reason, why a p-type semiconductor is electrically neutral, although nh >> ni.
Ans:- A p-type semiconductor, although having holes as majority carriers and electrons as minority carriers, is overall electrically neutral because the charge of holes is just equal and opposite to that of the ionised cores in the crystal lattice.

Q 22:- An n-type semiconductor has a large number of free (conduction) electrons, yet it is electrically neutral. Explain how.
Ans:- An n-type semiconducting crystal has a large number of free electrons, yet the crystal maintains its overall electric neutrality. It is because the charge of additional electrons supplied by dopant atoms is exactly equal to the positive charge of dopant ions produced in the crystal lattice.

Q 23:- Why is a semiconductor diode damaged on passing a strong current?
Ans:- On passing a strong current, beyond its rated value, the semiconductor diode is damaged or destroyed due to overheating.

Q 24:- How is it that the reverse current in Zener diode starts increasing suddenly at a relatively low breakdown voltage of 5 volt or so?
Ans:- In a Zener diode, the doping is heavy for both p- and n-sides. As a result, the depletion region formed is very thin and consequently, electric field is quite high even for a reverse bias voltage of only about 5 V or so. Such an high electric field pulls valence electrons from host atoms on p-side of the junction, which are then accelerated to n-side of the junction. Due to flow of these electrons, the current in the circuit suddenly increases leading to the breakdown stage of Zener diode.

Q 25:- Why are Si and Ga As preferred materials for solar cells?
Ans:- The solar radiation spectrum is a continuous spectrum having a peak at about 1.5 eV. For photoexcitation, energy of incident photon hv > Eg. Hence, semiconductor with band gap Eg ≃ 1.5 eV or lower is likely to give better conversion efficiency. Silicon has Eg ≃ 1.1 eV and for Ga As, Eg =≃ 1.53 eV. Hence, these two materials are preferred for fabricating solar cells. Out of these two, Ga As is better than Si because its absorption coefficient for solar radiation is relatively higher.

Q 26:- What is the function of base region of a transistor? Why is the base region of a transistor made very thin and lightly doped?
Ans:- The base region of a transistor controls the flow of majority charge carriers from emitter region to collector region. Base region is made very thin and lightly doped so that most of the charge carriers entering into the base from the emitter side may pass on to the collector side with minimum possible recombination (of electrons and holes) within the base region.

Q 27:- In a transistor, emitter-base junction is forward biased while the base-collector junction is reverse biased. Why?
Ans:- Emitter-base junction is forward biased so that majority charge carriers (say, electrons, for a n-p-n transistor) from heavily doped emitter may easily cross over the junction to the base side. These electrons are minority charge carriers in base region. The base-collector junction is reverse biased so that these charge carriers may further cross over to collector region because for drift of minority carriers, a reverse bias is needed.

Q 28:- What is the difference between hole current and electron current for a semiconductor?
Ans:- In a semiconductor, both free electrons as well as holes move and contribute towards electric conduction. On applying an external electric field, free electrons present in conduction band move towards the positive potential giving rise to an electron current. On the other hand, holes present in valence band move towards the negative potential giving rise to a hole current.

Q 29:- Under what condition does a junction diode work as (a) open, and (b) closed switches?
Ans:- A junction diode works as (a) an open switch in reverse bias, and (b) a closed switch in forward bias.

Q 30:- Can we take one slab of p-type semiconductor and physically join it to another n-type semiconductor to get p-n junction?
Ans:- We know that any slab, howsoever, flat it may be, has roughness much larger than the interatomic crystal spacing (≃ 2 to 3 Å). Hence, continuous contact at the atomic level is not possible when one slab of p-type semiconductor is physically joined to another n-type semiconductor. Consequently, the junction will possess discontinuity for the flowing charge carriers and will not behave as a p-n junction.

Q 31:- Can two p-n junctions joined back to back work as a transistor? Given reason too.
Ans:- Two p-n junctions joined back to back cannot work as a transistor because of the following reasons:-

(a) The surfaces of two p-n junctions, howsoever smooth they may be, have imperfections greater than the lattice constant for the semiconductor material. Hence, at the joint, there will always remain some discontinuity.

(b) In a transistor, the base region should be extra thin. When two p-n junctions are joined back to back, thickness of base region will be maximum. It will lead to recombination of holes and electrons in the base region itself and collector current will be practically zero.

Q 32:- A transistor is to be used as (a) an amplifier, (b) a switch. In which state the operating point of the transistor should be fixed in CE mode?
Ans:- (a) For use as an amplifier, the transistor must be set in the active state.

(b) For use as a switch, the transistor should be either in cut-off region or in saturation region of its transfer characteristics.