TFET-Based Pixel Source Follower of CMOS Image Sensor for Improved Linearity and High Signal-to-Noise Ratio

In the mobile industry, pixel shrinkage is becoming increasingly important for achieving high resolution and cost reduction, which requires scaling the source follower (SF) as a short channel. This article presents a comprehensive study of SF gain linearity, which is a major cause of nonlinearity in CMOS image sensor (CIS). We also analyze the effectiveness of using the tunneling field effect transistor (TFET) as a pixel SF through technology computer aided design (TCAD) modeling and simulations. In short channel devices, the output resistance, which used to be a very large value in pixel SF gain, decreases and becomes a key factor in the gain value of pixel SF and CIS nonlinearity. Based on our analysis, we found that TFET SF has superior SF gain value and linearity compared to the conventional NMOS SF due to its larger output resistance and lack of body effect. In addition, TFET offers better immunity to side effects caused by channel shortening and more uniform characteristics in process variations, resulting in very low pixel-to-pixel output variation. These advantages are particularly crucial for device scaling in large arrays, where high resolution and reduced device variation are required. Therefore, we propose the use of TFET as a pixel SF that can provide a high signal-to-noise ratio (SNR) and excellent linearity for mobile CIS applications.