14.6.1. p-n junction diode under forward bias

When an external voltage V is applied across a semiconductor diode such that p-side is connected to the positive terminal of the battery and n-side to the negative terminal, it is said to be forward biased.

In a forward-biased p-n junction diode, the p-side receives a positive charge from the battery while the n-side receives a negative charge. This setup allows charge carriers—holes from the p-side and electrons from the n-side—to move towards the junction.

Think of it like a water pipe in which the p-side is connected to a water source (positive terminal) pushing water (holes) into the pipe, while the n-side (negative terminal) helps remove water (electrons) from the other end.

The applied voltage mostly drops across the depletion region and the voltage drop across the p-side and n-side of the junction is negligible. The direction of the applied voltage (V) is opposite to the built-in potential V0.

The forward bias reduces the width of the depletion region—the area around the p-n junction that is devoid of charge carriers. Since the depletion region is where no charges are present and has a high resistance, a smaller region allows for easier flow of current.

Imagine a narrow river that runs through a valley. The application of forward bias is like widening the river, allowing more water to flow freely, thereby significantly increasing the current that can pass through.

Due to the applied voltage, electrons from the n-side cross the depletion region and reach the p-side (where they are minority carriers). Similarly, holes from the p-side cross the junction and reach the n-side (where they are minority carriers).

This movement of electrons and holes under forward bias leads to an increase in the overall current flowing through the diode. The process is called minority carrier injection, where carriers inject from one side to the other, increasing the charge carriers in the opposing side.

This is like a crowd at a concert where people (charges) are trying to push through an entrance (junction). If more people enter from the left side (n-side) while others leave from the right (p-side), the overall crowd density increases at the door, symbolizing higher current.

If the applied voltage is small, the barrier potential will be reduced only slightly below the equilibrium value, and only a small number of carriers will possess enough energy to cross the junction. Thus, the current will be small.

At low voltages, only those charge carriers with significant energy can overcome the energy barrier created by the depletion region, resulting in a low current. As the applied forward bias voltage increases, more carriers gain sufficient energy, leading to a significant increase in current flow.

Imagine trying to jump over a small fence (the barrier). If the fence is low (low voltage), only a few can easily get over (small current). But if you increase the pressure (apply more voltage), more people can jump over the fence easily.

The total diode forward current is the sum of hole diffusion current and conventional current due to electron diffusion. The magnitude of this current is usually in mA.

The overall current flowing through the diode during forward bias is a combination of two types of movements: one from the holes diffusing into the n-side and the other from electrons moving into the p-side. This results in a current measured in milliamperes (mA), indicating that the diode is effectively conducting.

Think of this current as a busy highway with cars (holes) moving towards a city (p-side) and trucks (electrons) moving towards a rural area (n-side). Both groups contribute to the overall traffic flow, exemplifying how the forward current builds up.

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