There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

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

Since the successful demonstration of a blue light-emitting diode (LED)1, potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p–n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate14. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p–i–n junctions is demonstrated at room-temperature.

Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO

An object is to provide a semiconductor device of which a manufacturing process is not complicated and by which cost can be suppressed, by forming a thin film transistor using an oxide semiconductor film typified by zinc oxide, and a manufacturing method thereof. For the semiconductor device, a gate electrode is formed over a substrate; a gate insulating film is formed covering the gate electrode; an oxide semiconductor film is formed over the gate insulating film; and a first conductive film and a second conductive film are formed over the oxide semiconductor film. The oxide semiconductor film has at least a crystallized region in a channel region.

Semiconductor device, and manufacturing method thereof

We report high-performance ZnO thin-film transistor (ZnO-TFT) fabricated by rf magnetron sputtering at room temperature with a bottom gate configuration. The ZnO-TFT operates in the enhancement mode with a threshold voltage of 19V, a saturation mobility of 27cm2∕Vs, a gate voltage swing of 1.39V∕decade and an on/off ratio of 3×105. The ZnO-TFT presents an average optical transmission (including the glass substrate) of 80% in the visible part of the spectrum. The combination of transparency, high mobility, and room-temperature processing makes the ZnO-TFT a very promising low-cost optoelectronic device for the next generation of invisible and flexible electronics.

Wide-bandgap high-mobility ZnO thin-film transistors produced at room temperature

We have fabricated high performance ZnO thin film transistors (TFTs) using CaHfOx buffer layer between ZnO channel and amorphous silicon?nitride gate insulator. The TFT structure, dimensions, and materials set are identical to those of the commercial amorphous silicon (a-Si) TFTs in active matrix liquid crystal display, except for the channel and buffer layers replacing a-Si. The field effect mobility can be as high as 7 cm2?V-1?s-1 for devices with maximum process temperature of 300?C. The process temperature can be reduced to 150?C without much degrading the performance, showing the possibility of the use of polymer substrate.

High Mobility Thin Film Transistors with Transparent ZnO Channels.

We model grain boundaries (GBs) for polycrystalline ZnO thin film transistors (TFTs). Experimental result shows a non-linear increase of drain current and gradual enhancement of field effect mobility with increasing gate bias. Our initial single GB model was unable to explain the experimentally obtained results, where we considered the peak defect distribution at the mid gap. Realizing from the experimentally obtained results, we remodeled the grain boundary considering the peak distribution close to the conduction band, which then better replicates the experimental observation. We describe here the transfer characteristic of experimental ZnO TFT in linear region with calculated potential profiles. Appropriate grain boundary modeling signifies that the slower decrease in potential barrier in grain boundary with applied gate voltage is responsible for such non-linear changes in drain current and gradual enhancement of mobility.

Modeling of grain boundary barrier modulation in ZnO invisible thin film transistors

We have fabricated field-effect transistors with single-crystalline ZnO channels consisting of high-quality epitaxial films grown on lattice-matched (0001) ScAlMgO4 substrates by laser molecular-beam epitaxy. Amorphous alumina gate insulators are deposited on the top of the ZnO films using either RF magnetron sputtering or electron-beam evaporation. The field-effect mobility (µFE) of the device prepared by the latter method is as high as 40 cm2V-1s-1, one order of magnitude higher than those typically observed for polycrystalline channel devices. However, hysteresis appears in transfer characteristics. This unfavorable effect is found to be eliminated by the thermal annealing of the entire devices in air. The much larger hysteresis and lower µFE are observed for the device with sputtered gate insulators. This is presumably due to dense surface states created by ion or electron bombardment during the sputtering.

https://iopscience.iop.org/article/10.1143/JJAP.44.L1193/pdf

High-Mobility Field-Effect Transistors Based on Single-Crystalline ZnO Channels

This paper reports on vertical GaN-based trench MOSFETs operating at 100 A for the first time. A current distribution layer (CDL) in a drift layer is employed for the high current operation. An effective insert position of the CDL is designed and, thereby, the current density of the MOSFET with the CDL is increased about 1.17 times higher than that of the MOSFET without the CDL. Large MOSFET chips with a drain current of up to 100 A are fabricated and their switching characteristics are demonstrated.

100 A Vertical GaN Trench MOSFETs with a Current Distribution Layer

We have developed a multi-channel measurement system for combinatorial investigation of thermoelectric materials. The measurement apparatus has ten series of pin-probe array which enables us to measure the Seebeck coefficient and electric conductivity of 10 samples simultaneously. A successful measurement on a composition-spread thin films library indicated that this measurement system is highly useful for the high-speed exploration of thermoelectric materials by combinatorial approach.

Junya Nishii

Papers

High Mobility Thin Film Transistors with Transparent ZnO Channels.

Modeling of grain boundary barrier modulation in ZnO invisible thin film transistors

High-Mobility Field-Effect Transistors Based on Single-Crystalline ZnO Channels

100 A Vertical GaN Trench MOSFETs with a Current Distribution Layer

High-speed evaluation of thermoelectric materials using multi-channel measurement system