Semiconductor diode
A two-terminal electronic device that utilizes the properties of the semiconductor from which it is constructed. In a semiconductor diode without a pn junction, the bulk properties of the semiconductor itself are used to make a device whose characteristics may be sensitive to light, temperature, or electric field. In a diode with a pn junction, the properties of the pn junction are used. The most important property of a pn junction is that, under ordinary conditions, it will allow electric current to flow in only one direction. Under the proper circumstances, however, a pn junction may also be used as a voltage-variable capacitance, a switch, a light source, a voltage regulator, or a means to convert light into electrical power. See Semiconductor
The conductivity of a semiconductor is proportional to the number of electrical carriers (electrons and holes) it contains. In a temperature-compensating diode, or thermistor, the number of carriers changes with temperature. See Thermistor
In a photoconductor the semiconductor is packaged so that it may be exposed to light. Light photons whose energies are greater than the band gap can excite electrons from the valence band to the conduction band, increasing the number of electrical carriers in the semiconductor. See Photoconductivity
In some semiconductors the conduction band has more than one minimum. This results in a region of negative differential conductivity, and a device operated in this region is unstable. The current pulsates at microwave frequencies, and the device, a Gunn diode, may be used as a microwave power source.
A rectifying junction is formed whenever two materials of different conductivity types are brought into contact. Most commonly, the two materials are an n-type and a p-type semiconductor, and the device is called a junction diode. However, rectifying action also occurs at a boundary between a metal and a semiconductor of either type. If the metal contacts a large area of semiconductor, the device is known as a Schottky barrier diode; if the contact is a metal point, a point-contact diode is formed. See Schottky effect
The contact potential between the two materials in a diode creates a potential barrier which tends to keep electrons on the n side of the junction and holes on the p side. When the p side is made positive with respect to the n side by an applied field, the barrier height is lowered and the diode is forward biased. Majority electrons from the n side may flow easily to the p side, and majority holes from the p side may flow easily to the n side. When the p side is made negative, the barrier height is increased and the diode is reverse-biased. Then, only a small leakage current flows: Minority electrons from the p side flow into the n side, and minority holes from the n side flow into the p side. The current-voltage characteristic of a typical diode is shown in the illustration. Rectifying diodes can be made in a variety of sizes, and much practical use can be made of the fact that such a diode allows current to flow in essentially one direction only.
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