Masato Ofuji

42.1: Invited Paper: Improved Amorphous In‐Ga‐Zn‐O TFTs

We review the features of amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs), as well as circuit operation based on these TFTs. We also report a novel TFT structure which improves environmental stability of the TFT operation by taking full advantage of the a-IGZO properties, where a conventional PECVD a-SiNX:H films serve not only as an effective barrier layer but also as a hydrogen source to form the coplanar source and drain.

Circuits using uniform TFTs based on amorphous In-Ga-Zn-O

— High-performance and excellent-uniformity thin-film transistors (TFTs) having bottom-gate structures are fabricated using an amorphous indium-gallium-zinc-oxide (IGZO) film and an amorphous-silicon dioxide film as the channel layer and the gate insulator layer, respectively. All of the 94 TFTs fabricated with an area 1 cm2 show almost identical transfer characteristics: the average saturation mobility is 14.6 cm2/(V-sec) with a small standard deviation of 0.11 cm2/(V-sec). A five-stage ring-oscillator composed of these TFTs operates at 410 kHz at an input voltage of 18 V. Pixel-driving circuits based on these TFTs are also fabricated with organic light-emitting diodes (OLED) which are monolithically integrated on the same substrate. It is demonstrated that light emission from the OLED cells can be switched and modulated by a 120-Hz ac signal input. Amorphous-IGZO-based TFTs are prominent candidates for building blocks of large-area OLED-display electronics.

Fast Thin-Film Transistor Circuits Based on Amorphous Oxide Semiconductor

Five-stage ring oscillators (ROs) composed of amorphous In/Ga/Zn/O (a-IGZO) channel thin-film transistors (TFTs) with the channel lengths of 10 mum were fabricated on a glass substrate. The a-IGZO layer was deposited by RF magnetron sputtering onto the unheated substrate. The RO operated at 410 kHz (the propagation delay of 0.24 mus/stage), when supplied with an external voltage of +18 V. This is the fastest integrated circuit based on oxide-semiconductor channel TFTs to date that operates faster than the ROs using conventional hydrogenated amorphous silicon TFTs and organic TFTs

Materials, Devices, and Circuits of Transparent Amorphous-Oxide Semiconductor

This paper presents the following recent investigations of transparent amorphous-oxide semiconductors (TAOS) from materials to devices and circuits. 1) Composition of metals in TAOS are widely explored with the aim of seeking semiconductors suitable for the channel layers of thin-film transistors (TFTs) composing backplanes for flat-panel displays. It is found in combinatorial approaches to the materials exploration that indium-based ternary TAOS (In-X-O) and their TFTs show the properties and the performance as good as those of the most popular material of amorphous In-Ga-Zn-O (alpha-IGZO) when X = Zn or Ge. 2) Defects and impurities in TAOS are investigated by theoretical approaches. The first-principle calculation of the electron states reveals that charge-neutral oxygen vacancy or interstitial forms the density of states around mid-gap level and does not generate carriers directly, while hydrogen impurity raises the Fermi level beyond the conduction-band minimum and acts as a donor in TAOS. 3) Device structures of TAOS-TFTs are also investigated extensively for better performance and stability. It is found in channel-etch type TFTs with bottom-gate inverse-stagger structures that the TFT characteristics and stability are significantly improved by chemically removing the back-channel layer in a wet-etching process. Coplanar homojunction (CH) structure is proposed as a novel device structure where conductive alpha-IGZO regions work as the source and drain electrodes to the channel region of semiconductor alpha-IGZO. The CH TFTs show excellent characteristics and stability, with low series resistance without any difficulty in making good electrical contact between metals and TAOS. 4) Circuits using TAOS-TFTs are demonstrated. A ring oscillator composed of fifteen-stage inverters with a buffer circuit operates as designed by circuit simulation with a TFT model for hydrogenated amorphous Si TFTs. Pixel circuits composed of three TFTs and one transparent capacitor successfully drive organic light-emission diode cells without a planarization layer on a 2-in diagonal panel having 176 times144 times 3 pixels.