High pH Resolution Extended Gate Type pH Image Sensors with the Charge Accumulation Circuit

In order to observe the pH distribution of biological cells, we propose a pH image sensor with a new structure enabling high density and high sensitivity imaging. In the new proposed pH image sensor we add the charge accumulation circuit to the conventional 2-Tr pixel structure. In the charge accumulation operation, the circuit amplifies a signal by repeatedly transferring the signal charge from the pixel to the capacitor and thus it is expected that the pH resolution can be improved from the viewpoint of Signal-Noise Ratio (SNR). Since the charge accumulation circuit is designed with three transistors and one capacitor and is arranged outside the pixel array, a new pH image sensor is designed without large increase in area. Image sensors with charge accumulation circuits were designed and fabricated. We evaluate the fabricated pH image sensors, observe sensor gain increased by the charge accumulation circuit and realized the performance that enabled pH imaging. From the measurement result, the input-referred sensor noise is reduced by 82% compared with the conventional 2- Tr pixel structure sensor. Therefore, it is possible to realize pH resolution less than 0.02pH.