Amorphous silicon twin photodiode structure for differential current measurements

In this paper we present a new amorphous silicon structure called “twin photodiode” constituted by two p–i–n series connected photodiodes biased at the same reverse voltage. The structure takes advantage of the differential current measurement to reveal very small variations of photocurrent in a large background current signal. The removal of the background allows us to increase the dynamic range of the input signal and the sensitivity of the detection system. In addition, the differential approach allows us to reduce the common mode signal due to the effect of temperature variations and instability of light source intensity.We have fabricated several twin structures for detection of ultraviolet radiation with different geometries utilizing a four mask-step process. Experimental results have demonstrated the ability of our structure to detect differential currents three orders of magnitude lower than the current of each sensor. The achieved common mode rejection ratio keeps constant with reverse bias voltage and increases with increasing wavelengths, varying from 30 dB nm at 254 nm to 42 dB at 365 nm.