The rectifying properties of selenium were first discovered in the 1876 and the Edison effect and rectifying properties related to the two pole vacuum tube were discovered eight years after that in 1884Thus the history of using semiconductors to create diodes actually goes back before that of the vacuum tube.

Initially, diodes like the crystal detectors and selenium rectifiers were made from natural copper oxides like galena and iron pyrites, but with advancements in technology and better refining techniques single crystalline semiconductors entered the scene and thus the age of silicon and germanium was startedBut as germanium offers less resistance to heat, modern day electronic components are based mainly on silicon semiconductors.

Diodes consist of a structure known as pn junctionOne terminal is attached to the p-type semiconductor and is known as positive electrode or anode, and another terminal is attached to the n-type semiconductor and referred to as negative electrode or cathodeCurrent flows from the anode to and current flow in the other direction is restrictedThis rectification phenomenon, the principle operation of the diode, can be used to covert alternating current to unidirectional currentThus a diode can be thought of as a valve for electrical current.

Whereas the pn junction in diodes is a junction with semiconductors on both sides, another type of junction that has been developed is the Schottky barrier junction, which consists of a semiconductor and metal junctionThe Schottky barrier junction has high speeds and low power requirements making it highly useful in many electronic components.

When the current is flowing from the anode to the cathode the diode is known to be forward biased, IF and VF are examples of thisConversely when a negative voltage is applied to anode and positive to cathode the flow of current is stopped and the diode characteristics at this point are known as reverse bias characteristics, IR and VR being examples of the same.