Bihar Board - Class 12 Physics - Chapter 14: Semiconductor Electronics Materials Devices And Simple Circuits Long Answer Question
Long Answer Type Questions
1. Classify the metals on the basis of conductivity.
Answer:
Classification of Metals:
On the basis of the relative values of electrical conductivity (σ) or resistivity (ρ = 1/σ ), the solids are broadly classified as:
(i). Metals
They possess very low resistivity (or high conductivity).
ρ ~ 10–2 – 10–8 Ω m
σ ~ 102 – 108 S m-1
(ii). Semiconductors
They have resistivity or conductivity intermediate to metals and insulators.
ρ ~ 10–5 – 106 Ω m
σ ~ 105 – 10-6 S m-1
(iii). Insulators
They have high resistivity (or low conductivity).
ρ ~ 1011 – 1019 Ω m
σ ~ 10-11 – 10-19 S m-1
2. Classify the metals on the basis of Energy Bands.
Answer:
Classification of Metals on the Basis of Energy Bands :
When the atoms come together to form a solid they are so close to each other that the fields of electrons of outer orbits from neighboring atoms overlap. This makes the nature of electron motion in a solid very different from that in an isolated atom. Inside the solid, each electron has a unique position and no two electrons have the same pattern of surrounding charges.
Hence, each electron has a different energy level. These energy levels are so closely packed that we call it an energy band. The energy band which includes the energy levels of the valence electrons is called the valence band. The higher energy band is called the conduction band.
(i). Metals
In metals, the conduction band and valence band are overlapped to each other. The electrons from the valence band can easily move into the conduction band. Normally, the conduction band is empty but when it overlaps with the valence band, electrons can move freely into it and it conducts electric current through it.
(ii). Semiconductors
In insulators, a large energy band gap exists between the valence band and conduction band. There are no electrons in the conduction band and hence, electrical conduction is not possible under ordinary circumstances. It means that the energy gap is so large that electrons cannot be excited from the valence band to the conduction band by thermal excitatio
(iii). Insulators
In semiconductors, a small and finite energy band gap exists. Because of the small energy band gap some electrons from the valence band, at room temperature, acquire enough energy to cross the energy gap and enter the conduction band. These electrons are very few and can move in the conduction band. Hence, the resistance of semiconductors is not as high as that of the insulators.
3. What is Intrinsic Semiconductor and Extrinsic Semiconductor?
Answer:
Intrinsic Semiconductor:
It is a pure semiconductor without any significant dopant species present. In lattice structures of Ge and Si, each atom is surrounded by four nearest neighbors. Si and Ge have four valence electrons. In its crystalline structure, every Si or Ge atom tends to share one of its four valence electrons with each of its four nearest-neighbor atoms. These shared electrons form a covalent bond.
In intrinsic semiconductors, the number of free electrons per unit volume (ne) is equal to the number of holes per unit volume (nh).
ne=nh=ni
Extrinsic Semiconductor:
When a few parts per million (ppm) of a suitable impurity is added to the pure semiconductor, the conductivity increases many times. Such materials are known as extrinsic semiconductors or impurity semiconductors.
In a doped semiconductor, the following relation holds
ne.nh=ni 2
4. Define n-Type Semiconductor and p-Type Semiconductor.
Answer:
n-Type Semiconductor:
When an impurity atom with 5 valence electrons is doped to a germanium crystal, it replaces one of the germanium atoms. Four of the five valence electrons form covalent bonds with one valence electron of four Ge atoms and the fifth valence electron becomes free to move in the crystal structure. This free electron acts as a charge carrier. Thus by introducing impurity in pure Ge, the number of free electrons increases, and hence the conductivity of the crystal increases. Since the majority of charge carriers in these crystals are negatively charged electrons, they are called n-type semiconductors.
p-Type Semiconductor:
When an impurity atom with 3 valence electrons is doped to a germanium crystal, it replaces one of the germanium atoms. The four germanium atoms surrounding the impurity atom can share one electron each with the impurity atom which has got three valence electrons. for every trivalent impurity atom added, an extra hole will be created. As the trivalent impurity atoms accept electrons from the germanium crystal, it is called acceptor impurity. The Ge crystal obtained is called a p-type semiconductor as it contains free holes
5. What is p-n Junction? Describe the formation of p-n Junction .
Answer:
p-n Junction:
When p- and n-type semiconductors are combined to form a p-n unit, a number of new characteristics appear, which make the combination a very useful device, called the p-n junction diode.
p-n Junction Formation:
In the n-region of a p-n junction, the concentration of free electrons is higher than that of holes, whereas in the p-region, the concentration of holes is much higher than that of free electrons. Therefore when a p-n junction is formed, some electrons from the n-region will diffuse into the p-region. Since the hole is nothing but the vacancy of an electron, an electron diffusing from the n- to the p-region simply fills this vacancy, i.e., it completes the covalent bond. This process is called electron-hole recombination.
As a result of electron-hole recombination, the electrons in the n-region are neutralized by holes, so in this small region, we are left with only ionized donor atoms. The positive and negative ions in a small region around the junction are bound and are, therefore, immobile. This small region in the vicinity of the junction which has been depleted of free charge carriers and has only immobile ions is called the depletion region.
6. Explain Semiconductor Diode .
Answer:
Semiconductor Diode:
A semiconductor diode is basically a p-n junction with metallic contacts provided at the ends for the application of an external voltage. The symbol for the simplest electronic device, namely the p-n junction is shown as. The direction of the thick arrow is from the p to the n-region. The p-side is called the anode and the n-side is known as the cathode
P-n junction Diode as Forward Bias:
If the positive terminal of the battery is connected to the p-side and the negative terminal to the n-side, the junction diode is said to be forward-biased
P-n junction Diode as Reverse Bias:
If the positive terminal of the battery is connected to the n-side and the negative terminal to the p-side, the junction diode is said to be reverse-biased
7. State briefly the underlying principle of a- transistor oscillator. Draw a circuit diagram showing how the feedback is accomplished by inductive coupling. Explain the oscillator action.
Answer:
Principle of transistor oscillator : “Sustained a.c. signals can be obtained from an amplifier circuit without any external input signal by giving a positive feedback to the input circuit through inductive coupling or RC/LC network.”
Oscillator action : In an ideal n-p-n biased transistor, when input base emitters junction and output base collector junction are forward and reverse biased respectively, a high collector current IC flows through the circuit. If in circuit switch S is on, this current IC will start flowing in the emitter circuit through the inductive coupling between coils T1 and T2, which provides the +ve feedback output to input and hence make IE maximum. In the absence of +ve feedback the IE thus decreases making the circuit back to its original state. This process continues and oscillations are produced.
The fR resonance frequency is thus given by
f=12LC
8. Describe V-I Characteristics of Junction Diode.
Answer:
V-I Characteristics of Junction Diode :
With increasing forward bias the current first increases non-linearly up to a certain forward-biased voltage called knee voltage or cut-in voltage and beyond which the current varies non-linearly.
In the case of reverse bias, the reverse current called reverse saturation current is independent of reverse bias voltage but depends only on the temperature of the junction. If we go on increasing the reverse bias voltage, for a particular value the reverse current increases abruptly. This voltage is called breakdown voltage of Zener voltage.
9. Explain Half-wave Rectifier.
Answer:
Half-wave Rectifier :
Construction:
The arrangement for a half-wave rectifier is shown in Fig. The AC input voltage is fed across the primary coil P of a suitable step-down transformer. The secondary coil S of the transformer is connected to the semiconductor p-n junction diode D and a load resistance RL.
Working Method :
Let during the first half of the AC input cycle, the end A of secondary S of the transformer be at positive potential and end B at the negative potential. In this situation, the diode is forward biased and a current flows in the circuit. Consequently, an output voltage across load RL is obtained.
During the second half of AC input, the end A of secondary S of the transformer is at negative potential and diode D is in reverse bias. So, no current flows through load RL and there is no output voltage across RL.
In the next positive half-cycle of AC input, we again get the output and so on. Thus, we get output voltage as shown in Fig. Here, the output voltage, though still varying in magnitude, is restricted to only one direction and is said to be rectified. Since the rectified output of the circuit is obtained only for half of the input AC wave, the device is called a half-wave rectifier.
10. Explain Full-wave Rectifier.
Answer:
Full-wave rectifier:
A full-wave rectifier is a rectifier that rectifies both halves of each AC input cycle and gives a unidirectional output voltage continuously.
Construction:
In a full-wave rectifier, we use two semiconductor diodes that operate in a complementary mode. The AC input supply is fed across the primary coil P of a center tap transformer. The two ds A and B of the second S of the transformer are connected to the p-ends of the Diodes D1 and D2 respectively. A load resistance RL is connected between the n-terminal of both the Diodes and the center tapping O of the second of the transformer. The DC output is obtained across load residence RL.
Working Method:
During the first half cycle of the input voltage, the terminal A is positive with respect to O while B is negative with respect to O. Diode first is forward bias and conducts while diode second is reverse bias and does not conduct, the current flow through RL from D To O. During the second half cycle, A is negative and B is positive with respect to O, thus diode first is reverse bias and diode second is forward biased. The current through RL is in the same direction as during the first half cycle. The resulting output current is a continuous series.
As we are getting output in the positive half as well as the negative half of the AC input cycle, the rectifier is called a full-wave rectifier. Obviously, this is a more efficient circuit for getting rectified voltage or current than a half-wave rectifier.
11. What is a Zener diode? Explain Zener diode as a voltage regulator.
Answer:
Zener diode:
The specially designed junction diodes which can operate in the reverse breakdown voltage region continuously without being damaged, are called Zener diodes. These are generally highly doped Silicon diodes. Silicon is preferred over germanium because of its higher thermal stability. A Zener diode is represented by the symbol shown as.
Zener diode as a voltage regulator:
An important application of the Zener diode is that it can be used as a voltage regulat. The regulating action takes place because of the fact that in the reverse breakdown region, a very small change in voltage produces a very large change in current. In the Zener region, the resistance of the Zener diode drops considerably.
Let us consider a Zener diode and a dropping resistor R connected to a fluctuating dc supply such that the Zener diode is reverse biased. When the applied voltage is such that the voltage across Zener is less than Zener voltage, the diode will not conduct. Hence, the output voltage will be proportional to the input voltage and is given by
Vout=RLRS+RL Vin
,but when the input voltage is such that the voltage developed across the Zener is more than Zener voltage, the diode will conduct and will offer very small resistance.
Hence, it will allow all the extra current, and the output voltage will be equal to Zener voltage i.e., Vout = Vz. But every Zener diode has a certain value of current limit and corresponding power limit. If the current in the Zener diode exceeds this limit, the diode will burn out. Zener diodes are always used in reverse bias.
12. Explain the Photodiode.
Answer:
A junction diode made from a photosensitive semiconductor is called a photodiode. In photodiodes one region is made so thin that incident light may reach the depletion region.
The photodiode is operated under reverse bias. When the photodiode is illuminated with energy greater than the energy gap (Eg) of the semiconductor, then electron-hole pairs are generated. The construction of a photodiode is such that electron-hole pairs are generated in or near the depletion region of the diode.
Inside the diode, the electric field is such that electrons are collected on N-side, and holes are collected on the P-side giving rise to an emf. Hence, when external resistance is connected than current flows through it. The photocurrent is proportional to incident light intensity. Photodiodes can be used as a photodetector to detect optical signals.
13. Explain the Light-Emitting Diode.
Answer:
Light-Emitting Diode (LED)
A light-emitting diode is a heavily doped p-n junction encapsulated with a transparent cover so that emitted light can come out. When the forward current of the diode is small the intensity of light emitted is small. As the forward current increases, intensity of light increases and reaches a maximum. Further increase in the forward current results in decrease of light intensity. LEDs are biased such that the light-emitting efficiency is maximum.
LEDs are used in remote controls, burglar alarm systems, optical communication systems, etc. Advantages of LEDs over low-power conventional incandescent lamps are that they have less operational voltages, less power consumption, fast action with no warm-up time, are nearly monochromatic, have long life and ruggedness, and have quick switching on-off capability.
14. Explain the Solar cell.
Answer:
Solar cell:
In a solar cell, one region is made very thin so that most of the light incident on it reaches the depletion region. In this diode when photons of visible light incident to the depletion region, electrons jump from the valence band to the conduction band producing electron-hole pairs. These free electrons under the influence of the barrier electric field move to the n region and holes move to the p region, so the potential of the p region increases, and that of the n region decreases. A net potential difference develops across the junction.
15. Draw V-I characteristics of a p-n junction diode.
Answer the following questions, giving reasons:
(i) Why is the current under reverse bias almost independent of the applied potential upto a critical voltage?
(ii) Why does the reverse current show a sudden increase at the critical voltage.
Name any semiconductor device which operates under the reverse bias in the breakdown region.
Answer:
(i) In reverse bias of p-n junction diodes the small current is due to the minority carrier and hence resistance is also very high. Increase in voltage leads to a very-very small increase in reverse bias currents so we conclude that in reverse bias reverse current is almost independent of applied potential upto a critical voltage because after this critical voltage, current increases suddenly.
(ii) In reverse bias, reverse current through junction diodes is due to minority charge carriers. As reverse bias voltage is increased, the electric field at the junction becomes significant. When reverse bias voltage becomes equal to zener voltage, electric field strength across the junction becomes high. Electric field across the junction is sufficient to pull valence electrons from the atom on the p- side and accelerate them towards the n-side. The movement of these electrons across the function account for high current which is observed at breakdown reverse voltage. Zener diode and photodiode operate under reverse bias.
16. Define junction Transistor and types.
Answer: n-p-n Transistor:It consists of the thin layer of p-type semiconductors developed between two small n-type semiconductors layers.
17. Explain working of p-n-p transistors .
Answer:
Working of p-n-p transistor:
The emitter base of the p-n-p transistor is forward-biased by connecting it to the positive pole of the emitter-base battery VEE and the collector is reverse-biased by connecting it to the negative pole of the collector-base battery VCC.
Holes being the majority carriers in emitters are repelled due to forward bias towards the base. As the base is thin and lightly doped, it has a low density of electrons. Therefore, when the holes enter the base region, only about 5% electron-hole combination takes place.
The remaining holes reach the collector under the influence of reverse collector voltage, an electron leaves the negative pole of collector-base battery ECB and neutralizes it. At the same time, an electron from some covalent bond in the emitter enters the positive terminal of EEB, creating a hole in the emitter. Thus, the current in the p-n-p transistor is carried by holes and at the same time, their concentration is maintained as explained above. In this case also,
Ie=Ib+Ic
18. Explain working of p-n-p transistors .
Answer:
Working of n-p-n transistor :
To understand the action of the n-p-n transistor, the n-type emitter is forward - biased by the help of battery VEE and the collector base is reverse-biased by the help of battery VCC.
The electrons being majority carriers in the emitter are repelled due to forward bias towards the base. The base contains holes as the majority of carriers and some holes and electrons combine in the base region but the base is lightly doped. Due to this, the probability of electron-hole combination in the base region is very small (< 5%).
The remaining electrons cross into the collector region and enter the positive terminal of the battery VCC connected to the collector. At the same time, an electron enters the emitter from the negative pole of the emitter-base battery VEE. Thus, in n-p-n transistors, the current is carried inside the transistor as well as in the external circuit by the electrons. If Ie, Ib and Ic are the emitter current, the base current, and the collector current, respectively, then
Ie=Ib+Ic
19. Explain Logic Gates,Truth table and Boolean expression.
Answer:
Logic Gates: A gate is a logic circuit that has one or more inputs but only one output. It follows a logical relationship between input and output voltages and for this reason, they are called logic gates.
Each logic gate has its characteristic symbol and its function is defined either by a truth table or by a Boolean expression. In digital circuits, low and high voltage is often represented by 0 and 1, respectively.
Truth table: It is a table that shows all input/output possibilities for a logic gate. It is also called a table of combinations.
Boolean expression: George Boole invented a different kind of algebra-based on the binary nature of logic. It was first applied to switching circuits, as a switch is a binary device.
20. Explain NOT ,OR and AND gate.
Answer:
NOT Gate:
This is the most basic gate, with one input and one output. It produces an inverted version of the input at its output i.e., it produces a ‘1’ output if the input is ‘0’ and vice versa. This is why it is also known as an inverter.
OR Gate :
In Boolean algebra, the addition symbol (+) is referred to as OR. The Boolean expression Y = A + B implies Y equals A OR B. The OR gate is a device that combines A with B to give Y as the result. The OR gate is two or more inputs and one output device.
AND Gate :
The multiplication sign [dot (.)] is referred to as AND in Boolean algebra. The Boolean expression Y = A . B implies Y equals A AND B. The AND gate is a device that combines A with B to give Y as the result. The AND gate is two or more inputs and one output device.
21. Explain NAN The NAND gate :
Answer:If the output Y' of the AND gate is connected to the input of the NOT gate, the gate obtained is called the NAND gate. Boolean expression for the NAND gate is
Y = A.B
The NOR gate:
If the output (Y') of the OR gate is connected to the input of a NOT gate, the gate obtained is called the NOR gate. Boolean expression for the NOR gate is
Y = A+B
22. (i) Explain with the help of a diagram the formation of depletion region and barrier potential in a pn junction.(ii) Draw the circuit diagram of a half wave rectifier and explain its working.
Answer:(a) (i) Depletion layer: The layer containing unneutralized acceptor and donor ions across a p-n junction is called depletion layer. It is called the depletion layer because it is depleted of mobile charge carriers.
(ii) Barrier potential: The electric field between the acceptor and donor ions is called the barrier. The difference of potential from one side of the barrier to the other side is called barrier potential.(i) The increase of doping concentration will reduce the width of the depletion layer in semiconductors.(ii) depletion layer widens under reverse bias and vice versa.
(b) Rectifier: A rectifier is a circuit which converts an alternating current into direct current.p-n diode as a half wave rectifier. A half wave rectifier consists of a single diode as shown in the circuit diagram. The secondary of the transformer gives the desired a.c. voltage across A and B.In the positive half cycle of a.c., the voltage at A is positive, the diode is forward biased and it conducts current.In the negative half cycle of a.c., the voltage at A is negative, the diode is reversed biased and it does not conduct current.Thus, we get output across RL during positive half cycles only. The output is unidirectional but varying.
23. It is required to design a (two-input) logic gate, using an appropriate number, of :(a) NAND gates that give a ‘low’ output only when both the inputs are ‘low’.(b) NOR gates that give a ‘high’ output only when both the inputs are ‘high’.Draw the logic circuits for these two cases and write the truth table, corresponding to each of the two designs.
Answer:(a) The ‘NAND’ gate that gives a ‘low’ output only when both its inputs are low, is an ‘OR’ gateThe required design and the truth table are as follow :Truth Table
(b) The ‘NOR’ gate that gives a high output only when both the inputs are high, is an ‘AND’ gate. The required
24. (a) Write the functions of the three segments of a transistor.(b) The figure shows the input waveforms A and B for ‘AND’ gate. Draw the output waveform and write the truth table for this logic gate.
Answer:(a) All the three segments of a transistor have different thickness and their doping levels are also different. A brief description of the three segments of a transistor is given below
Emitter: This is the segment on one side of the transistor. It is of moderate size and heavily doped. It supplies a large number of majority carriers for the current flow through the transistor.
Base : This is the central segment. It is very thin and lightly doped.
Collector : This segment collects a major portion of the majority carriers supplied by the emitter. The collector side is moderately doped and larger in size as compared to the emitter.
25. Explain how a depletion region is formed in a junction diode.
Answer:
As soon as a p-n junction is formed, the majority charge carriers begin to diffuse from the regions of higher concentration to the regions of lower concentrations. Thus the electrons from the n-region diffuse into the p-region and where they combine with the holes and get neutralized. Similarly, the holes from the p-region diffuse into the n-region where they combine with the electrons and get neutralized. This process is called electron-hole recombination.
The p-region near the junction is left with immobile -ve ions and the n-region near the junction is left with +ve ions as shown in the figure. The small region in the vicinity of the junction which is depleted of free charge carriers and has only immobile ions is called the depletion layer. In the depletion region, a potential difference VB is created, called potential barrier as it creates an electric field which opposes the further diffusion of electrons and holes.
(i) In forward bias, the width of the depletion region is decreased.
(ii) In reverse biased, the width of the depletion region is increased.
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