Wednesday, May 6, 2009

LED and photodiode

LED

LED physics: as shown in Fig. 1, current flows from the p-side (anode) to the n-side (cathode) when the diode is forward biased. When an electron meets a hole (electron-hole combination), it falls into a lower energy level, and releases energy in the form of a photon.


(Image is from: http://en.wikipedia.org/wiki/Light-emitting_diode)

The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction.

Photodiodes

Photodiodes are one type of photo detectors (other types includes thermal detectors, photoresistors, photomultipliers, etc.).

Same as LED, a photodiode is a semiconductor p–n junction device. But opposite to LED, in a photodiode light is absorbed in a depletion region and generates a photocurrent. There are two operation modes for a photodiode:

Photovoltaic mode: operated in zero bias. The illuminated photodiode generates a voltage which can be measured. This is photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is just an array of large area photodiodes.

Photoconductive mode: operated in reverse bias (opposite to LED). The resulting photocurrent can be measured. Detectors operated in this mode have high linearity and dynamic range.












MaterialSpectrum range
Silicon (Si)400-1000 nm
Germanium (Ge)900-1600 nm
Indium gallium arsenide phosphide (InGaAsP)1000-1350 nm
Indium gallium arsenide (InGaAs)900-1700 nm


References:
[1] http://en.wikipedia.org/wiki/Light-emitting_diode
[2] http://www.rp-photonics.com/photodiodes.html

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