Question

Distinguish between ‘intrinsic’ and ‘extrinsic’ semiconductor.

Answer 1

Hint: Semiconductor consists of two words- semi means imperfect/moderate and conductor means presence of free electron. Intrinsic meaning nothing from outside and extrinsic means something from outside.

Complete step by step answer:
Semiconductors are basically divided into two categories based on addition of impurity which are given below. We all know that semiconductors deal with holes and electrons present in valence and conduction bands. Semiconductors are the 4th group of elements in the periodic table. So its valency is four i.e. electrons in the outermost shell are four. Therefore its element is called tetravalent.

a.) Intrinsic semiconductor (pure form)
b.) Extrinsic semiconductor (impure form)

1.) Intrinsic semiconductor: It is one of the types of semiconductor. It is a pure form of semiconductor crystal because no external impurity has been introduced in it. Since it is pure form, therefore it contains only tetravalent elements. These elements are stable until external thermal energy has given to it since its octet is completely filled. In intrinsic semiconductor the number of holes in the valence band must be equal to the number of electrons in conduction band i.e. concentration of holes and electrons will be equal in intrinsic semiconductor and it is temperature dependent.

2.) Extrinsic semiconductor: it is an impure form of semiconductor because external impurity has been introduced. The process of addition is called doping. In an extrinsic semiconductor the electrons and holes concentration and hole concentration in conduction band and valence band respectively at a given temperature are different from the intrinsic carrier concentration at the temperature. Depending on the type of doping, extrinsic semiconductors again divide into two types.

1.) n-type
2.) p-type

In n-type extrinsic semiconductor, pentavalent impurity is doped in pure semiconductor. In pentavalent impurity the number of electrons in the outermost shell are five electrons. Therefore covalent bonds have formed with one excess electron. Such a doped semiconductor is called an n-type semiconductor and impurity atoms are called a donor. Here electrons are majority charge carriers and holes are minority charge carriers.

In p-type extrinsic semiconductor, trivalent impurity is doped in pure semiconductor, in trivalent impurity the number of electrons in outermost shell are three. Therefore covalent bonds are formed with one excess hole. Such a doped semiconductor is called a p-type semiconductor and impurity atoms are called an acceptor. Here holes are majority charge carriers and electrons are minority charge carriers.

Note: The main difference is of impurity. Concentration of doping must be considered because concentration of doping is responsible for change concentration of free charges which leads to flow of current. Note that a small amount of impurity must be added. Large amounts of impurity may lead to change in structure of elements.