Class 12 Physics Semiconductor Electronics Intrinsic semiconductor

Intrinsic semiconductor

  • A pure semiconductor, free from impurities is called intrinsic semiconductor
    • The electrical conductivity of pure semiconductor is called intrinsic conductivity
    • Structure – Consider pure Germanium and Silicon. Both have 4 valence electrons

Crystalline structure

At temperature 0K –

  • In the crystal structure, the four valence electron of the Ge atom forms four covalent bond by sharing of electron with the neighbouring atoms
  • Each covalent bond is made of two atoms, each one from each atom
  • By forming covalent bond, each Ge atom in the crystal behaves as if the outermost orbit of each atom is complete with 8 electrons, having no free electrons in the crystal

At room temperature

  • The conduction is possible if the electrons break away from the covalent bonds and are free by the thermal energy
  • When electron breaks away from the covalent bond, the empty space or vacancy left in the bond is called a hole
  • An electron from the neighbouring atom can break away and can be attracted by the hole, creating hole in the other place
  • In the crystal structure, thus, we can see, electrons break the covalent bond and keep moving. Similarly, due to attraction of hole and electron, hole also keeps moving in a crystal
  • Thus, breakage of a covalent bond produces one free electron and one hole in the crystal
  • In an intrinsic semiconductor, the number of holes = number of electrons. Thus ne = nh = ni

Energy band theory

There is an energy gap of about 1 eV between the valence and the conduction band

At temperature 0K –

  • In terms of energy band theory, the valence band is full and the conduction band is totally empty
  • As no electrons are available for conduction, the Ge crystal behaves like a electrical insulator

At room temperature -

  • The thermal vibrations of the atoms provide energy to the electrons in the valence band to cross the energy gap and move into the conduction band as free electrons
  • This results in electrical conductivity of the semiconductor
  • As electrons move from the valence band to the conduction band, a vacancy is created in the valence band. This vacancy is called a hole
  • As electrons move in the conduction band, the holes move in the valence band and electrical conduction in semiconductors is possible

  • At a higher temperature, when electric field is applied, the holes move towards the negative potential, giving rise to hole current and electrons move towards the positive potential giving rise to electron current. Thus I = Ie + Ih

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