Sunday, September 4, 2022
Chapter 1: Semiconductor Diodes : Semiconductor Devices & Basic Electronic Systems - F.Y. B.Sc. (Computer Science)
1.1 Semiconductors
Answer: Semiconductors are the materials whose conductivity falls between the conductivity of conductors and insulators.
Answer: There are two types of semiconductors:-
1. Intrinsic Semiconductors:- Intrinsic semiconductors are semiconductors in their purest possible form.
Example: Silicon and Germanium.
2. Extrinsic Semiconductors:- Extrinsic semiconductors are semiconductors obtained from intrinsic semiconductors by adding impurities to them.
Example: Silicon doped with arsenic.
Answer: The process of adding impurities to the intrinsic semiconductors is called as "Doping". It helps in increasing the conductivity of pure(Intrinsic) Semiconductors.
Answer: The material which is used as impurity in doping process is called as dopant.
Q. How does conduction occur in intrinsic semiconductors?
Answer: The conduction occurs in following ways:
1. With Increase in temperature electrons absorb thermal energy.
2. Electrons break the covalent bonds, they move from valence band to conduction band.
3. When electrons move to conduction band, a hole is created in valence band.
4. Both electrons and holes operate as charge carriers. Hole has +ve charge and electron has -ve charge.
5. These electrons and holes help in conduction of electricity.
Q. What are two forms of doping ?
Answer: There are two forms of doping :
Donor doping : When the dopant is a pentavalent atom, the type of doping is called as donor doping.
Acceptor Doping: When the dopant is a trivalent atom, the type of doping is called as acceptor doping.
Q. Define n-type and p-type semiconductors.
Answer:
N-type Semiconductors : An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor.
P-Type Semiconductors : An extrinsic semiconductor which has been doped with electron acceptor atoms is called a p-type semiconductor.
Q. How is n-type semiconductor created?
Answer: The n-type semiconductor is created as following:
1) The n-type semiconductor is created by adding pentavalent impurity to the intrinsic semiconductor.
2) For example, when Arsenic is added to Silicon, the 4 valence electrons of arsenic atom form four covalent bonds with electrons of silicon atoms.
3) The fifth electron is left and called as free electron which hels in conduction.
4) This creates an excess of negative (n-type) charge carriers.
Q. What are majority and minority carriers in n-type semiconductor?
Answer: In n-type semiconductor the majority carriers are free electrons and minority carriers are holes.
Q. How does conduction occur in n-type semiconductor?
Answer: In n-type semi-conductor, when external dc voltage is applied, the free electrons
move towards the positive terminal of the source and holes move towards the negative terminal.
The conduction majorly happens due to free electrons.
Q. How is p-type semiconductor created?
Answer: The p-type semiconductor is created as following:
1) The p-type semiconductor is created by adding trivalent impurity to the intrinsic semiconductor.
2) For example, when Gallium is added to Silicon, the 3 valence electrons of Gallium will form 3 covalent bonds with electrons of Silicon atoms.
3) The fourth covalent bond will remain incomplete due to which a hole is created.
4) This creates an excess of positive p-type charge carriers.
Q. What are majority and minority carriers in p-type semiconductor?
Answer: In p-type semiconductor the majority carriers are holes and minority carriers are electrons.
Q. How does conduction occur in p-type semiconductor?
Answer: In p-type semiconductor, when external dc voltage is applied, the holes move towards negative terminal of supply and electrons
move towards positive terminal of the supply. The conduction majorly happens due to holes.
Q. Explain formation of P-N junction diode ?
Answer: P-N junction diode is formed in following way:
1) The p-type and n-type semiconductors are joint together with special fabrication technique.
2) The p-side is called anode and n-side is called as cathode.
3) After formation of p-n junction, electrons from n-side and holes from p-side diffuse through the junction to the other side.
Q. Explain the formation of the depletion region in P-N junction diode?
Answer: Depletion region is formed in the following way:
1) After formation of p-n junction, free electrons from n side cross the junction and recombine with holes present in p side.
2) The atoms in p side become negatively charged after electrons recombine with them.
3) When electrons move from n side to p side, holes are formed in p side.
4) The atoms in n side become positively charged after the holes are formed.
5) Now, negatively charged atoms are formed near junction on p side and positively charged atoms are formed near junction on n side.
6) These atoms near junction are immobile (cannot move). So after some time diffusion stops and a depletion region is formed.
Q. What is the role of depletion layer in P-N junction diode?
Answer: Depletion layer in p-n junction diode has no free charge carriers. It contains immobile atoms. The depletion layer acts like barrier that prevents flow of electrons from n side to p side and flow of holes from p side to n side.
Q. What do you mean by potential barrier in PN junction?
Answer: The electric field that is created near the P-N junction due to presence of immobile positive and negative ions on both sides of junction, is known as barrier potential or cut in voltage. It is measured in volts.
Q. Define P-N junction diode? Draw it's symbol.
Answer: The p-n junction diode is a two terminal semiconductor device that allows the current to flow in only one direction, from anode to cathode. It dosen't allow the current to flow in reverse direction.
Q. How PN junction works in forward bias?
Answer: The PN junction works in foward bias as following:
1) When p-region is connected to positive terminal and n-region is connected to negative terminal of external DC source then the biasing is called as forward biasing.
2) As the external supply voltage is increased, more free electrons move from n-side towards the p-side and holes move from p-side towards n-side.
3) The holes in p side convert negative immobile ions into neutral atoms and electrons in n side convert positive immobile ions into neutral atoms so that depletion region reduces and collapses.
4) Now the current starts flowing due to majority carriers from anode to cathode.
5) The forward resistance of diode is very small.
Answer: The PN junction works in foward bias as following:
1) When p-region is connected to negative terminal and n-region is connected to positive terminal of external DC source then the biasing is called as reverse biasing.
2) As the external supply voltage is increased, holes in p region and electrons in n region move away from the junction.
3) Due to this, the width of depletion region increases.
4) Eventually, depletion region stops increasing and currents starts flowing due to minority carriers from cathode to anode. This is called reverse current.
5) The reverse resistance of diode is very large.
Answer: The V-I characteristics can be divided into two parts:
1) Forward Characteristics 2) Reverse Characteristics
1. Forward Characteristics:
1) It is a graph of anode to cathode forward voltage versus current flowing through diode.
2) In the beginning, forward voltage (Vf) is less than Cut in voltage, so forward current flowing through it is very small.
3) As forward voltage equals to cut in voltage, the current starts increasing rapidly.
2. Reverse Characteristics
1) It is a graph of cathode to anothe reverse voltage vs reverse current flowing through diode.
2) In the beginning, the reverse saturation current remain constant if temperature is constant. The current is not dependent on reverse voltage.
3) As reverse voltage equals to breakdown voltage, the current starts increasing rapidly due to reverse breakdown.
4) Reverse breakdown can damage the diode so it should be avoided.
Answer: A zener diode, also known as a breakdown diode is a semiconductor device that is designed to operate in reverse direction when a certain specified voltage is reached.
Q. Explain working principle of zener diode.
Answer: A Zener diode operates just like a normal diode when it is forward-biased. When connected in reverse biased mode the small leakage current flows through it. As the reverse voltage increases to the predetermined breakdown voltage (Vz), current starts flowing through the diode.
Q. Draw the circuit symbol of zener diode.
Answer:
Q. Draw the reverse characteristics of a zener diode.
Answer:
Reverse characteristics of zener diode:-
1. When a reverse voltage is applied to a Zener voltage, a small reverse saturation current flows across the diode.
2. This current is due to thermally generated minority carriers.
3. As the reverse voltage increases, at a certain value of reverse voltage, the reverse current increases drastically and sharply.
4.This is an indication that the breakdown has occurred. We call this voltage breakdown voltage or Zener voltage, and Vz denotes it.
Q. Explain avalanche breakdown/ breakdown in zener diode.
Answer: The avalanche breakdown occurs in the following steps.
1. Avalanche breakdown occurs when a large amount of reverse current flows through zener diode.
2. In reverse biased conditions, the conduction will take place only due to minority carriers.
3. As we increase voltage, kinetic energy of minority carriers also increases which then collide with stationary atoms.
4. This leads to valence electrons moving to conduction band, this carrier multiplication is called avalanche effect.
5. This leads to large amount of current flowing through the diode.
Q. Explain working principle of light emitting diode with diagram.
Answer:
1. When LED is forward biased, electrons from n-region recombine with holes in p-region.
2. In this process, electrons move from higher to lower energy level.
3. The excess energy is given away in form of light.
4. In this way LED emits light.
Q. Give applications of LED.
Answer: Applications of LED are as follows:
1. It is used in seven segment displays.
2. It is used as indicators in electronic devices.
3. It is used in infrared remote controls.
Q. Explain working principle of photodiode.
Answer: Working principle of photodiode:
1. It is always operated in reverse biased condition.
2. When light is incident on it, the photons incident will impart energy onto the ions.
3. More light intensity leads to more generation of electron-hole pairs.
4. These electrons and holes move towards positive and negative ends of source.
5. This way photocurrent is formed.
Q. Give applications of photodiode.
Answer: Applications of photodiode:-
1. As light detector.
2. In CD players.
3. In object counting system.
4. In light intensity meters.
Q. Explain working principle of optocoupler.
Answer:
Principle of operation of optocoupler:-
1. It is a combination of light source and light detector.
2. When input pulse is high, LED turns on and light is emitted on photodiode.
3. This leads to photocurrent flowing through photo diode.
4. When input is low, LED turns of leading to zero photocurrent.
5. This way signal is coupled from one point to another optically.
Q. Explain working of solar cell.
Answer:
Principle of operation of solar cell.
1. It is a large photodiode.
2. When light is incident on it, photons impart energy on valence electrons.
3. Now electron-hole pairs are optically generated which quickly move due to influence of external electric field.
4. This constitues photocurrent, and solar cell supplies power to the load.
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