The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Keynote Speech: Amorphous Oxide Semiconductors and their Social Implementation

Unpredictable amorphous oxide semiconductors are
creating a new display market
Hideo Hosono, honorary and Institute Professor , MDX Materials Research Center for Elements Strategy, Tokyo Institute ofTechnology

Summary• An indium-gallium-zinc-oxide (IGZO) thin-film transistor (TFT) *1, an amorphous oxide
semiconductor, has an electron mobility that is one order of magnitude higher and much
lower off-state current than that of silicon semiconductors (Figure 1).
• Owing to this performance, IGZO-TFTs are widely used in displays for smartphones, tablet
PCs, and OLED TVs, and the market is growing rapidly.
Amorphous oxides have a structure similar to that of glass; further, they are generally insulators and do not
conduct electricity. Nonetheless, Prof. Hosono created a transparent oxide semiconductor with excellent
electron transport properties based on an original material design.
At an international conference in 1995, Prof. Hosono presented the first transparent amorphous oxide
semiconductor (TAOS) *2. However, this development did not attract much attention because amorphoussilicon was at its peak at that time.
Over time, Prof. Hosono’s design guidelines were verified theoretically and experimentally. Then, a paper
reporting the use of crystallized TAOSs in TFTs (indium-gallium-zinc-oxygen (IGZO)-TFT) was published inScience in 2003, following which a paper reporting on amorphous IGZO-TFT was published in Nature in 2004.
These reports did attract attention and led to further research in this area, ultimately resulting in thedevelopment of today’s large OLED TVs.

Detail

Promoting the Spread of Large OLED TVs, with Memory Applications being the New Target
Prof. Hosono proposed and proved the novel idea of creating a semiconductor from a transparent
oxide, which is an insulator. After the publication of Prof Hosono’s papers, numerous studies
worldwide started investigating amorphous oxide semiconductors; consequently, Prof. Hosono’s twopapers have been cited approximately 10,000 times.
An amorphous indium-gallium-zinc-oxygen (IGZO) thin-film transistor (TFT) *3 is a type of TAOS
that has an electron mobility that is one order of magnitude more than that of amorphous silicon
hydride, which was used until then. This allows sufficient current to be drawn even when the pixel
size is reduced. Therefore, high resolution is possible, off-current is extremely low, and transparency
is high. LCDs with amorphous IGZO-TFTs thus offer the advantages of high resolution and low
power consumption. Their characteristics can be best utilized in driving large OLED TVs. Unlike
LCDs, that of the light emission intensity from OLEDs is proportional to the current flowing throughthe TFT. This made it difficult to drive large OLED TVs with amorphous silicon.
These characteristics have been analyzed in detail, and amorphous IGZO-TFTs were first used todrive LCD screens in smartphones and tablet PCs. Amorphous IGZO-TFTs are also suitable forlarge displays because they can be manufactured at low cost and can be easily scaled to larger
sizes. In particular, large OLED TVs were first made possible and commercialized through the useof IGZO-TFTs. Thus, amorphous IGZO-TFT is now becoming a global standard for both LCD andOLED.
Recently, there is a strong demand to apply oxide semiconductors to super high definition displays
and memory. To meet these demands, an electron mobility as high as that of polysilicon is required.
However, the higher the mobility of oxide semiconductor TFTs, the lower is their stability. Prof.
Hosono and his colleagues clarified the cause of this problem and realized a TAOS-TFT with both
good electron mobility and good stability. This report was published in Nature Electronics in 2021.
In addition, the first book on TAOSs with a focus on IGZO, titled “Amorphous Oxide Semiconductors”(co-edited by Hideo Hosono and Hideya Kumomi, Fig. 2) was published in May 2022. This book,written by experts from around the world, covers everything from the basics to applications in a single
volume. IGZO-based memory devices are expected to make great progress in the future becauseof their low power consumption and suitability for stacking with silicon semiconductors.
Not flinching in a fight over a patentProf. Hosono’s research results were achieved through the Japan Science and Technology Agency’s(JST) ERATO “Transparent Electron Activation Project” (1999–2004). Patents were granted for TFTsin 2003 and amorphous TAOS-TFTs in March 2004 in Korea, the US, Taiwan, China, India, Europe,and other countries worldwide, and JST has granted licenses.
Prof. Hosono said that “The developed material is only worthwhile if it is used. However, the many
domestic companies approached at the time for licensing were not interested”. Therefore, Prof.
Hosono changed the policy and decided to grant licenses to domestic and foreign companies without
any distinction. However, the process of obtaining a patent was not smooth. First, the patent wasrejected by the Korean patent office, and a lawsuit was filed to invalidate the decision, which waswon and granted. Then, when JST granted a license to Samsung Electronics, the Japanese press
was critical because a Korean manufacturer was chosen instead of a Japanese one.
Nevertheless, Prof. Hosono remained steadfast in his belief that “materials must be used,” further
saying that “Patents that have a large impact will be disputed. Patents that are not disputed are notprofitable.” Prof. Hosono ended all disputes over patents in 2019.
Aim for new markets with a spirit of flexibilityRegarding the Japan Society of Applied Physics, Prof. Hosono said that “The word ‘applied’ means‘hawkish’, and hawkishness is what makes our society attractive. I hope that society will maintain its
spirit of introducing new things as they become interesting and spreading them throughout the world
with a spirit of flexibility. After all, when I attended the semiconductor conference in 1995, I was coldly
told that this was no place for a glassmaker, but I am still continuing my research today.” Heemphasized the importance of a pioneering spirit in conducting research in a flexible manner withan eye toward social implementation.
Annotations*1 Thin-film transistor (TFT)
A transistor is a semiconductor device that amplifies and switches electrical signals in electronic circuits; it is an essential
part of modern electronic devices. In a display, each dot that makes up the screen is controlled by a TFT.

*2 Transparent amorphous oxide semiconductors (TAOS)
When a crystalline semiconductor with a regular arrangement of atoms is turned into an amorphous semiconductor with adisordered arrangement, the ease of electron movement (i.e., mobility) decreases by an order of magnitude. For example,electron mobility in silicon decreases by 2–3 orders of magnitude. However, in 1995, Prof. Hosono hypothesized that thespatial extent and symmetry of electron orbitals between metal ions and oxygen ions could lead to amorphous oxide
semiconductors with mobility comparable to that of crystals and named this group of materials transparent amorphousoxide semiconductors (TAOS). As a result, he realized semiconductors that are as transparent as glass, in which electrons
can move as easily as in crystals, with the ease of electricity flow being controllable.

*3 Amorphous IGZO-TFT
IGZO refers to an amorphous oxide containing indium (In), gallium (Ga), zinc (Zn), and oxygen (O); it is one of the TAOS
material systems proposed by Prof. Hosono. It is now recognized internationally. IGZO has 10–30 times higher electron
mobility than that of amorphous silicon, and its thin film transmits visible light, making it possible to fabricate transparent
thin films. Thin films can be easily fabricated by sputtering in a low-temperature process. Therefore, further application
expansion is expected owing to advantages such as the possibility of film deposition on plastic films, etc.

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FiguresFig.1 Orbital configuration near the conduction band minimum through which electrons flow in a TAOS. If the paththrough which electrons flow comprises metal spherical s-orbitals with a large spatial extent, electrons should flowsmoothly even though the spheres themselves are not arranged in a regular pattern. This basic idea has led to therealization of TFTs in which electrons move dozens of times better than in amorphous silicon.

Fig.2 Prof. Hosono’s book (co-edited with Hideya Kumomi), published in May 2022 as part of the Wiley Series in Display
Technology, covers the research on IGZO to date and its future prospects for use in logic circuits and memory.