Types of Semiconductors Std 8 to 10

3.3  TYPES OF S SEMICONDUCTORS

In a pure semiconductor, holes and electrons are always present in equal numbers and the resulting conductivity is called intrinsic conductivity. Pure semiconductors are called intrinsic semiconductors.

            The conductivity of semiconductors can be enhanced by the addition of minute traces of impurities (other elements) called dopants. The process of adding dopants is called doping and the resulting semiconductor is called doped semiconductor or extrinsic semiconductor. The conductivity in this case, is called extrinsic conductivity.

            There are two types of extrinsic semiconductors depending upon the types of impurities. They are classified as

(A) n-type semiconductors and

(B) p-type semiconductors.

These are used in the manufacture of diodes and transistors.

(A)   n-type semiconductors : if a very small amount of pentavalent impurity for example antimony (or arsenic or phosphorous), is introduced into the crystal of germanium, four out of five electrons of each impurity atom enters into bonds with the nearest germanium atoms, to form covalent bonds. The fifth electron is set free. Such free electrons act as current carries.

The added pentavalent impurity donates electrons to the semiconductor and the semiconductor becomes n-type semiconductor. The name n-type indicates that the majority charge carries are electrons. There will be free-electrons and holes in equal numbers due to breaking of some covalent bonds.

n-type Semiconductors

(B)   p-type semiconductors : When a trivalent impurity like boron, indium, gallium or aluminum is introduced into a germanium crystal, each impurity atom takes away one electron from the neighboring germanium atoms, to form covalent bonds. This results in the creation of  holes. Electrons from neighboring atoms can fill up these holes and there will be apparent motion of holes. Thus holes act as current carries. The added trivalent impurity accepts electrons from the semiconductor and the semiconductor becomes a p-type indicates that the majority charge carries are holes equivalent to positive charge. There will be free electrons and holes in equal numbers due to breaking of some covalent bonds.

p-type semiconductors