I would like to know why is it that a current flows in the attached diagram, even though the diode is reverse biased (having a gap in the middle)?

 

It just so happens that the reverse current in the photodiode is light sensitive. For this reason, if the diode junction can be exposed to light, as in the case of the packaging of a photodiode, the light will affect the reverse current. You will notice that the magnitude of this reverse current is very very low.

hgmjr

 

You can get an explanation of photodiodes at the end of this part of our Ebook - http://www.allaboutcircuits.com/vol_3/chpt_3/12.html

Photodiodes are normally operated with a reverse bias.

 

There is always some current in a reverse biased diode (all kinds), usually on the order of microamps.

The 'gap' in the middle of a diode, the depletion region, is always there to some extent so it must be able to conduct some current or the devices wouldn't work at all. Bipolar transistors require such a region to give you current gain where the base-collector junction must be reverse biased.

 

Oh I see. Dont diodes also steer current in a certain direction, so by making the photodiode reverse biased, does that mean that the current moves up? (even though this doesnt make sense to me as current always flows from + to -)

 

Hello,

A lot of information on silicon photo diodes can be found in this link:
http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm

This is a link from the EDUCYPEDIA page on photo diodes:
http://www.educypedia.be/electronics/composemiconductorsphoto.htm

Bertus