The conductivity of an intrinsic semiconductor depends on its temperature. At room temperature its conductivity is as low as insulator. In this condition they are of no importance. Thus, in order to make it useful it is necessary improve their conductivity. This is done by making use of impurities. When we add a small amount of a suitable impurity to the pure semiconductor, the conductivity of the semiconductor increases. This is extrinsic semiconductor. Also known as impurity semiconductor.
Doped Meaning- Doping in Semiconductor
Doping is the deliberate addition of a desirable impurity. And the impurity atoms that we add to intrinsic material are dopants. Such a material is also called a doped semiconductor.
The dopant we select is such that it does not distort the original lattice of pure semiconductor. It occupies only a very few of the original semiconductor atom sites in the crystal. A necessary condition to attain this is that the sizes of the dopant and the semiconductor atoms should be nearly the same.
Impurities Used in Doping
We use two types of dopants in doping the tetravalent Si or Ge:
- Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.
- Trivalent (valency 3); like Indium (In), Boron (B), Aluminium (Al), etc.
n Type Extrinsic Semiconductor
In a semiconductor doped with pentavalent impurity, electrons become the majority carriers and holes the minority carriers. These semiconductors are, therefore, known as n-type semiconductors.
Thus, for n type semiconductor ne>>nh.
In a doped semiconductor the total number of conduction electrons ne is due to the electrons contributed by donors and those generated intrinsically. Whereas the total number of holes nh is only due to the holes from the intrinsic source. But the rate of recombination of holes would increase due to the increase in the number of electrons. As a result, the number of holes would get reduced further.
p Type Extrinsic Semiconductor
A material, in which the holes are the majority carriers and electrons are minority carriers is p type semiconductor. Therefore, these are doped with trivalent impurity. For p-type semiconductors, the recombination process will reduce
the number of intrinsically generated electrons to ne.
Thus, for p type semiconductor nh>>ne.
Note that the trivalent foreign atom becomes effectively negatively charged when it shares fourth electron with neighbouring Si atom. Therefore, we treat the dopant atom of p-type material as core of one negative charge along with its associated hole as shown in below.
It is obvious that one acceptor atom gives one hole. These holes are in addition to the intrinsically generated holes while the source of conduction electrons is only intrinsic generation.
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