Physics · Semiconductor Electronics : Materials, Devices And Simple Circuits · NEET
Diffusion current is due to MAJORITY carriers. In the n-side electrons are the majority and in the p-side holes are the majority. Because one side is crowded with electrons and the other with holes, this concentration difference makes holes move from p to n and electrons move from n to p. This movement of majority carriers is the diffusion current. Drift current, on the other hand, is due to MINORITY carriers swept by the junction field.
Holes diffuse from the p-side to the n-side (p to n), because the p-side has many holes and the n-side has very few. Electrons diffuse from the n-side to the p-side (n to p), for the same reason. Both movements are because of the concentration gradient. Conventional current direction (diffusion current) is from p to n, because moving holes and electrons moving the opposite way both give a current pointing p to n.
When the junction forms, diffusion current is large at first and drift current is small. As carriers cross over, the depletion region and its electric field grow. This growing field increases the drift current. The process stops when the drift current becomes exactly equal to the diffusion current. Since the two currents are equal in size and opposite in direction, they cancel, so the NET current across the junction is zero. This is the equilibrium state with no external battery.
Drift current is caused by the built-in electric field inside the depletion region. This field points from the n-side (positive space charge) to the p-side (negative space charge). If a minority electron on the p-side or a minority hole on the n-side wanders near the junction, the field sweeps it across. So drift current is due to minority carriers being pushed by the field, and it flows from n to p (opposite to diffusion current).
No, they are in opposite directions. Diffusion current (majority carriers) flows from the p-side to the n-side. Drift current (minority carriers) flows from the n-side to the p-side. Because they oppose each other and become equal at equilibrium, the net current is zero. This opposite direction is the whole reason the two currents can cancel.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
When the junction just forms, the diffusion current is large and the drift current is small. As the depletion region and its electric field grow, the drift current increases until it equals the diffusion current.
The built-in field points from the n-side to the p-side, because the n-side has a positive space-charge layer (ionised donors) and the p-side has a negative space-charge layer (ionised acceptors). This field drives the drift current.
Yes, drift current is still present under forward bias, but it is very small (a few microamperes) compared with the large diffusion current from injected carriers (in milliamperes), so it is usually neglected there.
As electrons leave the n-side and holes leave the p-side, the n-side becomes positive and the p-side becomes negative. This difference of potential is the barrier potential, and it opposes further diffusion of carriers.
