Explanation:

Zener diodes are special types of diodes that are designed to operate in the reverse breakdown region. They are commonly used in voltage regulation applications. The voltage across a zener diode is constant in the reverse direction.

Reverse Breakdown Region:
When a diode is operated in the reverse bias mode and the reverse voltage applied across it exceeds a certain value, known as the breakdown voltage or zener voltage, the diode enters the reverse breakdown region. In this region, the diode allows current to flow in the reverse direction, and its voltage remains relatively constant.

Zener Voltage:
The zener voltage is a characteristic property of a zener diode. It is specified by the manufacturer and is typically in the range of a few volts to several hundred volts. When the reverse voltage across the zener diode exceeds the zener voltage, the diode starts conducting and maintains a relatively constant voltage across its terminals.

Constant Voltage:
In the reverse breakdown region, the voltage across a zener diode remains relatively constant regardless of the current flowing through it. This characteristic makes zener diodes useful for voltage regulation and voltage reference applications.

Applications:
Zener diodes are commonly used in voltage regulation circuits to maintain a constant output voltage. They can be used to protect sensitive electronic components from voltage spikes and transients. Zener diodes are also used as voltage references in precision measurement and control circuits.

Conclusion:
The voltage across a zener diode is constant in the reverse direction when operated in the reverse breakdown region. In the forward direction, the zener diode behaves like a regular diode and exhibits a voltage drop according to its forward characteristics.