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

Semiconductors For std 10 CBSE Course

Semiconductors

You know that metals are good conductors of electricity. Substances like glass, plastic, porcelain are insulators. Some substances like germanium and silicon are neither good conductors nor insulators. Their conductivity lies in between that of conductors and insulators. Such substances are called semiconductors.

Conductivity of semiconductors : you know that the atoms of silicon and germanium have four electrons each in their outermost shell. All these electrons are involved in covalent bonding and are not free. Hence these elements should behave like insulators.  

However in these cases, usually a small amount of energy (in the form of heat or light) is sufficient to break a covalent bond and  set and electron free to move. Thus even at room temperatures, a substantial number of electrons will be dissociated from their atoms and this number increases with temperature. This is the reason for these elements exhibiting conductivity.

  Further, when an electron is removed from a covalent bond, it leaves a vacancy, equivalent to positive charge. An electron from a neighbouring atom, can drop into this vacancy, leaving the neighbour with a vacancy. In this way, the vacancy usually called a ‘hole’ can move and serves as an additional ‘charge carrier’