What device is used as a stable voltage reference?

A Zener diode is a special type of semiconductor diode designed to reliably operate in its reverse-breakdown region. When reverse-biased at or above its specific Zener voltage, it will hold the voltage across its terminals remarkably constant even if the current flowing through it varies significantly. Because of this unique property of maintaining a steady voltage drop, the Zener diode is the fundamental component used to provide a stable voltage reference in power supplies and other electronic circuits. The other options serve completely different purposes. Analog-to-digital and digital-to-analog converters translate signals between continuous and discrete formats, while an SCR (Silicon Controlled Rectifier) is typically used for switching power or in overvoltage crowbar protection circuits.