# what is the meaning of drift and diffusion current of a diode?

what is the difference between drift and diffusion current of a diode? Both currents flow in the diode but the only difference is one is small (diffusion current) while other one is large (drift current).Whenever electric potential is applied across the terminals of the diode, there is established an electric field inside it. This electric field cause the free electrons to move in one direction and free holes into another direction. This drift of electrons and holes causes a certain amount of current and since it is caused by different types of charges that is why current components are added rather than being cancelled. This total current is called the drift current.

## factors affecting the drift current

This current depends not only the type of semiconductor material, but also on the ability of charge carriers to move through that semiconductor material. The measure of this ability of charge carriers to move through any semiconductor material is called drift mobility and it is represented by

µ. Its value for germanium and silicon is given below

Silicon                                                                           Germinium

µn=0.14m2/(V.s)                                                       µn=0.38m2/(V.s)

µp=0.05m2/(V.s)                                                       µp=0.18m2/(V.s)

The units for drift mobility are square meters per volt-second. It can also be named as carrier velocity per unit field intensity. (m/sec)/(v/m). So we can write it as Vn=E µn  .

Where E is the electric field intensity. The value of drift mobility depends on the temperature as well as on the electric field intensity.

Carrier Mobility can be used to calculate the value of total current density (J) when the value of E is known. Where current density is the current per unit cross sectional area.

J=Jn+Jp=nqn µnE+ nqp µpE

=nqnVn+pqpVp

The drift current is not sensitive to the height of the barrier potential. The reason for this is that the drift current is limited not by how fast the carriers are swept down the barrier, but rather how often. The direction of the conventional current is always opposite to the drifting electrons.

## What is the diffusion current?

Another type of current that flows through the semiconductor material is the diffusion current. It is a transient process so it is actually not for longer time. Basically it flows because of the difference of concentration of charges. Whenever there is a concentration of electrons or holes in one region is high as compared to the second region, the charges will start to flow from high density region to low density region. This process continues until the distribution of charges becomes uniform (ideally). Diffusion current is only found in semiconductor materials but not in the conductors.

The electron diffusion current at the junction is very small as compared to the hole diffusion current as shown in the figure below.

The hole diffusion current depends on the distance and decreases with increasing the value of x.