What describes a Zener diode's function?

A Zener diode is specifically designed to allow current to flow in the reverse direction when the voltage exceeds a certain level, known as the Zener breakdown voltage. This is a fundamental aspect of how it functions, particularly for voltage regulation applications. When connected in reverse bias, the Zener diode maintains a constant output voltage across its terminals, despite changes in the current flowing through it, making it highly effective in regulating voltage within a circuit.

In voltage regulation, the Zener diode essentially serves as a reference voltage element, ensuring that the voltage remains stable. This characteristic is crucial in power supply circuits where maintaining a steady voltage is necessary to protect sensitive electronic components.

The other options do not accurately describe the primary purpose of a Zener diode. While it does conduct current (but not exclusively in one direction, as both forward and reverse directions are involved under different conditions), it is not primarily for amplifying current or transforming AC to DC, which are functions associated with other types of diodes. Therefore, the description focusing on the Zener diode's ability to operate in the breakdown region for voltage regulation is the key characteristic that identifies its function.