Novel source follower transistor structure without lightly doped drain for high performance CMOS image sensor

Abstract

To realize high-resolution pixels in the CMOS image sensor, it is necessary to reduce low-frequency noise, particularly random telegraph signal (RTS) noise of the source-follower transistor (SFT). To achieve less relative variation of drain noise current, ΔID/ID, a metal–oxide–semiconductor field-effect transistor structure without the lightly doped drain (LDD) for the SFT transistor is proposed. Then, a comparison of RTS noise characteristics between the proposed SFT structure without LDD and the conventional SFT structure with LDD was conducted. Although the RTS noise occurrence probability of the proposed SFT structure without LDD is somewhat greater than that of the conventional SFT structure with LDD, the amplitude of relative variation of drain noise current of the proposed SFT structure is significantly less than that of the conventional SFT. Despite changes in several factors in the proposed SFT, such as effective channel length, trap depth profile in gate oxide, and random dopant fluctuation (RDF), it is believed that the change of trap depth profile is a primary factor for the improved RTS characteristic. Therefore, the proposed SFT is highly desirable for the high-resolution CMOS image sensor.