3 Extrinsic Semi-conductor
Extrinsic Semi-conductor
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 the majority carriers and holes are the minority carriers. Therefore, they are called n-type semi-conductors.
- For n-type semi-conductors, ne >> nh
- p-type semi-conductor
- Doped with trivalent atoms such as aluminium, boron, or indium, etc.
- Dopant has one valence electron less than Si or Ge. Therefore, the atom can form covalent bonds with three neighbouring Si atoms, but does not have any electron to offer to the fourth Si atom.
- Therefore,the bond between the fourth neighbour and the trivalent atom has a vacancy or hole.
- Hole is available for conduction. One acceptor atom gives one hole.
- Holes are the majority carriers and electrons are the minority carriers.
- Forp-type semi-conductor, nh >> ne
Energy band diagram of the semi-conductors at T > 0 K,
n-type semi-conductor
p-type semi-conductor