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,the current is produced due to flow of charge carriers.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 and/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 a free space, it will accelerate the electron in a straight line from the negative terminal to the positive terminal of the applied voltage.But same thing 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 the majority charge carriers across the junction, an electric field is developed across the junction, called barrier potential. The further diffusion of the majority charge carriers is hindered by this potential barrier. But at the same time, the minority carriers can cross the junction as it is a downhill journey for them. When the minority charge carriers are generated thermally near the junction, they drift slowly along the junction plane, aided by the electric field of the barrier potential. This current, which arises because of the motion of the drift of minority charge carriers, is the drift current in a p-n junction diode. It flows in a direction opposite to the diffusion current which arises owing to the motion of the majority charge carriers across the junction. At equilibrium in an unbiased p-n junction diode, 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 V_d, which is equal to the product of the mobility of the charge carriers and the applied Electric Field intensity E;

        Drift velocity Vd = mobility of the charge carriers × Applied Electric field intensity.

→ Holes move towards the negative terminal of the battery and electrons move towards the positive terminal of the battery. 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 centimeter area perpendicular to the direction of flow.

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

                              Jn = q n μn E (A / cm2)

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

                                                          JP = q p μp E (A / cm2)

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

P - Number of holes per cubic centimeter

μ_n – Mobility of electrons in cm2 / Vs

μ_p – Mobility of holes in cm2 / Vs

E – Applied Electric filed Intensity in V /cm

q – Charge of an electron = 1.6 × 10⁻¹⁹ coulomb.[1]

^[2]

References

1. ^ Halliday. Physics, Volume 2, 5Th Ed. Wiley-India, 2007. pp. 1115. ISBN 8126510897, 9788126510894. 
2. ^ Halliday. Physics, Volume 2, 5Th Ed. Wiley-India, 2007. pp. 1117. ISBN 8126510897, 9788126510894.