Semiconducting Materials - Questions and Answers

1. What is a semiconductor?

The materials in which the electrical conductivity lies between conductors and insulators are called semiconductors. They have resistivity value between 10^-4 to 0.05 ohm-m. The electrical conductivity of semiconductor increase when we add impurities and by increasing the temperature and it is contrary to the metals.

2. Define the properties of semiconductor?

The resistivity of semiconductors lies between that of a conductor and an insulator. It varies from 10^-4 to 0.5 Ω^-m.
At 0K, semiconductors behave as insulators.
They have an empty conduction band and almost filled valence band at 0K.
Semiconductors have negative temperature coefficient of resistance.
If impurities are added, it will increase the electrical conductivity also increase.
If we increase the temperature of semiconductor, its electrical conductivity also increases.

3. With increase of temperature the conductivity of semiconductor increases while that of metals decreases. Give reasons.

With increase of temperature more and more charge carriers are created and hence the conductivity of semiconductor increases. In the case of metals with increase of temperature the concentration of charge carriers remains the same. But due to increase of thermal energy the electrons make frequent collisions with lattice ions and hence the resistivity increases and conductivity decreases.

4. Mention any four advantages of semiconducting materials.

It can behave as insulators at $0K$ and as conductors at high temperatures.
It possess some properties of both conductors and insulators.
On doping we can produce both $n$ and $p$ -type semiconductors with charge carriers of electrons and holes respectively.
It possess many applications in electronic field such as manufacturing of diodes, transistors, LED’s, IC etc.

5. Distinguish between Elemental semiconductors and Compound semiconductors

S.No	Blemental semiendirefors	Compromit semit
1.	These are made from single element	These are made from compound (mixed)element
2.	These are made from IV group elements of periodic table	These are made from III and V II and VI group elements of periodic table.
3.	These are called as indirect band gap semiconductor (electron-hole recombination takes place through traps)	These are called as direct band gap semiconductor (electron-hole recombination takes place directly)
4.	Heat is produced in the recombination	Photons are emitted during recombination
5.	Life time of charge carriers is more due to indirect recombination.	Life time of charge carriers is less due to direct recombination.
6.	Current amplification is more	Current amplification is less
7.	These are used for making diodes, transistor, etc. Examples: Ge, Si	These are used for making LED, diodes, etc. Examples: GaA, GaP

6. What is an intrinsic semiconductors?

Semiconductor is an extremely pure form (without impurities) is known as intrinsic semiconductors.

7. What is pair production or electron – hole pair generation?

When an energy is supplied to the semiconductor, the covalent bonds are broken and the electrons are raised from valence band to conduction band and a vacant site is created in the valence band. It is called pair production or electron – hole pair.

8. Explain the concept of hole in semiconductor.

In intrinsic semiconductor, charge carriers are created due to breaking of covalent bonds. When a covalent bond is broken, an electron escapes to the conduction band leaving behind an empty space in the valence band. This missing electron is called a hole.

9. Define effective mass of the electron.

When an electron moves through a crystal lattice, it experience a periodic potential and hence an internal force. The effect of this force is incorporated in the mass of the electron and such a mass is called effective mass of the electron $m *$ .

10. State law of mass action.

It states that in an Extrinsic semiconductor, the product of electron concentration and hole concentration, produced by the doping process, is a constant.

ie., N_e N_h = n_i² = a constant

11. What is an extrinsic semiconductor?

A semiconducting material in which impurity atoms added
(doped) to the material to modify its conductivity is known as extrinsic semiconductors or impurity semiconductor.

12. What is an n-type semiconductor?

When a small amount of pentavalent inpurity is added to a pure semiconductor, it becomes extrinsic or impure semiconductor and it is known as n-type semiconductor.

13. What is an p-type semiconductor?

When a small amount of trivalent impurity is added to a pure semiconductor, it becomes extrinsic semiconductor and it is called p-type semiconductor.

14. What is meant by doping and doping agent?

The technique of adding impurities to a pure semiconductor is known as doping and the added impurity is called doping agent.

15. What is meant by donor energy level?

A pentavalent impurity when doped with an intrinsic semiconductor donates one electron which produces an energy level called donor energy level.

16. What is meant by acceptor energy level?

A trivalent impurity when doped with an intrinsic semiconductor accepts one electron which produces an energy level called acceptor energy level.

17. What are the differences between intrinsic and extrinsic semiconductor.

S.No	Intrinsic Semiconductors	Extrinsic semiconductors
1.	Semiconductor in a pure form is called intrinsic semiconductor.	Semiconductor which are doped with impurity is called extrinsic semiconductor.
2.	Here the charge carriers are produced only due to thermal agitation	Here the charge carries are produced due to impurities and may also be produced due to thermal agitation.
3.	They have low electrical conductivity	They have high electrical conductivity.
4.	They have low operating temperature.	They have high operating temperature.
5.	At 0K, the fermi level exactly lies between conduction band and valence band. Examples: Si, Ge, ect.	At 0K, Fermi level lies closer to conduction band in ‘n’ type semiconductor and lies near valence band in ‘p’ type semiconductor. Examples: Si and Ge doped with Al, In, P, As etc.

18. Differentiate N-type and P-type semiconductor.

S.No	n-type semiconductor	p-type semiconductor
1.	n-type semiconductor is obtained by doping an intrinsic semiconductor with pentavalent impurity.	p-type semiconductor is obtained by doping an intrinsic semiconductor with a trivalent impurity.
2.	Here electrons are majority carriers and holes are minority carriers.	Here holes are majority carriers and electrons are minority carriers.
3.	It has donar energy levels very close to conduction band.	It has acceptor energy levels very close to valence band.
4.	When temperature is increased, these semiconductors can easily donate an electron from donor energy level to the conduction band	When temperature is increased, these semiconductors can easily accept an electron from valence band to acceptor energy level.

19. Define Hall effect?

If a semiconductor or a conductor carrying current (I) is placed in a magnetic field (B), an electric field is produced in the direction perpendicular to both the current and magnetic field. The generated voltage is called the Hall voltage $(V H)$ and the field is called Hall field $(E H)$ .

20. What are the applications of hall effect?

For an $n$ -type semiconductor the Hall coefficient is negative whereas for a p-type semiconductor it is positive. Thus the sign of the Hall coefficient can be used to determine whether a given semiconductor is $n$ type or $p$ type.
By knowing the value of $R H$ , the Hall coefficient, the carrier concentration is to be determined, $\mathrm{n}_{\mathrm{e}}=-\left(\frac{1}{\mathrm{eR}}\right) \mathrm{n}_{\mathrm{H}}=\left(\frac{1}{\mathrm{eR}_{\mathrm{H}}}\right)$
If the conduction is due to one type of carriers and by measuring the conductivity and the Hall coefficient of a sample, the mobility of the charge carrier can be calculated.
Using a semiconductor sample of known hall coefficient, the magnetic flux density can be determined.
It is used to design magnetic flux meters and multipliers on the basis of Hall voltage.
It is used to find the power flow in an electromagnetic wave.