A two-step readout CMOS image sensor active pixel architecture

In this paper, we introduce a 5-transistor (5T) active pixel sensor (APS) structure and a specialized oscillator readout circuit. The pixel keeps a reasonable fill factor of 43% using n-well and p-sub photodiode with an area of 5 μm × 5 μm and generates a two-step signal response to the illumination. The pixel successfully extends output swing to 0.72 V. Measured pixel random noise is 2.5 mV, achieving 51 dB signal-to-noise ratio (SNR). A readout circuit is also implemented using a ring oscillator to replace the traditional design with analog-to-digital converter (ADC) circuitry. It generates frequency output and is recorded by counters to perform signal digitization. The design is implemented with an array of 32 × 92 pixels in a 0.13μm digital CMOS process and tested with a 1.25 V supply voltage.