Lateral Drift-Field Photodetector for high speed 0.35µm CMOS imaging sensors based on non-uniform lateral doping profile: Design, theoretical concepts, and TCAD simulations

A novel photodetector concept for high charge transfer speed and low image lag is presented. The key feature of the so-called Lateral Drift-Field Photodetector (LDPD) is a lateral electric drift field inside the photoactive area of the device. This results in a significant acceleration of the charge collection compared to conventional diffusion based detectors and, therefore, this device is an ideal candidate for high speed applications like 3D Time-of-Flight imaging. Furthermore, the LDPD concept shows a superior noise performance as the charge collection node is decoupled from the readout node. The LDPD device was developed in a standard 0.35µm CMOS process with only one additional mask step being necessary in order to implement the lateral doping gradient.

I. Introduction

Nowadays, CMOS image sensors based on active pixel sensors (APS) are considered the preferred technology for most imaging applications [1]. This success of CMOS imagers is owed in part to some of their intrinsic advantages compared to charge-coupled devices (CCD) such as x-y pixel addressing, in pixel amplification, and signal processing. Recently, these characteristics have also enabled a number of developments in the field of 3D Time-of-Flight (ToF) and low-light level imaging [2–8]. However, the use of high doping profiles, thin gate oxides, and low power supply voltages, necessary in modern sub-micron CMOS technologies, still adversely affects the performance of many analog circuits and photodetectors. This proves to be especially challenging for high speed imaging applications like 3D imaging which require large-area, low dark current, and low noise [2]. Taking into account these intrinsic constraints of CMOS technology a new, fast detector concept has been developed.