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Diode

In electronics, a diode is a two-terminal device (except that thermionic diodes may also have one or two ancillary terminals for a heater). Diodes have two active electrodes between which the signal of interest may flow, and most are used for their unidirectional current property. The varicap diode is used as an electrically adjustable capacitor.

The directionality of current flow most diodes exhibit is sometimes generically called the rectifying property. The most common function of a diode is to allow an electric current to pass in one direction (called the forward biased condition) and to block it in the opposite direction (the reverse biased condition). Thus, the diode can be thought of as an electronic version of a check valve. Real diodes do not display such a perfect on-off directionality but have a more complex non-linear electrical characteristic, which depends on the particular type of diode technology. Diodes also have many other functions in which they are not designed to operate in this on-off manner.

Early diodes included �cat�s whisker� crystals and vacuum tube devices (called thermionic valves in British English). Today the most common diodes are made from semiconductor materials such as silicon or germanium.

Shockley diode equation

The Shockley ideal diode equation or the diode law (named after transistor co-inventor William Bradford Shockley, not to be confused with tetrode inventor Walter H. Schottky) is the I�V characteristic of an ideal diode in either forward or reverse bias (or no bias). The equation is:


where I is the diode current, IS is a scale factor called the saturation current, VD is the voltage across the diode, VT is the thermal voltage, and n is the emission coefficient, also known as the ideality factor. The emission coefficient n varies from about 1 to 2 depending on the fabrication process and semiconductor material and in many cases is assumed to be approximately equal to 1 (thus the notation n is omitted).

The thermal voltage VT is approximately 25.85 mV at 300 K, a temperature close to �room temperature� commonly used in device simulation software. At any temperature it is a known constant defined by:


where q is the magnitude of charge on an electron (the elementary charge), k is Boltzmann�s constant, T is the absolute temperature of the p-n junction in kelvins

The Shockley ideal diode equation or the diode law is derived with the assumption that the only processes giving rise to current in the diode are drift (due to electrical field), diffusion, and thermal recombination-generation. It also assumes that the recombination-generation (R-G) current in the depletion region is insignificant. This means that the Shockley equation doesn�t account for the processes involved in reverse breakdown and photon-assisted R-G. Additionally, it doesn�t describe the �leveling off� of the I�V curve at high forward bias due to internal resistance.

Under reverse bias voltages the exponential in the diode equation is negligible, and the current is a constant (negative) reverse current value of -IS. The reverse breakdown region is not modeled by the Shockley diode equation.

For even rather small forward bias voltages the exponential is very large because the thermal voltage is very small, so the subtracted �1� in the diode equation is negligible and the forward diode current is often approximated as


The use of the diode equation in circuit problems is illustrated in the article on diode modeling.


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