What is Diffusion Current in Semiconductors and Its Derivations

The diffusion current is mainly generated in semiconductors where the doping is not consistent. So to make the doping consistent, the charge carriers flow within this takes place from the region of high concentration to low concentration. So this is known as diffusion current. Generally, this process doesn’t occur within conductors. The main function of this current within the semiconductor is due to the dominating current on the junction. At the stability condition, the net currents are zero as the forward current is unbiased through the reverse drift current however both the currents like drift & diffusion are present in the depletion region. This article discusses an overview of what do you mean by diffusion current and its formula.

What is Diffusion Current?

Definition: The diffusion current can be defined as the charge carriers within a semiconductor like holes or electrons flow from high concentration state to low concentration state. The region where a number of electrons can be present is known as higher concentration whereas the area where a low number of electrons can be present is known as low concentration. The flow of current can be generated because of the flow of charge carriers from high regions to low regions. The process of diffusion mainly occurs within a semiconductor when that is doped not-consistently.

Diffusion Current in N-type Semiconductor

The diagram of an n-type semiconductor is shown below. When we consider a non-consistently doped N-type semiconductor material, a number of electrons are present at a high-level region whereas the low number of electrons present at the low-level regions. The occurrence of the number of electrons at the high-level side in the semiconductor material can be more. Consequently, a repulsive force can be experienced from each other. The flow of electrons in the semiconductor material will be from a high region to a low region to get a consistent electron concentration.

Therefore, the material gets equivalent to electrons concentration. The electrons flow from the left region to the right region will form current. In this material, the process of diffusion mainly occurs in the same way. Both the currents like drift & diffusion will have occurred within semiconductor devices. This current can occur when the electric field is applied and it does not happen within a conductor. The direction of this current is similar or reverse when compare with drift current.

Diffusion Current Formula

The diffusion current formula for the concentration gradient and density equation is discussed below.

Concentration Gradient

In any semiconductor material, there is an existence of the electrons otherwise holes concentration. The disparity within this electron otherwise holes concentration can be called as a concentration gradient. The density is comparative to the concentration gradient.

If the concentration gradient’s value is high, subsequently the density of the current will be high. If the value of the concentration gradient is less, then the diffusion density is also low.

The equations in between the densities & concentration gradients can be written as

The equation of concentration gradient & current density of the N-type semiconductor is shown below.

Jn ∝ dn/dx

The equation of concentration gradient &  current density of the P-type semiconductor is shown below.

Jp ∝ dn/dx

Here, with respect to holes as well as electrons, it signifies the density

In the above equations, ‘Jn’ is the current density because of electrons

‘Jp’ is the diffusion of current density because of holes.

Diffusion Current Density Equation

The diffusion density due to the carrier concentration of electrons can be written by m²/V.s

Jn = +eDn dn/dx

Likewise, the diffusion density due to the carrier concentration of holes can be written as

Jp = -eDp dp/dx

The above equation is for the densities of diffusion densities with respect to electrons and holes but the overall density of the current of respective holes or electrons can be given by the sum of the diffusion & drift current.

In the above equations, ’Dn’ and ‘Dp’ are the diffusion coefficient of electrons as well as holes

The total diffusion density with respect to electrons is written as

Jn = Drift Current + Diffusion Current

Jn = enμnE +eDn dn/dx

The entire diffusion density of holes is given through the individual density equations of electrons & holes. So the density of total current can be written as

Jp = Drift current + Diffusion current

Jp = epμpE – eDp dp/dx

FAQs

1). What is Diffusion Current in Polarography?

An electrode like dropping mercury in polarography, the flow is controlled through the diffusion rate of the active solution types across the gradient concentration generated by removing molecules or ions at the surface of the electrode.

2). What is the diffusion length?

The average length of a carrier which flows between generation & recombination is known as diffusion length.

3). What is current?

It is the electric charge carrier’s flow rate.

4). What is the current formula?

The formula is I = V/R

Where,

‘I’ is the electric current

‘V’ is an electric voltage

‘R’ is the resistance of the wire

5). What Drift Means?

Drift current is the flow of charge carriers like electrons & holes because of the applied electric field or voltage.

Thus, this is all about an overview of diffusion current and the equations of these current densities can be described for electron as well as holes. Here is a question for you, what is the difference between drift & diffusion current?