Diodes are semiconductor devices that allow the flow of current in a single direction. Rectifier diodes are used to convert AC signals into DC. Schottky diodes consist of a semiconductor-metal junction, have a lower voltage drop and are typically used in switch mode power supplies. Zener diodes may conduct current in both directions but with a different voltage threshold, the reverse breakdown voltage is called zener voltage. Zener diodes are used as voltage references, voltage regulators and as overvoltage protection devices.