As a part of JST PRESTO program, Associate professor Masaharu Kobayashi, Institute of Industrial Science, the University of Tokyo, has developed a ferroelectric FET (FeFET) with ferroelectric-HfO2 and ultrathin IGZO channel. Nearly ideal subthreshold swing (SS) and mobility higher than poly-silicon channel have been demonstrated.
FeFET is a promising memory device because of its low-power, high-speed and high-capacity. After the discovery of CMOS-compatible ferroelectric-HfO2 material, FeFET has been attracting more attention. For even higher memory capacity, 3-D vertical stack structure has been proposed as shown in Fig. 1(a).
For 3-D vertical stack structure, poly-silicon is typically used as a channel material. However, poly-silicon has very low mobility in nanometer thickness region due to grain boundaries and extrinsic defects. Moreover, poly-silicon forms a low-k interfacial layer with ferroelectric-HfO2 gate insulator. This results in voltage loss and charge trapping which prevents low voltage operation and degrades reliability, respectively as shown in Fig. 1(b).