There  are  two  important  characteristics  of  photo- diode.

(i)   Reverse current-Illumination curve.

The below Fig shows the graph between reverse current (IR) and illumination (E) of a photo-diode.  The reverse current is shown on the vertical axis and is measured in μA. The illumination is indicated on the horizontal axis and is measured in mW/cm2. Note that graph is a straight line passing through the origin.

∴   IR    =  mE

where   m  =  slope of the straight line

The quantity m is called the sensitivity of the photo-diode.

(ii)  Reverse voltage-Reverse current curve.

The below Fig. shows the graph between reverse cur- rent (IR) and reverse voltage (VR) for various illumination levels.   It is clear that for a given reverse-biased voltage VR, the reverse current IR increases as the illumination (E) on the pn junction of photo-diode is increased.