Transparent electronics is today one of the most advanced topics for a wide range of device applications. The key components are wide bandgap semiconductors, where oxides of different origins play an important role, not only as passive component but also as active component, similar to what is observed in conventional semiconductors like silicon. Transparent electronics has gained special attention during the last few years and is today established as one of the most promising technologies for leading the next generation of flat panel display due to its excellent electronic performance. In this paper the recent progress in n- and p-type oxide based thin-film transistors (TFT) is reviewed, with special emphasis on solution-processed and p-type, and the major milestones already achieved with this emerging and very promising technology are summarizeed. After a short introduction where the main advantages of these semiconductors are presented, as well as the industry expectations, the beautiful history of TFTs is revisited, including the main landmarks in the last 80 years, finishing by referring to some papers that have played an important role in shaping transparent electronics. Then, an overview is presented of state of the art n-type TFTs processed by physical vapour deposition methods, and finally one of the most exciting, promising, and low cost but powerful technologies is discussed: solution-processed oxide TFTs. Moreover, a more detailed focus analysis will be given concerning p-type oxide TFTs, mainly centred on two of the most promising semiconductor candidates: copper oxide and tin oxide. The most recent data related to the production of complementary metal oxide semiconductor (CMOS) devices based on n- and p-type oxide TFT is also be presented. The last topic of this review is devoted to some emerging applications, finalizing with the main conclusions. Related work that originated at CENIMAT|I3N during the last six years is included in more detail, which has led to the fabrication of high performance n- and p-type oxide transistors as well as the fabrication of CMOS devices with and on paper.

Oxide Semiconductor Thin‐Film Transistors: A Review of Recent Advances

https://iopscience.iop.org/article/10.1088/1468-6996/11/4/044305/pdf

Present status of amorphous In–Ga–Zn–O thin-film transistors

Optical transparency, tunable conducting properties and easy processability make metal oxides key materials for advanced optoelectronic devices. This Review discusses recent advances in the synthesis of these materials and their use in applications. Metal oxides (MOs) are the most abundant materials in the Earth's crust and are ingredients in traditional ceramics. MO semiconductors are strikingly different from conventional inorganic semiconductors such as silicon and III–V compounds with respect to materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing and optoelectronic properties, thereby enabling both conventional and completely new functions. Recently, remarkable advances in MO semiconductors for electronics have been achieved, including the discovery and characterization of new transparent conducting oxides, realization of p-type along with traditional n-type MO semiconductors for transistors, p–n junctions and complementary circuits, formulations for printing MO electronics and, most importantly, commercialization of amorphous oxide semiconductors for flat panel displays. This Review surveys the uniqueness and universality of MOs versus other unconventional electronic materials in terms of materials chemistry and physics, electronic characteristics, thin-film fabrication strategies and selected applications in thin-film transistors, solar cells, diodes and memories.

Metal oxides for optoelectronic applications

We report a model of the carrier transport and the subgap density of states in a representative amorphous oxide semiconductor, amorphous InGaZnO4 (a-IGZO), for device simulation of a-IGZO TFTs. Compared to hydrogenated amorphous silicon, a-IGZO exhibits much lower densities of tail states and deep gap states, leading to the small subthreshold swings and high mobilities.

P‐29: Modeling of Amorphous Oxide Semiconductor Thin Film Transistors and Subgap Density of States

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.

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

We report a model of the carrier transport and the subgap density of states in amorphous InGaZnO4 (a-IGZO) for device simulation of a-IGZO thin-film transistors (TFTs) operated in both the depletion mode and the enhancement mode. A simple model using a constant mobility and two-step subgap density of states reproduced well the characteristics of the a-IGZO TFTs. a-IGZO exhibits low densities of tail states and deep gap states, leading to small subthreshold swings and high mobilities.

Modeling of amorphous InGaZnO4 thin film transistors and their subgap density of states

Oxide-semiconductor-based thin-film transistors (TFTs), particularly the amorphous ones, are becoming an important emerging technology. Since oxide semiconductors easily form polycrystalline phases, usually more complicated oxide mixtures are needed for growing amorphous phases. In this letter, we report that by simply reducing the thickness, ZnO can be intentionally grown into the amorphous phase. Furthermore, both top-gate and bottom-gate amorphous ZnO TFTs of micrometer scales were effectively implemented using fully lithographic and etching processes. Rather high field-effect mobilities of 25 and 4cm2∕Vs and on∕off current ratios of >107 and >106 were achieved for top-gate and bottom-gate configurations, respectively.

Amorphous ZnO transparent thin-film transistors fabricated by fully lithographic and etching processes

http://ntur.lib.ntu.edu.tw/bitstream/246246/148626/1/64.pdf

A new type of soluble pentacene precursor for organic thin-film transistors

We demonstrated flexible amorphous In-Ga-Zn-O (a-IGZO) thin-film transistors (TFTs) on fully transparent and high-temperature polyimide-based nanocomposite substrates. The flexible nanocomposite substrates were coated on the carrier glass substrates and were debonded after the TFT microfabrication. The adoption of the Ti/IZO stacked electrodes as source/drain/ gain electrodes significantly improved the etching compatibility with other material layers, enabling successful implementation of flexible a-IGZO TFTs onto the transparent nanocomposite substrates by conventional lithographic and etching processes. The flexible a-IGZO TFTs exhibited decent mobility and mechanical bending capability. Field-effect mobility of up to 15.9 cm2/V · s, a subthreshold swing of 0.4 V/dec, a threshold voltage of 0.8 V, and an on/off ratio of >; 108 were extracted from the TFT characteristics. The devices could be bent down to a radius of curvature of 3 mm and yet remained normally functional. Such successful demonstration of flexible oxide TFTs on transparent flexible substrates using fully lithographic and etching processes that are compatible with existing TFT fabrication technologies shall broaden their uses in flexible displays and electronics.

High-Performance Flexible a-IGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrates

Self-aligned techniques are often used in conventional CMOS and Si-based thin-film transistors (TFTs) technologies due to various merits. In this paper, we report self-aligned coplanar top-gate InGaZnO TFTs using PECVD a-SiNinfin:H patterned to have low hydrogen content in the channel region and high hydrogen content in the source/drain region. After annealing to induce hydrogen diffusion from a-SiNinfin:H into the oxide semiconductor, the source-drain regions become more conductive and yet the channel region remains suitable for TFT operation, yielding a working self-aligned TFT structure. Such fabrication involves neither back-side exposure nor ion implantation, and thus may be compatible with the typical and cost-effective TFT manufacturing.

Hsing-Hung Hsieh

Papers

Modeling of amorphous InGaZnO4 thin film transistors and their subgap density of states

Amorphous ZnO transparent thin-film transistors fabricated by fully lithographic and etching processes

A new type of soluble pentacene precursor for organic thin-film transistors

High-Performance Flexible a-IGZO TFTs Adopting Stacked Electrodes and Transparent Polyimide-Based Nanocomposite Substrates

Self-Aligned Top-Gate Coplanar In-Ga-Zn-O Thin-Film Transistors