Amorphous silicon balanced photodiode for application in biomolecular analysis

In this work we present an amorphous silicon/amorphous silicon carbide balanced photodiode structure suitable for differential photocurrent measurements in the ultraviolet (UV) and visible range. The device is a three-terminal structure constituted by two series-connected amorphous silicon p-i-n photodiodes. The structure takes advantage of the differential measurement to reveal very small variations of photocurrent in a large background current signal. Application of the device in detection of biomolecules based on the use of photosensors, can allow the increase of both the dynamic range and the sensitivity of the measurement. Several balanced structures with different geometries have been fabricated utilizing a four mask-step process. The devices have been characterized by measuring the common mode rejection ratio as a function of radiation intensity and wavelength and of bias voltage. Experimental results demonstrated that in dark condition differential currents three orders of magnitude lower than the current of each sensor are detected, while under ultraviolet illumination CMRR values around 40 dB have been achieved independent on the bias voltage. These performances are comparable with those obtained by crystalline differential photodiode structures.