A Low-Noise Dual-Stage a-Si:H Active Pixel Sensor

This paper presents a low-noise and low-current autozeroed (AZ) dual-stage active-pixel-sensor circuit with a noise figure of 830 input-referred rms electrons and compares its performance with the conventional pixel with a figure of 1950 electrons. Through the analysis, we show how the dual stage can increase the photoinduced signal transconductance gain while keeping the relative output-referred noise low by using lower bias currents. From the analysis and verified-via-noise measurements, we report a 55% reduction in the input-referred noise using the optimized readout. The new design performs reset autozeroing, which stabilizes the gain every reset period. Using a subthreshold-mode a-Si:H TFT photodetector, the designed AZ dual stage provided the maximum gain with transient light. The dual stage is also less sensitive to electrical degradation induced by stressing but increases the overall pixel size.