Drift current

In condensed matter physics and electrochemistry, drift current is the electric current, or movement of charge carriers, which is due to the applied electric field, often stated as the electromotive force over a given distance. When an electric field is applied across a semiconductor material, a current is produced due to flow of charge carriers.

The drift velocity is the average velocity of the charge carriers in the drift current. The drift velocity, and resulting current, is characterized by the mobility; for details, see electron mobility (for solids) or electrical mobility (for a more general discussion).

See drift–diffusion equation for the way that the drift current, diffusion current, and carrier generation and recombination are combined into a single equation.

Overview

In current, the positively charged particles called holes move with the electric field, whereas the negatively charged electrons move against the electric field. It is distinguished from diffusion current (manifested via thermal or density gradients), which results from the random Brownian motion of charge carriers independent of electrical stimulus. If an electric field is applied to an electron existing in free space, it will accelerate the electron in a straight line from the negative terminal to the positive terminal of the applied voltage. But this does not happen in the case of electrons available in good conductors. Good conductors have plenty of free electrons moving randomly in between the fixed positive ion cores. This random movement of electrons in a straight line is known as drift current. Drift current also depends on the mobility of charge carriers in the respective conducting medium.

Drift current in a p-n junction diode

In a p-n junction diode, electrons and holes are the minority charge carriers in the p-region and the n-region, respectively. Due to the diffusion of charge carriers, the diffusion current, which flows from the p to n region, is exactly balanced by the equal and opposite drift current.^[1] In a biased p-n junction, the drift current is independent of the biasing, as the number of minority carriers is independent of the biasing voltages. But as minority charge carriers can be thermally generated, drift current is temperature dependent.

When an electric field is applied across the semiconductor material, the charge carriers attain a certain drift velocity . This combined effect of movement of the charge carriers constitutes a current known as "drift current". Drift current density due to the charge carriers such as free electrons and holes is the current passing through a square centimeter area perpendicular to the direction of flow.

(i) Drift current density J_n, due to free electrons is given by:

$J_n=qn\mu_nE\quad(A/cm^2)$

(ii) Drift current density J_p, due to holes is given by:

$J_p=qp\mu_pE\quad(A/cm^2)$

Where: n - Number of free electrons per cubic centimeter.

p - Number of holes per cubic centimeter

$\mu_n$ – Mobility of electrons in $cm^2/Vs$

$\mu_p$ – Mobility of holes in $cm^2/Vs$

E – Applied Electric Field Intensity in V /cm

q – Charge of an electron = 1.6 × 10⁻¹⁹ coulomb.

^[2]

References

1. Halliday. Physics, Volume 2, 5Th Ed. Wiley-India, 2007. p. 1115. ISBN 978-81-265-1089-4.
2. Halliday. Physics, Volume 2, 5Th Ed. Wiley-India, 2007. p. 1117. ISBN 978-81-265-1089-4.