A novel CMOS pixel will be described that combines the sensitivity and speed of response at low light levels of an integrating pixel with the wide dynamic range of a logarithmic response. However, as with all pixels, the low-light sensitivity of this novel pixel is ultimately limited by the dark current that flows through the pixel in parallel with the photocurrent. Since the dark current increases rapidly with temperature, this effect is particularly important for cameras that operate in hot environments. Results are reported that show that the dark current in these novel pixels and the temperature dependence of the dark current can both be reduced by careful pixel layout. Alternatively, the user generated function that controls the response of the pixel can be changed to accommodate the dark current and significantly increase the low-light sensitivity of the pixel.
A wide dynamic image CMOS image sensor with a user adjustable logarithmic photo-response is presented. A pMOS switch and a time-dependent reference voltage are integrated into a three-transistor (3T) pixel structure to implement a logarithmic response. Several pixels have been manufactured using a 0.25μm standard CMOS technology. Compared to the conventional logarithmic response pixel based on a diode-connected transistor, the proposed pixel combines a wide dynamic range of 120dB with much higher responsivity (250mV/decade) and better dark response.