An optical comparator for particle detection

Thresholding is an important data reduction technique used to eliminate unwanted information, and is commonly used in applications such as particle detection for lab-on-achipsystems. This work presents a novel optical comparator which incorporates a photosensitive area into a five transistor comparator to perform thresholding. The optically active area is formed by enlarging the drain substrate p-n junction of each of the cross coupled NMOS transistors, and exposing one of them tothe light, while shielding the other with metal for symmetry and ark current suppression. The cross-coupled NMOS transistors provide positive feedback to enhance detection speed and latch the result, while the PMOS input transistors provide a biasing mechanism to set an optical threshold. The device was fabricated and tested in a commercially available 3-metal 2-poly CMOS process. Results show that the device can reliably discriminate an optical signal of power on the order of the defined threshold.