What is drift current?

Drift current is the electric current, or movement of charge carriers, which is due to an electric field applied to a circuit, or may be considered as due to a electromotive force over a certain distance. When an electric field that is a potential difference applied across a semiconductor material, the drift current is produced due to flow of charge carriers. The drift velocity is the average velocity of the charge carriers present in the drift current.

If an electric field is applied to an electron (−) existing in a free space, it will accelerate the electron in a straight line from the negative(−) terminal to the positive terminal(+) of the applied voltage source. But the same thing does not happen in the case of electrons available in good conductors ,that is in metals like copper, aluminium etc. Because good conductors have huge numbers of free electrons moving randomly & this random movement of electrons will drift according to the direction of applied electric field and 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 are the minority charge carriers in the p-region and holes(positive charges) are the minority charge carriers in the n-region. 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. 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 due to the charge carriers such as free electrons and holes is the current passing through a square centimetre area perpendicular to the direction of flow.

Drift current density, due to free electrons is given by:

Jn = qnE Amp / cm2

Drift current density, due to holes is given by:

Jp = qpE Amp / cm2

n - Number of free electrons per cubic centimetre.
p - Number of holes per cubic centimetre.
E – Applied Electric Field Intensity in V /cm.
q – Charge of an electron = 1.6 × 10−19 coulomb.