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