Forward and Reverse Bias — Definition

Definition

Imagine a p-n junction as a one-way street for electricity. This 'street' is formed when a p-type semiconductor (rich in 'holes' or positive charge carriers) is joined with an n-type semiconductor (rich in 'electrons' or negative charge carriers).

At this junction, electrons from the n-side diffuse into the p-side, and holes from the p-side diffuse into the n-side. This movement leaves behind immobile charged ions near the junction, creating a region devoid of free charge carriers, called the 'depletion region'.

This depletion region acts like an insulator and also sets up an internal electric field, creating a 'potential barrier' that opposes further diffusion of charge carriers. Think of this barrier as a small hill that charge carriers need to climb over to cross the junction.

Now, how do we make this one-way street work? We apply an external voltage, which is called 'biasing'. There are two main ways to bias a p-n junction: forward bias and reverse bias.

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Forward Bias — In forward bias, we connect the positive terminal of a battery to the p-type material and the negative terminal to the n-type material. Imagine pushing the charge carriers from both sides towards the junction. The positive terminal repels the holes in the p-type, pushing them towards the junction. Similarly, the negative terminal repels the electrons in the n-type, pushing them towards the junction. This external push effectively works against the internal potential barrier. It's like lowering the 'hill' that the charge carriers need to climb. As the external voltage increases and eventually overcomes the barrier potential (typically around 0.7V for silicon and 0.3V for germanium), the depletion region narrows significantly. This narrowing allows a large number of majority charge carriers (holes from p-side, electrons from n-side) to easily cross the junction, resulting in a substantial electric current flowing through the diode. The diode acts like a closed switch, allowing current to pass.

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Reverse Bias — In reverse bias, we do the opposite. We connect the negative terminal of the battery to the p-type material and the positive terminal to the n-type material. Now, the external voltage pulls the charge carriers away from the junction. The negative terminal attracts the holes in the p-type, pulling them away from the junction. The positive terminal attracts the electrons in the n-type, pulling them away from the junction. This action effectively widens the depletion region and increases the internal potential barrier. It's like making the 'hill' even taller and wider. With a wider, stronger barrier, it becomes very difficult for majority charge carriers to cross the junction. Consequently, only a very tiny current, called 'reverse saturation current' or 'leakage current', flows. This current is due to the movement of minority charge carriers (electrons in p-type, holes in n-type) which are swept across the junction by the strong electric field. The diode acts like an open switch, blocking most of the current. However, if the reverse bias voltage becomes too high, it can lead to 'breakdown', where the diode suddenly starts conducting a large current, often damaging it.