# Calculating photodiode current and voltage

1. Jul 24, 2010

### jollett

Hi I'm pretty new to the forums. I've been searching and can't find anything on this. Can anyone show me how to calculate the current/voltage produced by a photodiode?

2. Jul 24, 2010

### Bob S

Here are the specs for a typical Hamamatsu Si PIN photodiode.

http://jp.hamamatsu.com/resources/products/ssd/pdf/s3590-08_etc_kpin1052e07.pdf

Look up the spectral response for the wavelength you are interested in (see dotted line for 100% quantum efficiency* in plot). PIN photodiodes are essentially current sources, because the current output is nearly independent of the applied voltage. The depletion depth is much larger than the photon absorption length.

* 100% quantum efficiency is 1 photoelectron per incident photon

Bob S

3. Jul 31, 2010

### Mike_In_Plano

People tend to use the diodes in one of the following three ways:

1. Reverse biased and treated as a current source
2. Shunted into a low value impedance (typically 50 ohms)
3. Operated in the "photo-voltaic" mode

This first method is by and far preferred in that it gives you a current that's proportional to the number of photons. With the proper amplifier, this method is typically fairly sensitive and has a fair bandwidth.

The second method is often used for wide bandwidth applications. Most applications involving a 50 ohm load are based on photo diodes going into test equipment and the sensitivity is poor. However, the linearity is still pretty good - voltage is proportional to photons for a reasonable excitation.

The third method involves leaving the photo diode "unloaded." The following instrument or amplifier has a high input impedance and with even a little light, the diode reaches a saturation point where the output voltage is limited. Typically, you can get up to about .6v and it takes progressively more light to make a moderate increase output voltage.