Homework Help: Whats the characteristics of a photodiode?

"Resistance" is actually a rather poor concept to apply to a device like a photodiode, because diodes are highly nonlinear, yet the Ohmic concept of resistance involves current varying linearly with applied voltage. Diodes never behave like simple resistors.

Photodiodes are usually operated in reverse-bias, in which no current flows through them in darkness. (Well, there's always some leakage, but it's irrelevant.) A reverse-biased photodiode is thus an "infinite resistance," because essentially no current flows through it.

When you shine light on it, some charge carriers are liberated, and a current begins to flow. If the external applied voltage remains the same, the current increases, and so the "resistance" could be said to be lower.

When you shine more light on it, even more charge carriers are liberated, and a greater current flows. If the external applied voltage remains the same, the current increases, and so the "resistance" could be said to be even lower.

- Warren

 

Photodiodes work in the opposite manner of an LED(light-emitting diode). Shining light on a suitably arranged p-n junction can produce a current in the circuit that includes the juction.

As Chroot pointed out, you cannot use the concept of "resistance" in the normal sense when dealing with diodes. In case of photo-diode, PN junction is connected in reverse bias. The electron-hole pairs resulting from the photons incident on the PN-junction add to the minority carriers due to reverse bias. The radiation when incident will create electron-hole pairs on both sides of the junction.

Photon-induced electrons in the conduction band of the p-type will move across the junction down the potential hill with the theramlly generated minority carriers. Similarity, holes produced in the valence band of n-type add to the current by moving across the junction to the p-side. When no light is incident the reverse current is simply the reverse saturation current due the minority carriers.

 

i have concern about the photodiodes as well, can anyone tell me where i can find a graph of the characteristic of a photodiodes please?