Fabrication of Transparent Ferroelectric-Gate Thin Film Transistors with Nonvolatile Memory Operation
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0902-T10-54.1
Fabrication of Transparent Ferroelectric-Gate Thin Film Transistors with Nonvolatile Memory Operation Eisuke Tokumitsu, Masaru Senoo, and Etsu Shin Eisuke Tokumitsu, Takaaki Miyasako and Masaru Senoo Precision and Intelligence Laboratory, Tokyo Institute of Technology, R2-19, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT We demonstrate transparent thin film transistors (TFTs) with nonvolatile memory operation using Bi4-xLaxTi3O12 (BLT) as a gate insulator and indium tin oxide (ITO) as a channel. ITO is also used for the gate, source and drain electrodes. Drain current-drain voltage (ID-VD) characteristics of transparent ITO/BLT ferroelectric-gate TFTs exhibit excellent n-channel transistor operations. “On” current of 0.35 mA was obtained when the applied gate voltage is 6V. On the other hand, the “off” current of the device is as low as 10-10A, which indicates that the ITO channel is sufficiently depleted by the ferroelectric polarization. In addition, drain current-gate voltage (ID-VG) characteristics demonstrate clear counterclockwise hysteresis loop due to the ferroelectric gate insulator. Optical transmittance of the fabricated device is greater than 60% including the quartz substrate.
INTRODUCTION Thin film transistor (TFT) is one of the key devices for flat panel displays and system-onpanel applications and the flat panel displays using TFTs with amorphous Si or poly-Si channel are commercially available. However, these devices reduce the aperture rate of the displays. Recently, TFTs using conductive oxide or organic material have attracted much attention for future display applications. Advantages of these materials are that they are transparent and that they can be fabricated by the low-cost technique such as printing. Transistor operation of oxide-channel TFTs has been demonstrated for various conductive oxides [1-7], however, these devices usually have small “on”-current. In addition, the operation voltage of the most of these devices is still high. For example, ZnO-channel TFT with 100-nm-thick SiO2 gate insulator needs 50 V to obtain a drain current of 50 µA [2]. Prins et al. [6] first reported transparent ferroelectric-gate TFTs using Sb-doped SnO2 channel with PbZr0.2Ti0.8O3 gate insulator. However, “on”-current of their device was less than 40 µA. We have previously reported that the use of the ferroelectric gate insulator drastically improve the “on”-current even at low operation voltage [8,9]. The main reason for this improvement is that the ferroelectric gate insulator can induce much larger charge density than the conventional gate insulator such as SiO2 and Si3N4 even at low electric field because of the remanent polarization. Although the ferroelectric thin film has been considered as a material for the non-volatile memory applications, the capability to induce the huge charge density is an interesting advantage when it is used as a gate insulator of the field-effect transistors (FETs). In our previous work, (Bi,La)4Ti3O12 (BLT) and indium-tin-oxide (ITO) are used
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