Semiconductor materials can be categorized into two main classes: intrinsic semiconductors and extrinsic semiconductors. Intrinsic semiconductors are the pure form of semiconductor materials while extrinsic semiconductor materials are formed by adding some impurity to the pure semiconductors.
The main difference between intrinsic and extrinsic semiconductor is that intrinsic semiconductors are pure in form, no form of impurity is added to them while extrinsic semiconductors being impure, contains the doping of trivalent or pentavalent impurities.
These two classes of semiconductors can be differentiated on the basis of a number of factors like the addition of impurity or doping, electrical conductivity, the density of electrons, and holes in the semiconductor materials.
Difference between Intrinsic and Extrinsic Semiconductors – Table
Let’s have a look at the differences between the two with the help of a table.
| Basis of Comparison | Intrinsic Semiconductor | Extrinsic Semiconductor |
| Impurity present in the material | Intrinsic semiconductors are pure forms of semiconductors, hence they do not have a significant amount of impurity. | Extrinsic semiconductors are made by adding some impurity to the pure form of semiconductors. |
| Electrical conductivity | They exhibit poor electrical conductivity. | Electrical conductivity in the case of extrinsic semiconductors is significantly high as compared to intrinsic semiconductors. |
| The density of charge carriers | In intrinsic semiconductors, the number of free electrons in the conduction band is equal to the number of holes in the valence band. | The number of electrons and holes are not equal in extrinsic semiconductors and depends on the type of extrinsic semiconductor. |
| Dependency of electrical conductivity | The electrical conductivity of intrinsic semiconductors depends only on the temperature. | The electrical conductivity of extrinsic semiconductors depends on the temperature as well as the amount of doped impurity. |
| Position of Fermi level | In intrinsic semiconductors, the Fermi energy levels lie in the middle of the valence and conduction band. | In extrinsic semiconductors, the Fermi level shifts towards the valence or conduction band. |
| Examples | Examples include the crystalline forms of pure silicon or germanium. | Examples include Silicon (Si) and germanium (Ge) crystals with impurity atoms of As, Sb, P, etc., or In, B, Al, etc. |
What are intrinsic semiconductors?
Intrinsic semiconductors are the pure form of semiconductor materials. No external impurity is added or doped in intrinsic semiconductors. Since intrinsic semiconductors are made from highly pure semiconductor materials, they are also known as pure semiconductors.
In an intrinsic semiconductor, the number of free electrons in the conduction band is equal to the number of holes in the valence band.
For intrinsic semiconductors the band gap between the conduction band and valence band is small and the Fermi energy levels lie in the middle of the conduction and valence band.
Electrical conductivity for intrinsic semiconductors is very low and it only depends on the temperature. The conductivity increases exponentially with the temperature. Extrinsic semiconductors are classified into p-type and n-type semiconductors but intrinsic semiconductors are not classified further.
The free charge carriers in intrinsic semiconductors are generated by a mechanism called thermal generation and it is the breaking of some fraction of bonds between the adjacent atoms in the semiconductor solid.
One thing is to be noted here is that though the intrinsic semiconductors may have a very little amount of impurity, the impurity concentration should be appreciably lower than the concentration of free carriers.
What are extrinsic semiconductors?
It is a type of semiconductor in which an impurity is added at a controlled rate to make it conductive.
An intrinsic semiconductor can conduct a little amount of current even at room temperature, but this small current is not useful for the preparation of various electronic devices. So to make these semiconductors conductive a small amount of suitable impurity is added to the semiconductors.
Doping is a process by which an impurity is added to a semiconductor material. During the preparation of extrinsic semiconductor, the amount of impurity and type of impurity has to be analyzed carefully. The impurity is added to the semiconductors to increase the number of charge carriers (free electrons or holes) to make them conductive.
If we add a pentavalent impurity having 5 valence electrons to the pure semiconductor a large number of free electrons will exist.
If we add a trivalent impurity having three valence electrons, a large number of holes will exist in the semiconductor.
In extrinsic semiconductors, the concentration of electrons and holes are not equal. Extrinsic semiconductors are better in conductivity than intrinsic semiconductors. The conductivity of extrinsic semiconductors depends on temperature as well as impurity concentration. Unlike intrinsic semiconductors, extrinsic semiconductors are of two types: p-type and n-type. Extrinsic semiconductors are classified on the basis of the type of element doped to the semiconductor.
Conclusion
From the above discussion, we have concluded that extrinsic and intrinsic are two important types of semiconductor materials. Intrinsic semiconductors are pure semiconductor materials without any impurity in them while extrinsic semiconductors are doped with certain types of impurities to increase their conductivity.
The differences between intrinsic and extrinsic semiconductor are discussed and we have also explained the intrinsic and extrinsic semiconductors to make things more clear to you.
Author
Deepak Yadav
Aligarh Muslim University, Aligarh
