Tzu-Hsuan Chang

Highly Stretchable Carbon Nanotube Transistors with Ion Gel Gate Dielectrics

Field-effect transistors (FETs) that are stretchable up to 50% without appreciable degradation in performance are demonstrated. The FETs are based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes (CNTs) as the channel, a flexible ion gel as the dielectric, and buckled metal films as electrodes. The buckling of the CNT film enables the high degree of stretchability while the flexible nature of the ion gel allows it to maintain a high quality interface with the CNTs during stretching. An excellent on/off ratio of >10(4), a field-effect mobility of 10 cm(2) · V(-1) · s(-1), and a low operating voltage of <2 V are achieved over repeated mechanical cycling, with further strain accommodation possible. Deformable FETs are expected to facilitate new technologies like stretchable displays, conformal devices, and electronic skins.

Highly stretchable carbon nanotube transistors enabled by buckled ion gel gate dielectrics

Deformable field-effect transistors (FETs) are expected to facilitate new technologies like stretchable displays, conformal devices, and electronic skins. We previously demonstrated stretchable FETs based on buckled thin films of polyfluorene-wrapped semiconducting single-walled carbon nanotubes as the channel, buckled metal films as electrodes, and unbuckled flexible ion gel films as the dielectric. The FETs were stretchable up to 50% without appreciable degradation in performance before failure of the ion gel film. Here, we show that by buckling the ion gel, the integrity and performance of the nanotube FETs are extended to nearly 90% elongation, limited by the stretchability of the elastomer substrate. The FETs maintain an on/off ratio of >104 and a field-effect mobility of 5 cm2 V−1 s−1 under elongation and demonstrate invariant performance over 1000 stretching cycles.

Bottom-gate coplanar graphene transistors with enhanced graphene adhesion on atomic layer deposition Al2O3

A graphene transistor with a bottom-gate coplanar structure and an atomic layer deposition (ALD) aluminum oxide (Al2O3) gate dielectric is demonstrated. Wetting properties of ALD Al2O3 under different deposition conditions are investigated by measuring the surface contact angle. It is observed that the relatively hydrophobic surface is suitable for adhesion between graphene and ALD Al2O3. To achieve hydrophobic surface of ALD Al2O3, a methyl group (CH3)-terminated deposition method has been developed and compared with a hydroxyl group (OH)-terminated deposition. Based on this approach, bottom-gate coplanar graphene field-effect transistors are fabricated and characterized. A post-thermal annealing process improves the performance of the transistors by enhancing the contacts between the source/drain metal and graphene. The fabricated transistor shows an Ion/Ioff ratio, maximum transconductance, and field-effect mobility of 4.04, 20.1 μS at VD = 0.1 V, and 249.5 cm2/V·s, respectively.

High power fast flexible electronics: Transparent RF AlGaN/GaN HEMTs on plastic substrates

Heat dissipation is a major challenge for practical applications of fast flexible electronics, particularly using wide band gap semiconductors, due to the high power needed to achieve high frequency operation. Using an intrinsic GaN buffer layer as a heat conductive conductor, transparent, flexible RF GaN HEMTs with a device area of 400 × 350 um2 on plastic substrates (PET) are demonstrated with high thermal dissipation of 0.5 W. The device exhibits an fMAX of 115 GHz with no severe degradation of device performance compared with that made on a Si substrate. Low temperature plastic substrates also exhibited no thermal damage/melting. Our approach demonstrated that flexible single crystal material such as intrinsic GaN is a contender for thermal management of medium power RF flexible devices.

Radio-frequency flexible and stretchable electronics: the need, challenges and opportunities

Successful integration of ultrathin flexible or stretchable systems with new applications, such as medical devices and biodegradable electronics, have intrigued many researchers and industries around the globe to seek materials and processes to create high-performance, non-invasive and cost-effective electronics to match those of state-of-the-art devices. Nevertheless, the crucial concept of transmitting data or power wirelessly for such unconventional devices has been difficult to realize due to limitations of radio-frequency (RF) electronics in individual components that form a wireless circuitry, such as antenna, transmission line, active devices, passive devices etc. To overcome such challenges, these components must be developed in a step-by-step manner, as each component faces a number of different challenges in ultrathin formats. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed flexible active devices, including cost effective Si-based strained MOSFETs, GaAs-based HBTs and GaN-based HEMTs, performing at multi-gigahertz frequencies are presented. Furthermore, flexible or stretchable passive devices, including capacitors, inductors and transmission lines that are vital parts of a microwave circuitry are also demonstrated. We also present unique applications using the presented flexible or stretchable RF components, including wearable RF electronics and biodegradable RF electronics, which were impossible to achieve using conventional rigid, wafer-based technology. Further opportunities like implantable systems exist utilizing such ultrathin RF components, which are discussed in this report as well.

Biaxially stretchable carbon nanotube transistors

Biaxially stretchable field effect transistors (FETs) fabricated on elastomeric substrates are demonstrated incorporating a buckled network of polymer-wrapped semiconducting carbon nanotubes in the channel and a buckled layer of an ion gel as the gate dielectric. The FETs maintain an on/off ratio of >104 and a field-effect mobility of >5 cm2 V−1 s−1 for biaxial elongation up to 67% or uniaxial elongation either parallel or perpendicular to the channel. The performance is stable for at least 10 000 stretch-release cycles. Failure analysis shows that the extent of elongation is limited only by the magnitude of the pre-strain used during fabrication. This work is important because deformable FETs are needed for future technologies including stretchable electronics and displays.

Green microwave electronics for the coming era of flexible electronics

Novel fabrication techniques to manufacture various high performance devices, using both Si and III-V nanomembrane-form materials, that are essential for portable electronics on a biodegradable cellulose nanofibril (CNF) paper are presented. We have introduced a concept of natural biodegradation of the discarded electronic chips that could help reduce the accumulation of the massive amount of persistent electronic waste disposed of daily using CNF that is derived from wood, a natural sustainable resource. CNF paper offers properties as a flexible substrate for high performance electronics. Essential microwave electronic systems, such as Si-based transistor and GaAs-based transistor and diode, were fabricated to demonstrate the feasibility of our novel approach on the CNF substrate. A novel releasable device fabrication technology, along with deterministic assembly printing technique for these high-performance devices, that significantly decreased the amount of GaAs used compared to conventional chip based manufacturing are presented.

Radio-frequency flexible transistors on cellulose nanofibrillated fiber (CNF) substrates

RF performance flexible thin-film transistors toward green portable devices were realized. The cellulose nanofibrillated fiber (CNF) substrate combined with Si nanomembranes (Si NMs) printing technique enables to fabricate flexible, high-speed and bio-degradable devices. Flexible Si NM thin-film transistors (TFTs) built on the CNF substrate show mobility of 336 cm/v·s and fT and fmax of 2.4 GHz and 5.1 GHz, respectively. This demonstration paves the path to entire green portable devices so as to generate less waste and save more valuable resources.

The One-Pot Directed Assembly of Cylinder-Forming Block Copolymer on Adjacent Chemical Patterns for Bimodal Patterning

The direct self-assembly of cylinder-forming poly(styrene-block-methyl-methacrylate) (PS-b-PMMA) block copolymer is successfully assembled into two orientations, according to the underlying guiding pattern in different areas. Lying-down and perpendicular cylinders are formed, respectively, depending on the design of chemical pattern: sparse line/space pattern or hexagonal dot array. The first chemical pattern composed of prepatterned cross-linked polystyrene (XPS) line/space structure has a period (LS ) equal to twice the intercylinder period of the block copolymer (L0 ). The PS-b-PMMA thin film on the prepared chemical template after thermal annealing forms a lying-down cylinder morphology when the width of the PS strips is less than the width of PS block in the PS-b-PMMA block copolymer. The morphology is only applicable at the discrete thickness of the PS-b-PMMA film. In addition to forming the lying-down cylinders directly on the XPS guiding pattern, the cylinder-forming block copolymer can also be assembled in a perpendicular way on the second guiding pattern (the hexagonal dot array). The block copolymer films are registered into two orientations in a single directed self-assembly process. The features of the assembled patterns are successfully transferred down to the silicon oxide substrate.

Bendable MOS capacitors formed with printed In0.2Ga0.8As/GaAs/In0.2Ga0.8As trilayer nanomembrane on plastic substrates

An optimized approach is applied to realize the transfer printing of an In0.2Ga0.8As/GaAs/In0.2Ga0.8As trilayer nanomembrane (NM) onto a plastic substrate with high quality. Bendable metal-oxide-semiconductor capacitors (MOSCAPs) are fabricated on the transferred NM. A detailed COMSOL simulation study is conducted to investigate the mechanical bending behavior induced tri-principle stress of the NM on flexible substrates. The electrical characteristics of the fabricated MOSCAPs exhibit almost no hysteresis voltage of only 0.03 V, an extremely low gate leakage of 10-6 to 10-7 A/cm2, and low accumulation frequency dispersion, thus indicating the possibility of achieving high performance III-V MOS transistor operation. The impact of mechanical strains on the flatband voltages has been carefully investigated from the capacitance-voltage (C-V) measurements. The corresponding accumulation capacitance shows good robustness under tensile bending conditions. The results indicate an important step toward the realiz...