Hideya Kumomi

High-mobility thin-film transistor with amorphous InGaZnO4 channel fabricated by room temperature rf-magnetron sputtering

Thin-film transistors (TFTs) were fabricated using amorphous indium gallium zinc oxide (a-IGZO) channels by rf-magnetron sputtering at room temperature. The conductivity of the a-IGZO films was controlled from ∼10−3to10−6Scm−1 by varying the mixing ratio of sputtering gases, O2∕(O2+Ar), from ∼3.1% to 3.7%. The top-gate-type TFTs operated in n-type enhancement mode with a field-effect mobility of 12cm2V−1s−1, an on-off current ratio of ∼108, and a subthreshold gate voltage swing of 0.2Vdecade−1. It is demonstrated that a-IGZO is an appropriate semiconductor material to produce high-mobility TFTs at low temperatures applicable to flexible substrates by a production-compatible means.

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.

Combinatorial approach to thin-film transistors using multicomponent semiconductor channels: An application to amorphous oxide semiconductors in In–Ga–Zn–O system

A combinatorial approach was applied to thin-film transistors (TFTs) using amorphous In–Ga–Zn–O semiconductor channels. A large number of TFTs, having n-type channels with different chemical compositions, were fabricated simultaneously on a substrate. A systematic relation was clarified among the compositional ratio of In:Ga:Zn, oxygen partial pressure in film deposition atmosphere, and TFT characteristics. The results provide an experimental basis to understand the roles of each metallic element in the In–Ga–Zn–O system. This information leads to a guideline to tune the metallic compositions for required TFT specifications.

Two-dimensional numerical simulation of radio frequency sputter amorphous In-Ga-Zn-O thin-film transistors

We reported on a two-dimensional simulation of electrical properties of the radio frequency (rf) sputter amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs). The a-IGZO TFT used in this work has the following performance: field-effect mobility (μeff) of ∼12 cm2/V s, threshold voltage (Vth) of ∼1.15 V, subthreshold swing (S) of ∼0.13 V/dec, and on/off ratio over 1010. To accurately simulate the measured transistor electrical properties, the density-of-states model is developed. The donorlike states are also proposed to be associated with the oxygen vacancy in a-IGZO. The experimental and calculated results show that the rf sputter a-IGZO TFT has a very sharp conduction band-tail slope distribution (Ea=13 meV) and Ti ohmic-like source/drain contacts with a specific contact resistance lower than 2.7×10−3 Ω cm2.

Amorphous In–Ga–Zn–O coplanar homojunction thin-film transistor

A fabrication process of coplanar homojunction thin-film transistors (TFTs) is proposed for amorphous In–Ga–Zn–O (a-IGZO), which employs highly doped contact regions naturally formed by deposition of upper protection layers made of hydrogenated silicon nitride (SiNX:H). The direct deposition of SiNX:H reduced the resistivity of the semiconductive a-IGZO layer down to 6.2×10−3 Ω cm and formed a nearly ideal Ohmic contact with a low parasitic source-to-drain resistance of 34 Ω cm. Simple evaluation of field-effect mobilities (μsat) overestimated their values especially for short-channel TFTs, while the channel resistance method proved that μsat was almost constant at 9.5 cm2 V−1 s−1.

Sputtering formation of p-type SnO thin-film transistors on glass toward oxide complimentary circuits

Thin film transistors (TFTs) using polycrystalline tin oxides (SnO–SnO2) channels were formed on glass by a conventional sputtering method and subsequent annealing treatments. SnO-channel TFTs showed p-type operation with on/off current ratios of ∼102 and field-effect mobilities of 0.24 cm2 V−1 s−1. Incorporation of excess oxygen to SnO channel layers did not generate holes but did electrons, which in turn led to n-type operation. This result is explained by transformation to a local SnO2-like structure and finally to SnO2. We propose a simple method to fabricate complimentary circuits by simultaneous selective formation of p- and n-channel TFTs.

Density of States of a-InGaZnO From Temperature-Dependent Field-Effect Studies

Temperature-dependent field-effect measurements were performed on radio-frequency sputtered amorphous In-Ga-Zn-O thin film transistors (TFTs). We studied the effect of temperature on the TFT electrical properties. We observed that the field-effect mobility (mu) increases and the threshold voltage (V T) shifts negatively with temperature, while the current on-off ratio and subthreshold slope (S) remain almost unchanged. We also observed that the TFT drain current (ID) is thermally activated, and the relation between the prefactor (ID0) and activation energy (E a) obeys the Meyer-Neldel rule. The density of localized gap states (DOS) was then calculated by using a self-consistent method based on the experimentally obtained E a. The result shows good agreement with the DOS distribution calculated from SPICE simulations.

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

Electrical Instability of RF Sputter Amorphous In-Ga-Zn-O Thin-Film Transistors

Bias-temperature-stress (BTS) induced electrical instability of the RF sputter amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) was investigated. Both positive and negative BTS were applied and found to primarily cause a positive and negative voltage shift in transfer (<i>I</i> _DS-<i>V</i> _GS) characteristics, respectively. The time evolution of bulk-state density (<i>N</i> _BS) and characteristic temperature of the conduction-band-tail-states (<i>TG</i>) are extracted. Since both values showed only minor changes after BTS, the results imply that observed shift in TFT <i>I</i> _DS-<i>V</i> _GS curves were primarily due to channel charge injection/trapping rather than defect states creation. We also demonstrated the validity of using stretch-exponential equation to model both positive and negative BTS induced threshold voltage shift (Delta<i>V</i> _th) of the a-IGZO TFTs. Stress voltage and temperature dependence of Delta<i>V</i> _th evolution are described.