When a Si photodiode is operated in the dark (zero illumination), the current versus voltage characteristics observed are similar to the curve of a rectifier diode as shown by curve (1) in figure. This dark current in Si photodiodes range from 5 pA to 10 nA.
When light is incident on the photodiode, the curve shifts to (2) and increasing the incident illuminance (light level) shifts this characteristic curve still further to (3) in parallel. The magnitude of the reverse voltage has nearly no influence on the photocurrent and only a weak influence on the dark current. The normal reverse currents are in tens to hundreds of microampere range.
The I-V characteristics of a photodiode showing dark current and photocurrent for increasing illuminance
The almost equal spacing between the curves for the same increment in luminous flux reveals that the reverse current and luminous flux are almost linearly related. The photocurrent of the Si photo-diode is extremely linear with respect to the il- luminance. Since the total reverse current is the sum of the photocurrent and the dark current, the sensitivity of a photodiode is increased by minimizing the dark current.
