A diode is a semiconductor which has two terminals: an anode and a cathode. They are composed of two layers of semiconducting material, one side doped to make it an N-type semiconductor, and the other doped to make it a P-type semiconductor. The anode is on the side of the P-type, and the cathode is on the side of the N-type.(More information about types of semiconductors can be found here: ‘https://tphelectronics.com/2016/09/30/semiconductors/‘).
One of the main characteristics of diodes is that they only allow current to flow in one direction. This means that they can be used to filter out alternating current, or even rectify it with the use of a diode bridge.
They’re used both in forward bias, and reverse bias. This refers to the direction in which current flows (forward being anode to cathode, and reverse being cathode to anode).
When forward biased no current flows through the diode until more than the minimum voltage is applied (This is because of the depletion region). This means that it can be used to filter out voltages below this value (e.g noise). The value itself is dependent on the type of diode.
When reverse biased only a few nanoamps flow through the diode.However, If the voltage applied is too large, it’ll cause the diode to breakdown. In most cases this will destroy the diode, but the breakdown in zener diodes do not destroy it (this is used to regulate voltage).