S-H Jo, M. Bae, B-S Choi, P. Choi, J-K Shin
Kyungpook National University,
Keywords: linear-logarithmic, active pixel sensor, wide dynamic range, CMOS image sensor
Summary:Image sensor technology has progressed from advanced semiconductor technology. The complementary metal–oxide–semiconductor (CMOS) process offers the capability of integrating smart functions on imagers in a single chip. As CMOS technology scales down, the pixel size is reduced for higher resolution sensors; thus, creating high-performance CMOS image sensors has become more difficult. A lower saturation level due to the small supply voltage and a higher noise level due to mismatch of the devices cause the dynamic range to become a very significant problem. The dynamic range of image sensors is defined as the ratio of the saturation signal level to the noise level. Natural scenes have a very wide range of illumination with light intensities varying over the 100 dB range or wider. To capture natural scenes with such a dynamic range, an imager with wide dynamic range (WDR) is required. However, the dynamic range of current commercial cameras without applying a special technique is limited to less than 60 dB. Various approaches have been developed to acquire WDR images. Logarithmic sensors may extremely widen their dynamic range in the high-illumination region by compressing the image signal. However, the conventional logarithmic technique that operates in the sub-threshold region suffers from low sensitivity at low light intensity. In this work, we developed a linear-logarithmic two-transistor active pixel sensor (APS) with wide dynamic range. The proposed APS uses a gate/body-tied MOSFET transistor type photodetector (GBT-PD) for high sensitivity. Also, we applied the negative feedback structure, which offers significant advantages and allows obtaining WDR without complex circuitry. The proposed APS using an output voltage feedback structure allows a significant extension of the dynamic range while maintaining high sensitivity at low illumination. The sensitivity of the proposed APS is 8.23 V/lux·s in the linear region and 0.32 mV/lux·s in the logarithmic region, while the dynamic range reaches over 100 dB.