Semiconductors have conductivity greater than the insulator and less than the conductor. To increase the conductivity of intrinsic semiconductors, it is doped with some impurities. And the manufactured material is an extrinsic semiconductor, which may be an n-type semiconductor or a p-type semiconductor.
Based on the group of impurity material added to the pure semiconductor, the extrinsic semiconductor is further classified into two types n-type semiconductor and p-type semiconductor. Pure semiconductor consists of group-4 material and the impurity material added to increase its conduction capabilities are of group-5 and group-3.
An n-type semiconductor is made of doping group 5 material into the group 4 material. The pentavalent impurity donates electrons (negatively charged) for the conduction, such a conductor is called an N-type semiconductor.
What is an n-type semiconductor?
An N-type semiconductor is defined as the extrinsic semiconductor which is formed by doping of pentavalent impurities such as arsenic, phosphorus, etc. to the pure semiconductor silicon or germanium.
As the nature of an atom is to complete its valence shell. To complete its outermost orbit the atoms of pure semiconductor and the pentavalent impurity form a covalent bond between them. But the outermost orbit of Silicon (used as pure semiconductor) has space for only 4 electrons (as the outermost orbit of Si has a total capacity of 8 electrons and it already had 4 electrons in them). The pentavalent impurity has 5 from which 4 of them occupy the vacant space and leave 1 electron behind.
So the pentavalent impurity atom donated 1 free electron to the conduction band that’s why it is called donor impurity.
Energy band diagram of n-type semiconductor
The energy band diagram of n-type semiconductor is shown below:
The pentavalent impurity atom leaves one electron which is known as a free electron. Though each impurity provides only 1 free electron yet an extremely small amount of impurity provides enough atoms to supply millions of free electrons.
So when the semiconductor is placed at room temperature, the energy generated at room temperature is enough to donate one free electron to the conduction band and these donated electrons are called excess electrons.
The energy required to break that covalent or to move the free electrons to the conduction band is 0.05ev for silicon and 0.01ev for germanium. Those excess electrons jump to the conduction band from the valence band leaving a vacant space behind known as a hole.
Conduction in an n-type semiconductor
The excess electrons produced by the pentavalent impurity moves to the conduction band making electrons responsible for the conduction of electric current in the semiconductor.
When an electric field is applied to the semiconductor, the excess electrons donated by pentavalent impurity atoms move towards the positive terminal of the battery. Which constitutes an electric current. The electrons (negatively charged) are mainly responsible for the conduction so this type of conductivity is called the negative conductivity or N-type conductivity.
Electrons are the majority charge carriers and the holes are the minority charge carriers.