What is Diffusion Current?
The term ‘diffusion’ signifies a process in which there is a motion of molecules of substance from a region of higher concentration of lower concentration. Diffusion Current is defined as the current produced as a result of the motion of charged carriers due to the difference in concentration between two regions.
In simplest terms, we can say the movement of charged particles due to their non-uniform concentration in the two regions, results in the current known as diffusion current.
We know the current is associated with the motion of charged carriers.
Previously we have discussed drift current in a semiconductor material is generated when an external electric field is applied to the material. As there the charged carriers undergo motion due to the presence of external electric potential.
However, unlike drift current, in diffusion current, no external electric field is required for the motion of the charged carriers. As here the charges undergo motion to balance their concentration in the two regions.
Where diffusion current is noticed?
Generally, diffusion current is noticed in non-uniformly doped semiconductor materials. In such semiconductors, the charges move in order to get uniformly distributed. Hence, their motion results in the current through the material.
You must note here that conductors never possess diffusion current. However, drift current is noticed in semiconductors as well as conductors.
Generation of Diffusion Current
Suppose we have a semiconductor bar which is non-uniformly doped with pentavalent impurity thereby forming n-type semiconductor material. The non-uniform doping causes the concentration of charged carriers to be different at the two ends of the material.
We know that the type of majority charged carriers present in the semiconductor depends on the type of impurity with which the material is doped. So, the carriers undergoing diffusion can be either electrons or holes according to the type of semiconductor material.
But as here we have considered an n-type of semiconductor material thus majority carriers will be the electrons.
Diffusion current is mainly due to the majority charged carriers. Thus, in the above figure, we have represented only electrons as the charged carriers in order to understand the process of diffusion.
So, as the charged carriers are of the same type (i.e., electrons) thus there exists a repulsive force between them. Due to this in the non-uniform substrate, the carriers (electrons) gradually move and get diffused from a region where the concentration is high to the region where the concentration is low. Thereby resulting in the process of diffusion to take place.
Due to diffusion, the charged carriers when move generates current referred to as diffusion current.
You must note a point over here that the diffusion of charged carriers continues till the time the carriers get uniformly distributed. Also, a non-uniform semiconductor is necessary for the flow of this current.
If the non-uniform doping generates a p-type semiconductor material then by the same process, holes get diffused from a higher concentration region to lower one. Resultantly generating diffusion current.
The figure given below will help you to understand the non-uniform distribution of electrons:
Here x represents the length of the material. It is clear that, with the increase in x, the doping concentration shows reduction. Suppose n represents the concentration of electrons and because of non-uniform doping it varies w.r.t x. This is shown in the graph below:
From the above two figure, it is clear that the concentration of electrons in the material is changing with x. This means the above-given slope corresponds to the ratio of change in concentration of carriers with a change in the distance.
This rate of change of concentration with distance is known as the concentration gradient.
Hence is represented as:
Similarly, for p-type non-uniformly doped semiconductor,
Diffusion Current Density
The diffusion current density for either n or p-type of semiconductor material is proportional to its respective concentration gradient.
: Jn is the diffusion current density due to electrons
Further, on removing the sign of proportionality,
: Dn represents the constant of diffusion for electrons
: Jp is the diffusion current density due to holes
So, on removing the sign of proportionality
: Dp represents the diffusion constant for holes
Here, as we can see that we have a negative slope for the concentration gradient. The electrons are already of negative polarity but as holes are of positive polarity thus to represent the negative slope for concentration gradient of holes, a negative sign is used in the above equation.
Total Current Density
The overall current density in a semiconductor is due to both diffusion as well as drift current.
Previously, we have seen, the drift current due to electrons and holes is given as:
Also, recently we have derived the diffusion current density for electrons and holes as:
Hence, the total current density due to electrons will be:
Moreover, the total current density due to holes:
Thus, J represents the overall current density in a semiconductor material due to electrons and holes given as:
So, coming to the end of this discussion, we can say that, the diffusion current is the result of the movement of carriers in a non-uniform semiconductor material and is majorly related to the majority charged carriers.