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Important Questions and Answers: Semiconducting Materials

   Posted On :  02.12.2016 04:48 pm

Physics - Semiconducting Materials - Important Questions and Answers: Semiconducting Materials


SEMICONDUCTING MATERIALS

 

1. What are elemental semiconductors? Give some important elemental semiconductors.

 

Elemental semiconductors are made from single element of the forth group elements of the periodic table.

 

It is also known as indirect band gap semiconductor.

 

Example: Important elemental semiconductors germanium and silicon.

 

 

2.What are the properties of semiconductors?

 

(i) They are formed by covalent bond.

 

(ii) They have empty conduction

 

band.

 

(iii)They have almost filled valance band.

 

(iv)These materials have comparatively narrow energy gap.

 

 

3. Mention any four advantages of semiconducting materials.

          (i) It can behave as insulators at 0K and as conductors at      high temperature. 

(ii) It possess some properties of both conductors and insulators.

 

(iii) On doping we can produce both N and P-type Semiconductors with charge carriers of electrons and holes respectively.

 

(iv)It possess many applications in electronic field such as manufacturing of diodes, transistors, LED’s,IC etc.

 

 

4.       What are compound semiconductors? Give some important compound semiconductors.

 

Semiconductors which are formed by combining third and fifth elements or second and sixth group elements in the periodic table are called compound semiconductors.

 

Important compound semiconductors are


 

5. What are the differences between elemental semiconductors and compound semiconductors?


S.No Elemental Semiconductors

 

1. They are made of single element Eg: Ge,Si

2. They are called as indirect band gap semiconductors.i.e., electron-hole recombination takes place through traps, which are present in the band gap.

3. Here, heat is produced during recombination.

4. They are used for the manufacture of diodes and transistors., etc.

 

Compound Semiconductors

 

They are made of compounds Eg: GaAs, GaP, MgO etc

They are called as direct band gap semiconductors.i.e., electron-hole recombination takes place directly with each other.

Here, the photos are emitted during recombination.

They are used for making LED’s, laser diodes,IC’setc.

6. What is Fermi level in a semiconductor?

 

Fermi level in a semiconductor is the energy level situated in the band gap of the semiconductor. It is exactly located at the middle of the band gap in the case of intrinsic semiconductor.

 

7. Define Hall-effect and Hall voltage.

 

When a conductor (metal or semiconductor) carrying a current (I) is placed in a transverse magnetic field (B),a potential difference (electric field) is produced inside the conductor in a direction normal to the directions of both the current and magnetic field.

 

This phenomenon is known as Hall-effect and the generated voltage is called Hall-voltage.

 

Hall field per unit current density per unit magnetic induction is called hall coefficient.

 

8. Mention the applications of Hall Effect.

 

It is used to,

 

i.           Find type of semiconductor.

 

ii.           Measure carrier concentration.

 

iii.           Find mobility of charge

 

carrier.

 

iv.          Measure the magnetic flux density using a semiconductor sample of known Hall coefficient.

 

9. What is a semiconductor?

 

Semiconductor is a special class of material which behaves like an insulator at o K and acts conductor at temperature other than 0K. Its resistivity lies in between a conductor and an insulator.

 

10. What is an intrinsic semiconductor?

 

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

 

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 semiconductor or impurity semiconductor.

12. What is meant by intrinsic semiconductor and extrinsic semiconductor? What are the

 

differences between intrinsic and extrinsic semiconductor.


S.No Intrinsic Semiconductor

 

1. Semiconductor in a pure form is called intrinsic semiconductor.

2. Here the change carriers are produced only due to thermal agitation.

3. They have low electrical conductivity.

4. They have low operating temperature.

5. At 0K, Fermi level exactly lies between conduction band and valence band.

Examples: Si,Ge,etc.

 

Extrinsic Semiconductor

 

Semiconductor which are doped with impurity is called extrinsic semiconductor

Here the change carriers are produced due to impurities and may also be produced due to thermal agitation.

They have high electrical conductivity.

They have high operating temperature.

At 0K, Fermi level exactly lies closer to conduction band in n”“ypetsemiconductor and lies near valence band in “p”type semiconductor.

Examples: Si and Ge doped with Al, In,P,As etc

 

13. What is an n-type semiconductor?

 

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

 

14. What is a p-type semiconductor?

 

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

 

15. 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.

 

16. 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.

 

17. 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.

 

18. Compare n-type and p-type semiconductors.

 


 

S.No N-type semiconductors

 

1. N-type semiconductor is obtained by doping and intrinsic semiconductor with pentavalent impurity.

2. Here electrons are majority carriers and holes are minority carriers.

3. It has donor energy levels very close to CB

4. When the temperature is increased, these semiconductors can easily donate an electron from donor energy level to the CB

 

P-type semiconductors

 

P-type semiconductor is obtained by doping and intrinsic semiconductor with trivalent impurity.

Here holes are majority carriers and electrons are minority carriers

It has acceptor energy levels very close to VB.

When the temperature is increased, these semiconductors can easily accept an electron from VB to donor energy level .

19. Define impurity range, exhaustion range and intrinsic range in n-type Semiconductors.

 

 

Impurity range: This range is due to the transfer of electrons from the donor energy Level to CB. Here the electron concentration in the CB steadily increases due to ionization of donor atoms.

 

Exhaustion range: When all the electrons are transferred from donor energy level to conduction band, the electron concentration remains constant over certain temperature and is called exhaustion range.

 

Intrinsic range: In this range the n-type semiconductor practically behaves like the Intrinsic semiconductor. Therefore if the temperature is increased the electrons concentration in the conduction band increases rapidly due to the shifting of    electrons from valence band to conduction band.

 

 

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