An Amorphous Silicon Photo TFT with Si3N4/Al2O3 or HfO2 Double Layered Insulator for Digital Imaging Applications

Abstract This paper focuses on amorphous silicon photo thin-film transistors with double layered insulator using Si3N4/Al2O3 or HfO2 as candidates for the succession of Si3N4 as a traditional insulator in the fabrication of hydrogenated amorphous silicon thin-film transistors. Whether for industry or for research, there is a need to investigate the use of thin gate insulators for these devices to overcome leakage current. Our investigations included direct and transfer characteristics in dark and under illumination, generated photocurrents, external quantum efficiency and responsivity. Performance is evaluated in terms of the dielectric thickness and nature. Improvements in the proposed structures regarding off-current, responsivity and quantum efficiency are achieved via these materials. Comparing with Si3N4/HfO2 transistor, the Si3N4/Al2O3 device shows the lowest off-current. The HfO2 device presents the highest on-current when illuminated. The generated photocurrent is higher for Si3N4/HfO2 transistor revealing a lower amount of trapped charge. Under illumination and for very thin thicknesses, both devices enhance the Si3N4 device off-current and reach Si3N4 single layer dielectric based phototransistor performance. external quantum efficiency and responsivity are higher in HfO2 devices comparing with Al2O3 devices. The results are promising and may support further investigations in order to develop high k gate insulators for MIS photo thin-film transistors.