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Reverse bias improving Bandwidth?

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asadi.siyavash

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Hi I am working on photo diodes I read in many literature for high frequency use photo diode with reverse bias and for linearity and precision application and low frequency use it as photovoltaic, I know in photovolatic we don't have dark current so we have more precision result, but why the response frequency is worse than Photoconductive mode?
Any help would be greatly appreciated,
Siyavash.
 

Even in photovoltaic mode you have the same "dark current"
mechanisms consuming some of the photogenerated carriers.
You just don't see any of it, when it's dark. You will however
see a low-light-level nonlinearity in current vs flux.

Reverse bias creates a field which makes carrier transport
faster (drift) and also improves the charge separation and
reduces geminate recombination (and time-over-target
recombination in general). You also minimize junction
capacitance, by depletion width, while in photovoltaic
mode you maximize it (forward bias). This directly affects
bandwidth / pulse response.
 

The PHOTOCONDUCTIVE or reversed biased p-n device is designed to detect high speed light pulses or the high frequency modulation of a continuous light beam. The reverse voltage increases the junction field strength to accelerate electron/hole transit times and reduces the junction capacity, thereby minimizing capacitive loading effects on the frequency response. PC photodiodes operate over a frequency range from DC to 100 MHz with rise times in the 3 to 12 nanosecond range. The noise current generated by the PC photodiode is a combination of shot noise, excess noise, and, in the case of a guard ring device, Johnson noise. Shot noise is produced by the reverse bias current and exhibits a l/f excess noise characteristic below 1 kHz. The Johnson noise is generated by the channel resistance between the active and guard ring diodes.

The PHOTOVOLTAIC or zero bias detector is designed for ultra low noise, low frequency, instrument applications. The PV frequency response, shunt resistance and junction capacity are active area dependent. The equivalent noise current generated by the device at zero voltage is virtually a flat Johnson noise spectrum from DC to the cutoff frequency.

The design decision to use a PV or a PC photodiode is predicated primarily on the frequency response requirements of the given application. Below 100 kHz, the PV photodiode provides better signal-to-noise performance than that obtained from an equivalent active area PC photodiode; below 1 kHz, the PV silicon photodiode is far superior in signal-to-noise performance.
Click to expand...
Fig1.gif



Ref
http://www.bluehaze.com.au/modlight/modlightrx.htm

LIke all diodes and varicaps reverse voltage reduces junction capacitance and at high impedance this means wider bandwidth but more noise.
 

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