Differential amorphous silicon photosensors for biomolecular applications

This talk presents an amorphous silicon/amorphous silicon carbide structure designed for differential measurements in order to reveal very small variations of photocurrent in a large background current signal. This situation occurs for example during biomolecular recognition, when DNA or micotoxin detection is performed under UV radiation utilizing a-Si:H photosensor. The removal of the background current allows to increase the dynamic range of the input signal and the sensitivity of the detection system. In addition, the differential approach allows to reduce the common mode signal due to the effect of temperature variations and instability of light source intensity. The presented structure called “twin diode” is constituted by two n-i-p series connected junctions fabricated with a four mask-step process. Experimental results, achieved biasing at the same reverse voltage the two diodes, have demonstrated the ability of our structure to detect differential currents three orders of magnitude lower than the current of each sensor. We found that the common mode rejection ratio keeps constant with reverse bias voltage and increases with increasing wavelenghts, varying from 30 dB nm at 254 nm to 42dB at 365 nm.