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Semiconductor Electronics: Materials, Devices And Simple Circuits

Extrinsic Semiconductor

  • Metals − Possess very low resistivity (or high conductivity)
    Their resistivities lie in the range: (10–2 Ω m to 10–8 Ω m)
    Their conductivities lie in the range: (102 S m–1 to 108 S m–1)

  • Semi-conductors − Possess resistivity or conductivity intermediate to metals and insulators
    Their resistivities lie in the range: (10–5 Ω m to 106 Ω m)
    Their conductivities lie in the range: (105 S m–1 to 10–6 S m–1)

  • Insulators − Possess high resistivity (or low conductivity)
    Their resistivities lie in the range: (1011 Ω m to 1019 Ω m)
    Their conductivities lie in the range: (10–11 S m–1 to 10–19 S m–1)

  • Semi-conductors are of two types:

  • Elemental semi-conductor − Example: Si and Ge

  • Compound semi-conductor − Examples:
    • Organic: doped pthalocyanines, anthracene, etc.
    • Organic polymers: polypyrrole, polythiophene, etc.
    • Inorganic: CdS, GaAs, CdSe, etc.

  • Energy band diagram of metals or conductors

  • Conduction band is partially filled and the valence band is partially empty or the conduction and valence band overlap.

  • Due to overlap, electrons can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction.

  • When the valence band is partially empty, electrons from their lower levels can move to higher levels making conduction possible.

  • Energy band diagram for insulators

  • Large band gap Eg exists. (Eg > 3 eV)

  • Since there are no electrons in the conduction band, no electrical conduction is possible.

  • The electron cannot be excited from the valence band to the conduction band by thermal excitation.

  • Energy band diagram for semi-conductors

  • Energy band gap Eg is small. (Eg < 3 eV)

  • At room temperature, some electrons from valence band cross the energy gap and enter the conduction band.

     

  • A pure semi-conductor which is free of every impurity is called intrinsic semi-conductor.

Example − Ge and Si

The crystal structure of Ge/ Si in 2-D is shown above.



  • In intrinsic semi-conductors, the number of free electrons ne is equal to the number of holes nh.

That is, ne = nh = ni

Where,

ni Intrinsic carrier concentration

  • An intrinsic semi-conductor will behave similar to an insulator at T = 0 K

  • Eg is the energy band gap between valence band and conduction band.
  • The value of Eg for Si and Ge are 1.1 eV and 0.7 eV, respectively.
  • Metals − Possess very low resistivity (or high conductivity)
    Their resistivities lie in the range: (10–2 Ω m to 10–8 Ω m)
    Their conductivities lie in the range: (102 S m–1 to 108 S m–1)

  • Semi-conductors − Possess resistivity or conductivity intermediate to metals and insulators
    Their resistivities lie in the range: (10–5 Ω m to 106 Ω m)
    Their conductivities lie in the range: (105 S m–1 to 10–6 S m–1)

  • Insulators − Possess high resistivity (or low conductivity)
    Their resistivities lie in the range: (1011 Ω m to 1019 Ω m)
    Their conductivities lie in the range: (10–11 S m–1 to 10–19 S m–1)

  • Semi-conductors are of two types:

  • Elemental semi-conductor − Example: Si and Ge

  • Compound semi-conductor − Examples:
    • Organic: doped pthalocyanines, anthracene, etc.
    • Organic polymers: polypyrrole, polythiophene, etc.
    • Inorganic: CdS, GaAs, CdSe, etc.

  • Energy band diagram of metals or conductors

  • Conduction band is partially filled and the valence band is partially empty or the conduction and valence band overlap.

  • Due to overlap, electrons can easily move into the conduction band. This situation makes a large number of electrons available for electrical conduction.

  • When the valence band is partially empty, electrons from their lower levels can move to higher levels making conduction possible.

  • Energy band diagram for insulators

  • Large band gap Eg exists. (Eg > 3 eV)

  • Since there are no electrons in the conduction band, no electrical conduction is possible.

  • The electron cannot be excited from the valence band to the conduction band by thermal excitation.

  • Energy band diagram for semi-conductors

  • Energy band gap Eg is small. (Eg < 3 eV)

  • At room temperature, some electrons from valence band cross the energy gap and enter the conduction band.

     

A semi-conductor with impurity atom added to it is called extrinsic semi-conductor.

Two types of extrinsic semi-conductors are:

n-type semi-conductor

p-type semi-conductor

  • n-type semi-conductor

  • Doped with pentavalent atoms such as arsenic or phosphorous or antimony or bismuth

  • Impurity atom has five valence electrons, which will replace Si or Ge atoms.

  • Pentavalent dopant donates one extra electron for conduction and hence, is known as donor impurity.

  • Total number of conduction electrons ne is due to the electrons contributed by donors and those generated intrinsically while the total number of holes nh is only due to the holes from the intrinsic source.

  • The rate of recombination of holes would increase due to increase in the number of electrons. Therefore, the number of holes would get reduced further.

  • Electrons are theā€¦

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