SEMICONDUCTOR

DEFINITION

A semiconductor is a substance which has resistivity (10-4 to 0.5 ohm m) in between conductors and insulators.

OR

A semiconductor is a substance which has almost filled valence band and nearly empty conductionband with a very small energy gap (equivalent to 1 eV) separating the two.

e.g. germanium, silicon, selenium, carbon etc.

BONDS IN SEMICONDUCTORS

CO-VALENT BONDS

(i) Co-valent bonds are formed by sharing of valence electrons.(ii) In the formation of co-valent bond, each valence electron of an atom forms direct bond with the valence electron of an adjacent atom. In other words, valence electrons are associated with particular atoms.

For this reason, valence electrons in a semiconductor are not free.

PROPERTIES OF SEMICONDUCTORS

(i) The resistivity of a semiconductor is less than an insulator but more than a conductor.(ii) Semiconductors have negative temperature co-efficient of resistance(iii) When a suitable metallic impurity (e.g. arsenic, gallium etc.) is added to a semiconductor, itscurrent conducting properties change appreciably.

COMMONLY USED SEMICONDUCTORS

• Germanium (Ge)

• Silicon (Si)

EFFECT OF TEMPERATURE ON SEMICONDUCTORS

• At absolute zero: behaves as a perfect insulator

  • the valence band is filled and there is a large energy gap between valence band and conduction band. Therefore, no valence electron can reach the conduction band to become free electron.

  • It is due to the non-availability of free electrons that a semiconductor behaves as an insulator.

• Above absolute zero: behaves as slightly conductive

  • When the temperature is raised, some of the covalent bonds in the semiconductor break due to the thermal energy supplied.

  • The breaking of bonds sets those electrons free which are engaged in the formation of these bonds.

  • The result is that a few free electrons exist in the semiconductor.

INTRINSIC SEMICONDUCTOR

A semiconductor in an extremely pure form, which has little current conduction capability at room temperature is known as an intrinsic semiconductor

EXTRINSIC SEMICONDUCTOR

A semiconductor in whose conductivity is increased by adding impurities by a process called doping, is known as an extrinsic semiconductor.

Depending upon the type of impurity added, extrinsic semiconductors are classified into:

(i) n-type semiconductor

(ii) p-type semiconductor

N-TYPE SEMICONDUCTOR

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

Majority Carriers: Electrons

Minority Carriers: Holes

Pentavalent impurities

1. Arsenic (At. No. 33)

2. Antimony (At. No. 51).

CHARACTERISTICS

1. Many new free electrons are produced by the addition of pentavalent impurity.

2. Thermal energy of room temperature still generates a few hole-electron pairs

3. The current conduction in an n-type semiconductor is predominantly by free electrons i.e. negative charges and is called n-type or electron type conductivity.

P-TYPE SEMICONDUCTOR

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

Majority Carriers: Holes

Minority Carriers: Electrons

Trivalent impurities

1. Gallium (At. No. 31)

2. Indium (At. No. 49).

Such impurities which produce p-type semiconductor are known as acceptor impurities because the holes created can accept the electrons.

ELECTRICAL CHARACTERISTICS OF SEMICONDUCTORS

Both n-type as well as p-type semiconductor are electrically neutral.