Photodiode & EMF: Is This Correct?

[sanado]

Just thought i would ask, when a photodiode is acting in reverse bias, it allows current to pass but does not generate voltage. In fact, it has a voltage drop. When in forward bias, it allows current to pass but also generates an EMF.

Is this correct, any response would be appreciated.

 

[marcusl]

You have it backwards. When forward biased the diode conducts just like any pn junction. Under reverse bias the diode does not conduct (no current) in the dark. Photons generate minority carriers that are swept into the extrinsic (doped) regions, thus photons generate a current. A voltage appears across the junction in both cases.

 

[sanado]

But what about a solar cell, isn't that a photodiode in forward bias? Doesnt that produce a voltage/

 

[marcusl]

Well, yes a solar cell is technically a photodiode and it will produce a current in forward bias as well as no bias at all. But one usually calls solar cells "solar cells" or "photovoltaic cells", and "photodiodes" are usually different.

Photodiodes are usually run under reverse bias to improve efficiency and to quickly extract the charge generators created by incident light. The reverse bias also increases the depth of the depletion region which is where the photons are best absorbed. The time response of photodiodes is typically very fast (while solar cells are sluggish). The fastest photodiodes are p-i-n, with an intrinsic (undoped) silicon layer at center that permits complete depletion of charge carriers under modest reverse bias. Photons absorbed in the i region produce electron-hole pairs that are quickly separated due to the bias field. The carriers are rapidly accelerated to the p and n regions where they form the current.