To make any semiconductor device the very first step is to obtain a semiconductor in its purest form and a semiconductor is said to be pure when its ratio of impurity in semiconductor material is like one part of impurity in 100 million parts of semiconductor material. So the resulting semiconductor is know as Intrinsic semiconductor.
Define as: An extremely pure semiconductor is known as Intrinsic semiconductor.
On the basis of energy band diagram:
At absolute zero, intrinsic semiconductor behaves as an insulator. It’s conduction band is empty (i.e. there is no free electron in CB )and valance band is completely filled.(fig.1)
At room temperature, it’s temperature is raised then some of the valence electrons are lifted to conduction band leaving behind holes in valence band. (fig.2)
Electrons reaching at the conduction band are free to move at random. Holes created in the crystal also move at random in the crystal. So conductivity increases when temperature is raised. This behavior shows that they have negative temperature co- efficient of resistance i.e. when temperature increase resistivity decreases and vice versa. (so,conductivity increases, when temperature raised. )
When an electric potential difference is applied across a pure semiconductor kept at room temperature,current conduction took place by two kind of charge carriers (electrons & holes).
The free electrons drift towards the positive terminal and holes drift towards negative terminal of battery.Total current is sum of current by electrons and holes.This current is called drift current.
Drift current is defined as :
The flow of current in the semiconductors,constituted by the drift of free electrons in the conduction band and holes in the valence band due to external energy applied to them is known as drift current.