Drift and Diffusion

DRIFT AND DIFFUSION

The net current flows across a semiconductor has two components:

(i) Drift current

(ii) Diffusion current.

Drift Current

Definition

The electric current produced due to the motion of charge carriers under the influence of an external electric field is known as drift current.

When electrical voltage is applied to a material as shown in fig. 3.13, electric field is produced at every point within the material.

The charge carriers are forced to move in a particular direction due to the electric field. This is known as the drift motion and the current is known as drift current.

Drift current density in a semiconductor due to electrons

J_n (drift) = n µ_ne E    ...(1)

Drift current density due to hole

J_p (drift) = n µ_p e E    ...(2)

where n and p are number of electrons and holes per unit volume. μ_n and u_p are the mobilities of electrons and holes respectively, e ts charge of electrons and E is electric field.

So total drift current density

J = J_n (drift) + J_p (drift)

J = ne μ_n E + pe μ_p E ...(3)

For intrinsic semiconductor

J = n_i e (μ_n + μ_p ) E   (n = p = n_i)

Diffusion Current

Definition

The non-uniform distribution of charge carriers creates the regions of uneven concentrations in the semiconductor.

The charge carriers move from the regions of higher concentration to the regions of lower concentration. This process is known as diffusion. The current is known as diffusion current.

Consider a semiconductor having a concentration gradient of electrons dn / dx within the semiconductor.

The electrons diffuse from high concentration to low concentration due to the concentration gradient as shown in Fig. 3.14.

Rate of flow of electrons through unit area ∝ - (dn / dx)

Here, negative sign denotes that the electrons are diffusing from higher concentration to lower concentration region.

Rate of flow of electrons through unit area = D_n (dn / dx)

where D_n is a proportionality constant and it is known as diffusion coefficient of electrons.

Rate of flow of electrons through unit area

= - e × -D_n (dn / dx)

Rate of flow electrons through unit area is the diffusion current density of electrons J_n (diffusion)

J_n (diffusion) = e D_n (dn / dx)

Similarly, the diffusion current density of holes is given by

J_p (diffusion) = -e D_p (dp / dx)

where D_p is diffusion constant of holes.