A Tunable CMOS Image Sensor with High Fill-Factor For High Dynamic Range Applications.

Several CMOS imager sensors were proposed to obtain a high dynamic range imager (>100dB). However, as a drawback these imagers implement a large number of transistors per pixel resulting in a low fill factor, high power consumption, and high complexity CMOS image sensors. In this work, a new operation mode for 3T CMOS image sensors is presented for high dynamic range (HDR) applications. The operation mode consists of biasing the conventional reset transistor as an active load to photodiode generating a reference current. The output voltage achieves steady-state when the photocurrent becomes equal to the reference current similar to the inverter operation in the transition region. At a specific bias voltage, the output swing from o to Vdd in a small light intensity range, however, a high dynamic range is achieved using multiple readouts at different bias voltage. For high dynamic range operation, different values of bias voltage can be applied from each one the signal can be captured and then compose a high dynamic range image. Compared to high dynamic range architectures in the literature the proposed CMOS image pixel shows as an advantage high fill-factor and lower complexity. Moreover, the pixel does not operate in integration mode allowing higher speed operation. A prototype was fabricated at 0.35µm CMOS technology. Experimental results show that by applying five different control voltages it is possible to obtain dynamic range about 100dB.