Yei Hwan Jung

Stretchable silicon nanoribbon electronics for skin prosthesis

Sensory receptors in human skin transmit a wealth of tactile and thermal signals from external environments to the brain. Despite advances in our understanding of mechano- and thermosensation, replication of these unique sensory characteristics in artificial skin and prosthetics remains challenging. Recent efforts to develop smart prosthetics, which exploit rigid and/or semi-flexible pressure, strain and temperature sensors, provide promising routes for sensor-laden bionic systems, but with limited stretchability, detection range and spatio-temporal resolution. Here we demonstrate smart prosthetic skin instrumented with ultrathin, single crystalline silicon nanoribbon strain, pressure and temperature sensor arrays as well as associated humidity sensors, electroresistive heaters and stretchable multi-electrode arrays for nerve stimulation. This collection of stretchable sensors and actuators facilitate highly localized mechanical and thermal skin-like perception in response to external stimuli, thus providing unique opportunities for emerging classes of prostheses and peripheral nervous system interface technologies.

High-performance green flexible electronics based on biodegradable cellulose nanofibril paper

Todays consumer electronics, such as cell phones, tablets and other portable electronic devices, are typically made of non-renewable, non-biodegradable, and sometimes potentially toxic (for example, gallium arsenide) materials. These consumer electronics are frequently upgraded or discarded, leading to serious environmental contamination. Thus, electronic systems consisting of renewable and biodegradable materials and minimal amount of potentially toxic materials are desirable. Here we report high-performance flexible microwave and digital electronics that consume the smallest amount of potentially toxic materials on biobased, biodegradable and flexible cellulose nanofibril papers. Furthermore, we demonstrate gallium arsenide microwave devices, the consumer wireless workhorse, in a transferrable thin-film form. Successful fabrication of key electrical components on the flexible cellulose nanofibril paper with comparable performance to their rigid counterparts and clear demonstration of fungal biodegradation of the cellulose-nanofibril-based electronics suggest that it is feasible to fabricate high-performance flexible electronics using ecofriendly materials.

Origami silicon optoelectronics for hemispherical electronic eye systems

Digital image sensors in hemispherical geometries offer unique imaging advantages over their planar counterparts, such as wide field of view and low aberrations. Deforming miniature semiconductor-based sensors with high-spatial resolution into such format is challenging. Here we report a simple origami approach for fabricating single-crystalline silicon-based focal plane arrays and artificial compound eyes that have hemisphere-like structures. Convex isogonal polyhedral concepts allow certain combinations of polygons to fold into spherical formats. Using each polygon block as a sensor pixel, the silicon-based devices are shaped into maps of truncated icosahedron and fabricated on flexible sheets and further folded either into a concave or convex hemisphere. These two electronic eye prototypes represent simple and low-cost methods as well as flexible optimization parameters in terms of pixel density and design. Results demonstrated in this work combined with miniature size and simplicity of the design establish practical technology for integration with conventional electronic devices.Hemispherical format has been adopted in camera systems to better mimic human eyes, yet the current designs rely on complicated fabrications. Here, Zhang et al. show an origami-inspired approach that enables planar silicon-based photodetector arrays to reshape into concave or convex geometries.

X-Band Compatible Flexible Microwave Inductors and Capacitors on Plastic Substrate

Microwave inductors and capacitors compatible with low temperature processes form a route to high-frequency electronics on flexible substrates in conjunction with high speed thin film transistors. We report here the process by which one can fabricate passive components with X-band compatible performance on polyethylene terephthalate substrate by changing dielectric layer material and optimized design, where the method can be applied with most of the active device technologies developed up to date on flexible substrates. High resonance frequencies were obtained, comparing with former results, confirming the effectiveness of the approach with flexible dielectric materials. Studies on bending effects for the spiral inductors and metal on insulator capacitors show miniscule difference in performance related to bending radius. Performance enhancements compared to previously reported passive elements enable higher radio-frequency electronics on plastic substrates.

Flexible and Stretchable Microwave Microelectronic Devices and Circuits

Electronic systems built on flexible plastic films and stretchable rubber sheets have attracted new applications in many emerging fields. Integration of high-speed electronics such as microwave power amplifiers and switches can extend the applications even further with wireless capabilities. As such, flexible and stretchable microwave electronics represent opportunities for future electronics where remote capabilities are desired. Here, we review advances in numerous types of microelectronic devices used for fast, flexible, and stretchable electronic devices, as well as flexible and stretchable passive elements and circuitries. We first introduce the challenges associated with design and fabrication, and the characteristics required for high-frequency operation of the devices on foreign substrates. Second, we review the recent efforts that were made utilizing different types of high-performance semiconductors, which are ideal for high-speed flexible and stretchable electronics, such as silicon, compound semiconductors, and 1-D and 2-D materials. Third, passive electronic components fabricated on such substrates, including inductors, capacitors, and transmission lines, are reviewed. Finally, we discuss the flexible and stretchable microwave electronics at the circuit level and review the recent advances in making numerous types of flexible and stretchable microwave circuits for diverse applications.

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.

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.

Wireless Applications of Conformal Bioelectronics

Conformal bioelectronics in flexible or stretchable format that make direct contact to the skin or tissues have contributed extensively to diverse clinical applications. Wireless modules in such minimally invasive forms have developed in parallel to extend the capabilities and to improve the quality of such bioelectronics, in assurances to offer safer and more convenient clinical practice. Such remote capabilities are facilitating significant advances in clinical medicine, by removing bulky energy storage devices and tangled electrical wires, and by offering cost-effective and continuous monitoring of the patients. This chapter provides a snapshot of current developments and challenges of wireless conformal bioelectronics with various examples of applications utilizing either wireless powering or communication system. The chapter begins with near-field wirelessly powered therapeutic devices owing to the simplicity of power transfer mechanism followed by far-field powering systems which require integration of numerous electrical components. In the later sections of the chapter, sensors in conformal format that transfer clinical data wirelessly are discussed and ends by reviewing the developments of wireless bioelectronics that utilize integrated circuits for advanced capabilities in clinical applications.

Materials and design considerations for fast flexible and stretchable electronics

Flexible and stretchable high-speed devices that operate at microwave level may enable wireless functionalities for many applications. The rise of flexible or stretchable electronic systems using inorganic semiconducting materials allow integration of high-performance electronic devices on foreign substrates, such as plastic or rubber. Here, we report on materials and design considerations for fabricating flexible and stretchable electronics systems that operate in the microwave level. High-speed active devices, including cost effective Si-based transistors and high-power GaN-based transistors, performing at multi-gigahertz frequencies are fabricated on either plastic or rubber substrates. Furthermore, important passive components, such as the capacitors, inductors and transmission lines that can together complete a microwave integrated circuit, are also demonstrated. All of the components presented here have comparable high-performances to their rigid counterparts, which can be combined to form microwave integrated circuits that can be used for many applications, where wireless functionalities are desired.