A Novel Bipolar Tunnel Field-Effect Transistor With High On–Off Current Ratio and Steep Subthreshold Swing for CMOS Image Sensor

Tunnel field-effect transistors (TFETs) have been proven to enhance the dynamic range, reduce power consumption, and improve the nonuniformity among pixels of CMOS image sensors (CISs). However, conventional TFETs suffer from low on-state current and significant bipolar effects, which degrade their on-off current ratio and subthreshold swing, limiting the application of TFETs in CISs. Thus, a novel bipolar TFET is proposed and studied in this article. The novel TFET integrates a gate-biased TFET with a PNP transistor to enhance the on-state current (Ion). Besides, with the help of the lightly doped N-type base of the inner PNP, the leakage current (Ioff) at the off state can also be suppressed efficiently. Thus, a large Ion/Ioff ratio is achieved, resulting in a reduced subthreshold swing (SS) compared with conventional TFETs. Simulation results show that the proposed TFET demonstrates an Ion/Ioff ratio of more than ten decades with Ion up to $9.54~\mu $ A/ $\mu $ m at ${T}_{\text {Si}} =70$ nm, Ioff more than four decades lower than conventional TFET, and SS lower than 40 mV/decade for more than four decades of drain current, exhibiting excellent potential for CIS applications. Moreover, the output characteristics of the proposed device are examined and studied in detail.