A diode is reverse-biased when its n-type region is connected to the positive terminal of the supply voltage and its p-type region is connected to the negative terminal.

At lower reverse bias voltages, the small leakage current through the diode is primarily due to minority carriers, and the IV curve in this region is almost flat.

As the reverse bias voltage increases beyond a threshold voltage, known as the breakdown voltage or the knee voltage, a steep rise in the current is observed for a small variation in the voltage.

In heavily doped diodes at high reverse voltages, the electric field across the depletion region is strong enough for the valence electrons to break free from their bonds. This creates electron-hole pairs, resulting in a significant increase in current.

In lightly doped diodes, high reverse voltages accelerate the minority carriers across the depletion region. They collide with atoms, creating additional electron-hole pairs. This process cascades, leading to a rapid increase in current.

In lightly doped diodes, the current increases gradually compared to the heavily doped diodes.