- Intrinsic semi conductors has a little current conduction capability.
- This capability can be increased by many times by just adding a very small amount of impurity in (1 atom per 1 million pure atoms) the process of crystallization.
- This process is called as doping
- Si and Ge are tetravalent (4 electrons in Valance Band) atoms. So the impurity may be either tri valent (or) pentavalent.
- Depending on the type of impurity added extrinsic semi conductors are divided into 2 types
i. Donor type (or) N-type.
ii. Acceptor type (or) P-type.
The detailed explanation about N-type and P-type semi conductors are given in the following table.
| N-type Semi conductors | P-type Semi conductors |
| When a small amount of pentavalent impurity such as Arsenic, Antimony, Bismuth (or) Phosphorus is added to a pure semi conductor during the crystal growth. The resulting semi conductor is called N-type semi conductor. | When a small amount of trivalent impurity such as Boron, Gallium, Indium (or) Aluminium is added to a pure semi conductor in the resulting crystal . The resulting semi conductor is called P-type semi conductor. |
| Where N stands for negative. | Where P stands for positive. |
| When a pentavalent (or) a donor atom is added to Si the impurity atom forms 4 co-valent bonds with 4 Si atoms and fifth valance electron is left free. Which is loosely bound to the Antimony atom. | When a trivalent (or) a acceptor atom is added to Pure Si (or) Ge semi conductor. Boron has 3 valance electrons in its valance shell and pure Si has 4 valance electrons in its valance shell. The 3 electrons of Boron atoms forms 3 covalent bonds with 3 Si atoms and a whole is left free which is loosely bound to Boron. |
| One impurity atom provides one free electron yet an extremely small amount of impurity provides enough atoms to supply millions of free electrons. | simillarly a small amount of impurity provides millions of wholes. |
| In this type of semi conductor majority carriers are electrons which are responsible for conduction of current. | Majority carriers are holes and are responsible for conduction of current. |
| Number of holes are very less in N-type when compared to electrons. Hence holes are known as minority carriers. | Simillarly number of electrons less compared to number of holes in P-type. Minority carriers are electrons. |
N-type is preferred over P-type , mobility of electron is high compared to hole mobility.
