Types of Semi-conductor
February 1, 2019

Atomic structure of Silicon and Germanium

Silicon atomic number is 14. It means there are 14 electrons present in one Silicon atom. So, electronic configuration of Silicon is K=2, L=8, M=4.

Germanium atomic number is 32. So, electronic configuration of Germanium is K = 2, L=8, M = 18, N = 4.

It has four valence electrons (lone pair of electrons) in their outermost shell (valence shell).  These lone pair electrons combine with other lone pair electrons and make covalent bond. The atoms are arranged in an ordered array in tetrahedral crystal structure.


Types of Semiconductors:

On the basic of purity semi-conductors are divided into two parts:

  1. Intrinsic Semi-conductor (Pure Semi-conductor)
  2. Extrinsic Semi-conductor (Impure Semiconductor)

Intrinsic Semi-conductor: It is a purest form of semi-conductor. The number of free electrons in the conduction band and the number of holes in valence band is exactly equal and less in quantity. So, electrical conductivity is low and its conductivity is function of temperature. For pure germanium and silicon forbidden energy gap is 0.72eV and 1.1eV respectively. So, pure germanium valence electrons need only 0.72eV energy to jump from valance band to conduction band. In pure semi-conductor, electric conduction is due to the thermally-generated electron hole pairs. Fermi energy level lies in the middle of the energy gap.

Extrinsic Semi-conductor: It is prepared by doping a small quantity (about 1 part in 10^8) of impurity atoms to the pure semi-conductor. In this type of semi-conductor number of holes and electrons are never equal due to which electrical conductivity is high. Its electrical conductivity is function of temperature as well as quantity of impurity doped in it. Fermi energy level may be near to conduction band or valence band.

Question asked in competitive examinations:

  1. Single-element semi-conductor are characterized by atoms with --- valence electrons
    1. 3
    2. 4
    3. 2
    4. 1

  2. What is forbidden energy gap of pure silicon
    1. 1.1eV
    2. 0.72eV
    3. 0.92eV
    4. None

  3. What is forbidden energy gap of pure germanium
    1. 1.1eV
    2. 0.72eV
    3. 0.92eV
    4. None

  4. Semiconductor materials have ------ bonds
    1. Ionic
    2. Covalent
    3. Mutual
    4. Metallic

  5. Silicon has Z=14. Its outermost orbit is
    1. Partially filled
    2. Half filled
    3. Completely occupied
    4. Empty

  6. Major part of the current in an intrinsic semiconductor is due to
    1. Conduction-band electrons
    2. Valence-band electrons
    3. Holes in the valence band
    4. Thermally-generated electron

  7. Conduction electrons have more mobility than holes because they
    1. Are lighter
    2. Experience collisions less frequently
    3. Have negative charge
    4. Need less energy to move them

  8. Doping materials are called impurities because they
    1. Decrease the number of charge carriers
    2. Change the chemical properties of semiconductors
    3. Make semiconductors less than 100 percent pure
    4. Alter the crystal structures of the pure semiconductors.

  9. Current flow in a semiconductor depends on the phenomenon of
    1. Drift
    2. Diffusion
    3. Recombination
    4. All of the above

  10. The most widely used semiconducting material in electronic devices is
    1. Germanium
    2. Silicon
    3. Copper
    4. Carbon

  11. Electron-hole pairs are produced by
    1. Recombination
    2. Thermal energy
    3. Ionization
    4. Doping

  12. Recombination takes place when
    1. An electron falls into a hole
    2. A positive and a negative ion bond together
    3. A valence electron becomes a conduction
    4. A crystal is formed

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