In this article, we shall discuss the difference between Drift Current and Diffusion Current. The concept of both the drift current and the diffusion current can be easily understood by considering a semiconductor device like a PN junction diode.
As we know from the basic theory of semiconductor physics, every semiconductor material is made up of two types of charge carriers namely electrons and holes. Where the electrons are the negative charge carriers and holes are the carriers of positive charge.
Depending on the majority and minority, i.e. concentration of charge carriers, the semiconductors are of two types namely N-type semiconductors and P-type semiconductors. In the N-type semiconductor, the majority charge carriers are electrons and minority charge carriers are holes, whereas in the case of P-type semiconductors, the majority charge carriers are holes and minority charge carriers are electrons.
When an electric field is applied to a semiconductor, the majority and minority charge carriers are directed to flow in respective directions, as a result, an electric current start flowing through the semiconductor which is known as drift current. Since both majority and minority charge carriers are present in the same crystal of the semiconductor, hence there is also a gradient in the charge concentration which causes the diffusion of charge carriers from higher concentration to lower concentration of charge. The current due to the diffusion of charge carriers due to the difference in concentration is called diffusion current.
We will discuss the basics of drift current and diffusion current before going to discuss the differences between them.
What is Drift Current?
When the charge carriers move in a semiconductor due to the applied electric field, the resulting current is known as drift current.
In the case of semiconductors, two types of charge carriers exist namely electrons and holes. When an external electric field is applied to the semiconductor with the help of a battery, the holes are accelerated towards the negative terminal of the battery and the electrons are accelerated towards the positive terminal of the battery under the influence of the electric field. Hence, due to the movement of charge carriers caused by the electric field, an electric current flows through the semiconductor which is known as drift current.
What is Diffusion Current?
When there is a difference in the concentration of charge carriers in a single crystal of semiconductor, the movement of charge carriers takes place and establishes an electric current in the semiconductor. This electric current due to the charge concentration gradient is known as diffusion current.
The charge carriers have a tendency to move from the region of higher concentration to the region of lower concentration and this movement of charge carriers causes the flow of diffusion current in the semiconductor.
Difference between Drift Current and Diffusion Current
The following table gives all the major differences between drift current and diffusion current.
| Parameter | Drift Current | Diffusion Current |
| Definition | The electric current in a semiconductor caused by the movement of charge carriers due to the external electric field is known as drift current. | The electric current in a semiconductor caused by the flow of charge due to the concentration gradient of charge is known as diffusion current. |
| Need of external source of energy | An external source of electrical energy is required for the flow of drift current through the semiconductor. | There is no need for an external source of electrical energy for the existence of diffusion current. It is because the diffusion current flows due to the difference in charge concentration. |
| Direction of flow | The direction of flow of the drift current is the same as the direction of the applied electric field. | The direction of flow of diffusion current depends on the slope of charge carrier concentration. |
| Law obeyed | Drift current obeys Ohm’s law, i.e. the drift current is directly proportional to the applied voltage. | Diffusion current obeys Fick’s law, i.e. the rate of diffusion of charge carriers in a semiconductor crystal is proportional to the concentration gradient. |
| Change in direction of flow | If the direction of an externally applied electric field is changed, the direction of the drift current through the semiconductor will also change. | The direction of diffusion current is almost independent of the applied electric field. It mainly depends on charge concentration. |
| Dependency on permittivity | Drift current depends on the permittivity of the material. | Diffusion current does not depend on the permittivity of the material. |
Conclusion
In the above table, we discussed all the major differences between drift current and diffusion current. One major difference between the two is that the drift current is due to the movement of charge carriers caused by force applied by the external field. In contrast, the diffusion current is due to the diffusion of charge carriers from the region of high density into a region of low charge density.
