Why does the reverse bias configuration lead to a very small current?

A semiconductor junction (diode) is formed when a p-type semiconductor (having positive holes as majority carriers) is connected to an n-type semiconductor (with negative electrons as majority carriers). A diode has two terminals, known as the anode (connected to the p-type side) and cathode (connected to the n-type side).

There are two bias configurations for a diode: forward bias and reverse bias. In forward bias, the anode (p-type side) is connected to the positive terminal of the voltage source, and the cathode (n-type side) is connected to the negative terminal. This allows the majority carriers (holes and electrons) to flow easily, resulting in a high current flow. In reverse bias, the anode (p-type side) is connected to the negative terminal of the voltage source, while the cathode (n-type side) is connected to the positive terminal. This configuration makes it difficult for the majority carriers to flow, resulting in a very small current.

In reverse bias configuration, the electric field produced by the battery opposes the diffusion of majority carriers (holes and electrons) across the junction. The external electric field attracts the majority carriers away from the junction, resulting in a wider depletion region (insulating region with no charge carriers) and a higher-energy barrier that prevents the majority carriers from flowing across the junction. Therefore, the current flow in reverse bias is minimal.

Though majority carriers are unable to cross the junction in reverse bias, a very small number of minority carriers (holes in the n-type side and electrons in the p-type side) possess enough thermal energy to overcome the energy barrier and flow across the junction. This small current is called reverse saturation current, and it is almost independent of the applied voltage. However, this current is significantly smaller than the forward current in forward bias, usually in the range of microamperes (μA) or nanoamperes (nA) for typical silicon and germanium diodes. To summarize, the reverse bias configuration leads to a very small current because the external electric field opposes the flow of majority charge carriers, widening the depletion region, and raising the energy barrier. Only a small number of minority carriers with sufficient thermal energy can cross the junction, causing a very small reverse saturation current.