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Characteristics of a Photodiode
Spectral Response
The magnitude of the photocurrent generated by a photodiode is dependent upon the wavelength of the incident light. Silicon photodiodes exhibit a response from the ultraviolet through the visible and into the near infrared part of the spectrum. The spectral response peaks in the near infrared region between 800 nm and 950 nm. The shape of thespectral response curve, especially in the blue and UV part of the spectrum, can be altered by choosing among a number of manufacturing processes developed at PerkinElmer. An example of this is the “blue enhanced” photodiode whose sensitivity to light from the short wavelength part of the spectrum has been increased. Optical filters can also be added to change the spectral response. There areseveral popular ways to characterize the spectral response, following are brief descriptions of the most common ones. Re = ISC/Ee where: Re = responsivity (µA/[µW/cm2]) ISC = short circuit photocurrent (µA) Ee = irradiance (µW/cm2) The VTP1188S has a typical responsivity of 0.15 µA/(µW/cm2) @ 940 nm.

Quantum Efficiency - Q.E.
If the photodiode operated at 100% efficiency each photon of lightstriking the detector would result in one electron being added to the photocurrent. The Q.E. relates, as a percentage, the energy per photon and the quantum yield, electrons per photon. Q.E. = (124 SR)/λ where: Q.E. = quantum efficiency (%) SR = radiometric sensitivity (A/W) λ = wavelength of light (micrometer) The VTP process produces devices with a Q.E. of 75% @ 940 nm. The responsivity of a siliconphotodiode varies with temperature dependent upon the wavelength of the light. Curves in the data sheet section show plots of the responsivity temperature coefficient versus wavelength of light. The temperature coefficient is negative at the shorter wavelength of light and becomes positive in the infrared part of the spectrum. Of interest is the fact that for most diodes there is a range ofwavelengths where the temperature coefficient is effectively zero.

Radiometric Sensitivity - SR
Sr is the ratio of the short-circuit photocurrent generated by the photodiode (A-amps) divided by the energy of the incident light (Wwatts). For photodiodes made with the VTS process, a typical radiometric sensitivity of 0.6 A/W can be expected at the peak spectral response wavelength of 925 nm. Curves ofthis measure of sensitivity are often plotted with the SR normalized in order to show relative spectral response. SR = ISC / Φ where: SR = radiometric sensitivity (A/W) ISC = short circuit photocurrent (A) Φ = radiant flux (W)

Responsivity - Re
Re is a measure of sensitivity which takes into account the active area of the photodiode chip. This parameter is obtained by dividing the short-circuitlight current (mA-microamps) by the energy of the light per unit area (µW/cm2).


Characteristics of a Photodiode
One of the standard methods used to specify the sensitivity of a photodiode is to state its short circuit photocurrent (ISC) at a given light level from a well defined light source. The most commonly used source is an incandescent tungsten lamp running at a colortemperature at 2850 K. At 100 fc, ISC can vary over a range from microamps to milliamps depending on chip size and package employed. Photodiodes have unity internal gain. In order to increase their sensitivity to light one can either increase the active area of the photodiode chip itself or use lenses to increase the effective active area. The relationship between active area and sensitivity tendsto be linear: doubling the active area doubles the output current. If the external terminals are shorted together a short circuit photocurrent ISC will flow. When this happens, for the ideal case, VF = 0. ISC = IP (ideal case) For the real world case: IO = IP – IF – ISH ISC = IP – ISAT ( e where: KT/q = 0.026 @ 25°C and since VO = 0, thus: VF = ISC RS ISC = IP – ISAT ( e
q ( I S C R S ) ⁄ KT...
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