In this paper we demonstrate design, fabrication and characterization of polycrystalline silicon (poly-Si) photodetectors monolithically integrated on top of a silicon oxynitride (SiON) passive photonic circuit. The devices are developed for operation at the wavelength of ~850nm. Interdigitated PIN structures were designed and compared with conventional lateral PIN detectors. The devices, fabricated in standard CMOS technology, exhibit low dark current values of few nanoamperes. The best responsivity of 0.33A/W under a reverse bias of 9V was achieved for lateral PIN detectors with 3-μm interelectrode gap, coupled vertically to the optical waveguide. The applicability of devices for lab-on-chip biosensing has been proved by demonstrating the possibility to reproduce the sensor's spectral response.