Wide dynamic range photogate-type active pixel sensor using a self-regulation principle

A wide dynamic range photogate-type active pixel image sensor based on a self-regulation principle is reported. A 5 x 5 element prototype sensor was fabricated using a 5 mu m single-poly, single-metal negative channel metal oxide semiconductor (NMOS) silicon integrated circuit technology. The key techniques are based on the feedback of a floating diffusion (FD) potential to the photogate applied voltage and on the control of the sub-threshold characteristic of the photogate using an ion implantation process. The proposed sensor makes it possible to automatically adjust the sensitivity of each pixel using only three transistors. The conversion of the irradiated light to the output voltage is linear under low-light conditions, while it is nonlinear under bright light. The photo-electric conversion characteristics are similar to those of a linear-logarithmic image sensor. The dynamic range can be expanded without the need for an additional capacitor, an analog-to-digital (A/D) converter or any complex circuitry. Compared with a conventional photogate-type image sensor that was fabricated on the same chip, the dynamic range was expanded by 28 dB.