Acta Photonica Sinica, Volume. 51, Issue 5, 0523001(2022)
Design and Experiment of Low-voltage 4T-PPD Active Pixel
The 4T Pinned Photodiode (4T-PPD) active pixel is the most widely used pixel structure for CMOS Image Sensor (CIS). In recent years, as the application of CIS has gradually expanded to the Internet of Things (IoT) and Artificial Intelligence (AI) fields, there is an increasing demand for low energy consumption. The basic theory and commonly adopted approach to reduce power consumption are to lower the power supply voltage, while the supply voltage of 4T-PPD is traditionally greater than 2.8 V. In 2016, the study published in JSSC suggested that by improving the timing, the 4T-PPD active pixels could work at 0.9 V, but the readout noise was as high as 83e-rms and the dynamic range was only 50 dB, which could only meet low-quality imaging.Several studies have been conducted on the charge transfer characteristics of traditional high-voltage 4T-PPD. In 2003, FOSSUM E R simulated the charge transfer from PPD to Floating Diffusion (FD) node based on thermionic emission theory. Based on this work, in 2016, HAN Liqiang et al. included non-ideal factors such as the reverse charge injection from FD to PPD. Additionally, in 2019, CAPOCCIA R et al. added an estimate of the thermionic emission barrier height based on the findings of the aforementioned studies. However, these theories were not fully applicable to low-voltage 4T-PPD, since they all assumed a complete photo-generated charge transfer inside the PPD. When the voltage drops, the electrons far away from the transfer gate lack a lateral electric field and stay in the photosensitive area, causing image lag, which will seriously affect the imaging quality.In this paper, a low-voltage 4T-PPD active pixel was designed. First, a theoretical analysis of the internal charge transfer mechanism of PPD was proposed. Three charge transfer mechanisms operate inside the PPD, namely thermal diffusion, self-induced drift, and fringe-field drift. As the charge transfer by fringe-field drift is much faster than thermal diffusion or self-induced drift, the charge transfer time inside the PPD depends predominantly on the distance where the fringing field is absent. According to the derived equations, when the photogenerated charge to the full-well charge
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Wenjing XU, Jie CHEN, Zhangqu KUANG, Li ZHOU, Ming CHEN, Chengbin ZHANG. Design and Experiment of Low-voltage 4T-PPD Active Pixel[J]. Acta Photonica Sinica, 2022, 51(5): 0523001
Category: Optical Device
Received: Dec. 30, 2021
Accepted: Jan. 25, 2022
Published Online: Jun. 27, 2022
The Author Email: CHEN Jie (jchen@ime.ac.cn)