When a diode is in the "reverse bias" state, it means that the voltage applied across the diode is in the opposite direction of its normal forward current flow. In other words, the negative terminal of the voltage source is connected to the diode's anode, and the positive terminal is connected to its cathode.
In this condition, the behavior of a diode depends on its type: either a semiconductor diode or a Zener diode.
Semiconductor diode (e.g., Silicon diode):
When a semiconductor diode is in reverse bias, it behaves as an insulator or an open circuit. The majority carriers (electrons in N-type material and holes in P-type material) are forced away from the junction, creating a depletion region with no charge carriers near the junction. As a result, there is a significant increase in the width of the depletion region.
In this state, only a tiny reverse current called the "reverse leakage current" flows through the diode. This current is very small compared to the forward current in the diode's normal operating mode. The reverse leakage current is typically in the microampere range for standard semiconductor diodes.
A Zener diode is specifically designed to operate in the reverse breakdown region. When a Zener diode is in reverse bias, it operates in the Zener breakdown region. In this state, the diode experiences a controlled avalanche effect, where the electric field across the depletion region becomes strong enough to free electrons from their covalent bonds, resulting in a flow of current through the diode in the reverse direction.
Unlike regular diodes, Zener diodes are designed to withstand and control this reverse breakdown effect, maintaining a relatively constant voltage across the diode. This makes Zener diodes useful for voltage regulation and protection in electronic circuits.
In summary, a standard semiconductor diode in reverse bias behaves as an insulator with only a small leakage current flowing through it. On the other hand, a Zener diode operates in the reverse breakdown region, allowing a controlled reverse current flow to maintain a constant voltage drop across the diode.