From Understanding Lasers

9.2.3 Diodes, Junctions, and Recombination

The simplest semiconductor device is called a diode, a name meaning that it has two electrical terminals. The first electronic diodes were vacuum tubes that transmitted current in one direction but not the other. Semiconductor diodes, likewise, normally transmit current in only one direction, although there are exceptions.

A semiconductor diode consists of regions of p- and n-type material which meet at a junction layer. In practice, a diode often is made by diffusing an excess of one type of dopant into a slab of semiconductor doped with the other type. For example, an electron acceptor such as aluminum or gallium can be diffused into a slab of n-type silicon, forming a top layer of p-type material in which the holes outnumber the electrons. A junction region in which holes and electrons are equal in number separates the p-type region from the n-type region. The junction typically is only 0.1 to 1 μm thick, and it is the place where the current flow changes and important things happen. Exactly what happens depends on the voltage applied across the junction.

If there is no bias across the junction, charge carriers...


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Diodes
Diodes are electronic components that conduct electric current in only one direction, functioning as a one-way valve. Diodes are manufactured using semiconductor materials such as silicon, germanium or selenium and are used as voltage regulators, signal rectifiers, oscillators and signal modulators / demodulators.
Schottky Diodes
Schottky diodes in their simplest form consist of a metal layer that contacts a semiconductor element. The metal / semiconductor junctions exhibit rectifying behavior (i.e., the current passes through the structure more readily with one polarity than the other).
High Voltage Diodes
High voltage diodes are designed for use in high-voltage applications.
Tunnel Diodes
Tunnel diodes are heavily doped P-N diodes in which electron tunneling from the conduction band in the N-type material to the valence band in the P-type region produces a region of negative resistance. This negative-resistance region is the most important area of operation. As the voltage is increased, the current decreases. This feature makes tunneling diodes especially useful in oscillators and radio frequency (RF) applications.
General Purpose Diodes
General-purpose diodes are electric components that conduct electric current in only one direction, functioning similarly to a one-way valve.

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