Design And Research Of3T And4T CMOS Image Sensor Pixel TCAD Simulation

Recently, with the continuous development of semiconductor technology,performance of CMOS image sensor continues to improve. As the low power consumption,high performance, low cost, CMOS image sensors have been widely implemented invarious applications, such as social life and industrial production etc. CMOS imagesensors via their superiority, gradually become a viable alternative to CCDs in greatmajority domain. Photodiode is the most commonly used, in which the current responserate is usually0.4~0.5μA/μW and the spectral response with the range of0.4~1.1μm.Four transistor (4T) active pixel is the mainstream structure of CMOS image sensors. Thepixel surface dark current can be inhibited by adopted the pinned photodiode (PPD), so itis important to research of simulation and optimization of the4T pixel.This paper combines the working mechanism of3T and4T and uses the simulationtool Medici-TCAD to simulate and research dark current, photocurrent generated in thephotosensitive elements and the transient process of the pixel circuit. The dark current of3T pixel photodiode at3V reverse voltage is less than10nA, and the photocurrent canreach tens of microamperes and linear with illumination. This paper simulation to theelectron concentration and potential distribution of4T active pixel before and after thetransfer gate TG opening, in order to have an intuitive understanding for the lightabsorption of4T pixel and the issues of residual charge. This paper also simulate to thedark current of PPD and the result proves that the dark current reduced greatly comparedwith photodiode. For an extra ion implantation to the N-buried layer, it can eliminate thebarrier, decrease the residual charge concentration after the charge transfer and to optimizethe image lag phenomenon.