Diodes are semiconducting components which allow the flow of electrons or current only in one direction. A diode is composed of a P-type semiconductor and an N-type semiconductor. Current flows only from the P-type to the N-type semiconductor. Under these conditions, the diode is said to be forward biased. And when current tries to flow from N-type to P-type material, it is called reverse bias. In an actual diode, the two terminals are labeled as cathode (K) and anode (A).
Diodes can be used to rectify ac voltage into dc voltage. Diodes can also be used as logic gates in electronic circuits. You can make AND gates or OR gates with diodes.
In the forward biased region of operation, practical diodes have a voltage drop of 0.7 V. This is not the actual response of a diode but it is a good approximation of the actual responce.
In the reverse biased region, diodes are supposed to act as open circuits. No current will flow through the diode and appears as if is not even there. However, all practical diodes have a maximum reverse bias voltage. If it exceeds this maximum voltage, the diode breaks down and will not perform as it is supposed to. There are however, types of diodes which makes use of this property of diodes in its operation.
The characteristics of the diode can be seen in the specification sheet for the particular diode. These specification sheets are released by the manufacturer of the diodes. On these specification sheets, the complete characteristics of the diode are listed. Its minimum current, maximum current, minimum voltage, etc. There are also several types of diodes depending on their usual region of use. The diode which is used in its reverse bias region are called Zener Diodes.
