(i) Dark current: The current associated with a photo-diode with an applied reverse bias during operation in the dark (zero illumination) due to background radiation and thermally excited minority saturation current. It is of the order of picoamperes to nanoamperes. Larger active areas or increase in temperature and reverse bias result in higher dark current.
(ii) Dark resistance : The ratio of maximum withstandable reverse voltage to the dark current of a photo-diode is called dark resistance of that diode.
Dark resistance, Rd = \(\cfrac{(maximum \,reverse \,voltage)}{( dark \,current )}\)
