Question

When P-N junction diode is in forward biased condition, the flow of current is mainly due to (A) both by drift and diffusion of electrons (B) the drift of electrons (C) the diffusion of electrons (D) none of the a above

Short answer

In a forward-biased P-N junction diode, the main cause of current flow is due to the diffusion of electrons and holes across the junction. Drift current is negligible because the electric field generated by the external power source opposes the built-in electric field. Therefore, the correct answer is (C) the diffusion of electrons.

Step-by-step solution

Understand the forward-biased P-N junction diode

In a forward-biased P-N junction diode, the P-side of the diode is connected to the positive terminal of the power source while the N-side is connected to the negative terminal. As a result, the electric field generated inside the diode by the external power source is in the opposite direction to that of the built-in electric field.

Drift and diffusion of electrons

There are two processes responsible for the movement of carriers (electrons and holes) within a semiconductor: drift and diffusion. Drift refers to the movement of carriers due to the applied electric field, whereas diffusion refers to the natural movement of carriers from a high concentration region to a low concentration region in an attempt to equalize the concentration.

Current flow due to drift

When an electric field is applied to the P-N junction diode, the drift of electrons and holes occurs due to the force experienced by the carriers in the presence of the electric field. In the case of a forward-biased diode, the drift current is negligible due to the fact that the electric field generated by the external power source is in the opposite direction to that of the built-in electric field. Therefore, the drift of electrons does not contribute significantly to the current flow in a forward-biased diode.

Current flow due to diffusion

The diffusion of electrons and holes plays a major role in the current flow of a forward-biased P-N junction diode because the concentration of carriers in a forward-biased diode is higher near the junction (where P and N regions meet), resulting in a gradient between the P and N regions. As a result, electrons and holes tend to move from their high concentration regions to low concentration regions, driven by the concentration gradient.

Determine the final answer

Since drift of electrons is not significant in the forward-biased P-N junction diode condition and diffusion of electrons and holes contributes the most to the current flow, we can conclude that the correct answer is: (C) the diffusion of electrons

Key concepts

Forward Bias in a P-N Junction Diode

A P-N junction diode becomes forward-biased when the P-side, or the positively doped side, is connected to the positive terminal of a power supply, and the N-side, or negatively doped side, is connected to the negative terminal. This setup reduces the potential barrier allowing charges to flow across the junction.
As soon as the diode is forward-biased, the built-in electric field, which normally opposes charge movement, is countered by the external power supply's electric field.
This alignment causes the barrier to decrease further, making it easier for charge carriers to cross the junction. The external voltage ensures a continuous flow of charges, facilitating current flow through the diode. This current flow is an essential aspect of semiconductor devices and contributes significantly to their widespread applications.

Drift and Diffusion in Semiconductors

Two main mechanisms govern the movement of charge carriers in semiconductors: drift and diffusion.
Drift is the movement of charge carriers, such as electrons and holes, under the influence of an external electric field. This mechanism usually contributes more to current flow in reverse-biased junctions. However, in a forward-biased diode, the drift current is minimal, as the external field weakens the built-in field rather than strengthening it.

• Drift: Movement due to an electric field.
• Diffusion: Move freely from higher to lower concentration.

Diffusion occurs naturally as particles spread from areas of high concentration to low concentration, aiming to establish equilibrium. In a forward-biased P-N junction, diffusion is much more dominant. The concentration gradient created causes electrons and holes to move across the junction, which facilitates the majority of current flow.

Current Flow in Semiconductors

Current flow in semiconductors, particularly in a forward-biased P-N junction, primarily results from the process of diffusion. In this condition, an increased concentration of electrons accumulates on the N-side and holes on the P-side. These carriers move towards regions of lower concentration, across the junction.
The flow of electrons from the N-side to the P-side and holes from the P-side to the N-side generates the actual current known as diffusion current. Although drift can also contribute to current under certain circumstances, in the specific case of a forward-biased diode, the drift current is negligible.
Thus, in semiconductor devices, understanding and controlling diffusion is crucial, as it is a major contributor to the operational current flow.