No abstract

Thin‐film transistors (TFTs) grown on a flexible glass substrate using single‐crystal‐like germanium (Ge) channel to simultaneously achieve high carrier mobility, high performance characteristics, mechanical flexibility, and cost‐effective large‐area manufacturing are reported. High‐crystalline‐quality materials of biaxially textured CeO2 deposited at room temperature by ion‐beam‐assisted deposition followed by single‐crystal‐like Ge epitaxially grown at 550 °C by plasma‐enhanced chemical vapor deposition on an amorphous substrate are developed. p‐type Ge with {111} surface shows well‐aligned grains in both out‐of‐plane and in‐plane directions, as characterized by reflection high‐energy electron diffraction, X‐ray diffraction, and Raman spectroscopy. The material structures are fabricated to transistor devices with top‐gate geometry. The devices (channel width and length = 80 and 14 μm) exhibit performance characteristics with on/off ratio of ≈106, a field‐effect mobility of ≈105 cm2 V−1 s−1, and saturation current levels of ≈3.5 mA, which are significantly higher than performance metrics of other state‐of‐the‐art TFTs based on amorphous Si, organic semiconductors, and semiconducting oxides. This development can open a new avenue for next‐generation TFTs beyond the display applications.

Piezoelectric generators (PEGs) are a promising power source for future self-powered electronics by converting ubiquitous ambient mechanical energy into electricity. However, most of the high-output PEGs are made from lead zirconate titanate, in which the hazardous lead could be a potential risk to both humans and environment, limiting their real applications. III-Nitride (III-N) can be a potential candidate to make stable, safe, and efficient PEGs due to its high chemical stability and piezoelectricity. Also, PEGs are preferred to be flexible rather than rigid, to better harvest the low-magnitude mechanical energy. Herein, a high-output, lead-free, and flexible PEG (F-PEG) is made from GaN thin film by transferring a single-crystalline epitaxial layer from silicon substrate to a flexible substrate. The output voltage, current density, and power density can reach 28 V, 1 μA·cm, and 6 μW·cm, respectively, by bending the F-PEG. The generated electric power by human finger bending is high enough to light commercial visible light-emitting diodes and charge commercial capacitors. The output performance is maintained higher than 95% of its original value after 10 000-cycle test. This highly stable, high-output, and lead-free GaN thin-film F-PEG has the great potential for future self-powered electronic devices and systems.

We investigate thermo-electronic behaviors of flexible AlGaN/GaN heterostructure field-effect transistors (HFETs) for high-power operation of the devices using Raman thermometry, infrared imaging, and current-voltage characteristics. A large negative differential conductance observed in HFETs on polymeric flexible substrates is confirmed to originate from the decreasing mobility of the two-dimensional electron gas channel caused by the self-heating effect. We develop high-power transistors by suppressing the negative differential conductance in the flexible HFETs using chemical lift-off and modified Ti/Au/In metal bonding processes with copper (Cu) tapes for high thermal conductivity and low thermal interfacial resistance in the flexible hybrid structures. Among different flexible HFETs, the ID of the HFETs on Cu with Ni/Au/In structures decreases only by 11.3% with increasing drain bias from the peak current to the current at VDS = 20 V, which is close to that of the HFETs on Si (9.6%), solving the problem of previous flexible AlGaN/GaN transistors.

Single-crystal-like silicon (Si) thin films on bendable and scalable substrates via direct deposition are a promising material platform for high-performance and cost-effective devices of flexible electronics. However, due to the thick and unintentionally highly doped semiconductor layer, the operation of transistors has been hampered. We report the first demonstration of high-performance flexible thin-film transistors (TFTs) using single-crystal-like Si thin films with a field-effect mobility of ∼200 cm/V·s and saturation current, I/l > 50 μA/μm, which are orders-of-magnitude higher than the device characteristics of conventional flexible TFTs. The Si thin films with a (001) plane grown on a metal tape by a "seed and epitaxy" technique show nearly single-crystalline properties characterized by X-ray diffraction, Raman spectroscopy, reflection high-energy electron diffraction, and transmission electron microscopy. The realization of flexible and high-performance Si TFTs can establish a new pathway for extended applications of flexible electronics such as amplification and digital circuits, more than currently dominant display switches.