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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.

     

  • 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.
  • 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.

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

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