Unit – X B
Semiconductors in Solid State
- Semiconductors are the substances whose conductivity lies that between the conductors and insulators. e.g. Germanium, Silicon etc.
- These elements are members of fourth group of the periodic table with outer orbit configuration ns2 np2 and valency 4.
- They are covalent solids.
- These elements have partially filled conduction band and partially filled valence band.
- There are no free electrons for conduction in semiconductors at low temperature (absolute zero). Thus germanium crystal acts as an insulator at absolute zero.
- As the temperature increases, the width of the energy gap reduces and some electrons jump to the conduction band. Thus the conductivity of semiconductors increases with the increase in the temperature. Such semiconductors are also referred as extrinsic or pure semiconductors.
- The conductivity of semiconductors can be increased by purposely adding pentavalent or trivalent impurity (doping) to their crystal in small traces.
The structure of Pure Silicon (Semiconductor) Crystal:
n – type:
- If pentavalent impurity like arsenic or phosphorous is added, then at some sites arsenic replaces silicon in the crystal lattice and donate its non-shared electron for conduction in the crystal.
- Thus electrons are available for conduction at room temperature. Such semiconductor formed is called n-type semiconductor.
p – type:
- If trivalent impurity like boron or indium is added to semiconductor, then at some sites it replaces silicon in the crystal lattice.
- Due to the availability of only three electrons in Boron’s orbit, the fourth covalent bond does not have a pair of electron. Hence a hole is created at this point. It acts as a positive charge carrier. Then such semiconductor formed is called p-type semiconductor.
- Various combinations of n-type and p-type semiconductors are used for making electronic components.
- Diode is a combination of n-type and p-type semiconductors and is used as a rectifier.
- Transistors are made by sandwiching a layer of one type of semiconductor between two layers of the other type of semiconductor. npn and pnp type of transistors are used to detect or amplify radio or audio signals.
- The solar cell is an efficient photodiode used for conversion of light energy into electrical energy. It consists of both p – type and n – type semiconductors.