Based on the purity level semiconductor is classified into two classes intrinsic semiconductor and extrinsic semiconductor.
As extrinsic semiconductors are the ones in which impurity is added to increase the conduction capabilities of the semiconductor. On the other side, intrinsic semiconductors are pure or undoped semiconductors.
What is an intrinsic semiconductor?
An intrinsic semiconductor is a semiconductor in which no amount of impurity is doped.
There are many semiconductor materials like silicon, germanium, boron, tellurium, etc. but silicon and germanium are most widely used in the electronics industry. Silicon and germanium ( atomic no. 14 & 32 ) are from group IV elements of the periodic table i.e. at their outer-most shell there are 4 electrons are available and these valence electrons are responsible for conduction.
As a silicon atom has 4 valence electrons, it tends to form a bond with adjacent silicon atoms ( and that bond is known as a covalent bond) and complete its valence shell with 8 electrons. The amount of energy required to break their covalent bond is small (1.12eV for Si & 0.7eV for Ge).
But at absolute zero temperature, the conduction band is empty while the valence band is full of electrons. And no electron has enough energy at 0K to move to conduction band from valence band. Thus, it performs as an insulator at absolute temp.
However, at room temperature, the surrounding heat provides sufficient energy to help electrons move from the valence band to the conduction band. Hence, at room temperature, an intrinsic semiconductor is capable of conducting electric current. As energy required to break the covalent band of Si (1.12eV) is greater than the Ge (0.7eV). therefore, the conductivity of germanium is greater than the silicon.
Holes and free electrons generation
When the covalent bond of the semiconductor breaks, the electron leaves its place and creates a vacancy known as a hole. Whenever a free electron is generated, a hole is simultaneously created. Thus, free electrons and holes are generated in pairs. So, the number of free electrons and the number of holes are equal in the intrinsic semiconductor.
N = P
Conduction in an intrinsic semiconductor
When the covalent bond breaks and the free electrons are generated. Then these free electrons move randomly through the crystal. So, when the external electric field is applied to the intrinsic semiconductor the free electrons in the conduction band moves towards the positive terminal and holes move towards the negative terminal. This means conduction in the intrinsic semiconductor is both by electrons and holes.