Tunneling-based Source Follower for Low Noise Image Sensor

Abstract

Controlling in-pixel noise is important for taking clear images in a low illumination. Previous research has studied the use of a buried channel (BC) that separate the current path away from the surface [1]. However, channels far from the surface make gate control difficult, reduce voltage gain (Av), and impairs linearity. The non-uniformity between pixels generates fixed pattern noise, and as the signal becomes smaller, the noise is amplified along with the signal. Therefore, excellent Av and noise characteristics are simultaneously required. P-doped source of tunneling field-effect transistor (TFET) eliminates body effect enough to disregard body effect transconductance (gmb). Also, TFET has higher output resistance (ro) than NMOS because of no channel length modulation [2]. The low gmb and high ro cause TFET to achieve higher Av than NMOS. However, the use of TFET is difficult to achieve good output swing. This problem can be solved by the tunneling SF (TSF) as shown in Fig. 1(a). A p-doped grounded area ensures sufficient tunneling current at low VGS. In addition, TSF employs BC to achieve low noise. Interestingly, despite forming BC, TSF has high Av. This is because TSF forms an L-shaped BC. High gate controllability is obtained with surface attached region where the most gate control is performed. Even though the current path is adjacent to the surface in the BTBT generation area, the high electric field rapidly accelerates electrons and allows them to be transmitted with less influence from traps.