The most common kind of diode in modern circuit design is the semiconductor diode, although other diode technologies exist. The semiconductor diode is cut from a single crystal of a semiconductor material (such as silicon or germanium) to which special impurities has been added during manufacture so that the crystal is now composed of two distinct regions. One region contains semiconductor material of the p-type, which contains more positive charge carriers than negative; the other contains material of the n-type, which has more negative charge carriers than positive. The region of contact between the two types is called the p–n junction (depletion layer), and it is this that acts as the barrier preventing current from flowing.
The semiconductor diode has the valuable property that electrons only flow in one direction across it and as a result it acts as a rectifier. As it has two electrodes it receives its name - diode. In view of this, it is one of the most fundamental structures in semiconductor technology. The bipolar junction transistor, junction FET and many more all rely on the PN junction for their operation. This makes the semiconductor PN junction diode one of the key enablers in today's electronics technology.
