Characterization and Optimization of a Single-Transistor Active Pixel Image Sensor with Floating Junction Connected to Floating Gate

Xin-Yan Liu,Peng-Fei Wang,Jun Wu,Xi Lin,Xiao-Yong Liu,Chun-Min Zhang,Shuai Zhang,David Wei Zhang

doi:10.1155/2015/167145

Abstract

The application of semifloating gate transistor (SFGT) as the single-transistor active pixel image sensor (APS) is investigated in this paper. This single-transistor (1T) APS can realize the functions of the conventional 3T CMOS image sensor. The device operation mechanism, optimization methods, and transient behavior measurements will be discussed. Because the floating junction of this device is connected to the floating gate, special behaviors such as floating gate voltage pinning effects were observed. The transient time measurement emulating the exposure procedure also confirmed the light sensing function as a single-transistor image sensor.

Highlights

Active pixel image sensor (APS) is a concept of structure, which realizes photon absorption, signal charge storage, signal charge to voltage conversion, and signal voltage amplification inside the pixel
The increased number of transistors leads to decreased fill factor (FF) and in return limits the exposure speed and photodiode capacitance (CPD)
The signal charge to voltage conversion and signal voltage amplification are realized by a single device

Summary

Introduction

Active pixel image sensor (APS) is a concept of structure, which realizes photon absorption, signal charge storage, signal charge to voltage conversion, and signal voltage amplification inside the pixel. Compared with passive pixel sensor (PPS), an APS converts the accumulated signal charges to a potential in the gate. By introducing amplification at a pixel, APS improves image quality compared with PPS [1]. Circuits with shared transistors have been widely used in image sensor to reduce the transistor number. A semifloating gate transistor (SFGT) having imaging function has been proposed. SFGT has the active imaging function and FF of pixel array can be improved. In this paper, this structure concept will be investigated by device simulation and device measurements

Device Structure and Cell Operation

Device Fabrication and Measurements

Conclusion