A Diode is a semiconductor device that essentially acts as a one-way switch for current. It allows current to flow easily in one direction but severely restricts current from flowing in the opposite direction. Diodes are also known as rectifiers because they change alternating current (ac) into pulsating direct current (dc). Diodes are rated according to their type, voltage, and current capacity. Diode has polarity, determined by an anode (positive lead) and cathode (negative lead). Most diodes allow current to flow only when the positive voltage is applied to the anode.
E.g.: Avalanche, Gunn, Constant current, laser, thermal and more.