Lab-on-chip (LoC) device [1] is an example of system where several laboratory functions are integrated onto a single substrate, yielding a sensor-like system requiring minimal quantities of biological samples with a fast response time and high stability. Miniaturization of the system is obtained by micro fluidic structures, while the detection, in most cases, is done off-chip. On-chip optical detection is still a challenge for improving sensitivity and compactness. One of the most promising materials to this aim is amorphous silicon (a-Si:H) and its alloy. The low deposition temperature (below 250 °C) and its physical characteristics prompt the use of this material in different device such as solar cells [2], electronic switching [4], strain sensors [3], and photosensors [5]. The use of thin film a-Si:H photosensors for the detection of biomolecules has been already developed by different research groups [6]. In particular, detection of biomolecules by optical absorbance measurements in the UV range [7] or by measuring the analyte fluorescence [8] has already been demonstrated. In these experiments, a very low current variation (in the order of picoamps) had to be measured with a background current of several orders of magnitude higher. In this case a trade-off between effective dynamic range and resolution has to be considered. Differential measurement is extensively used to reject large common signals and to amplify only their difference. Here, we present an amorphous silicon balanced photosensor structure, integrated with a microfluidic network to perform on-chip detection with high dynamic range in biomedical applications.
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