Huilong Zhang

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.

Highly stretchable and sensitive piezoresistive carbon nanotube/elastomeric triisocyanate-crosslinked polytetrahydrofuran nanocomposites

Piezoresistive polymer nanocomposites are highly desirable for flexible mechanical sensing applications. In this study, a family of multi-walled carbon nanotube (CNT)/elastomeric triisocyanate-crosslinked polytetrahydrofuran (ETC-PTHF) nanocomposites that are highly stretchable and highly sensitive to mechanical stimuli were designed, synthesized, and characterized. The CNTs in the CNT/ETC-PTHF nanocomposites were initially dispersed in the ETC-PTHF matrix uniformly, leading to a relatively high electrical conductivity. Upon stretching, both the degree of CNT alignment along the stretching direction and the degree of PTHF crystallinity increased consistently with the tensile strain. The strain-induced microstructure change adversely affected the CNT conducting pathways, thereby reducing the electrical conductivity of the nanocomposites. For instance, the electrical conductivity of the 15 wt% CNT/ETC-PTHF nanocomposites decreased by approximately 7.3%, 29.2%, and 19.76, 169.2 and 1291 times when the tensile strain was 1%, 5%, 50%, 250%, and 500%, respectively. The nanocomposite film was able to detect a mechanical stimulus (poking) weaker than the landing force of a mosquito. Furthermore, the nanocomposite film demonstrated rapid and highly sensitive responses to continuous finger motion. These new piezoresistive CNT/ETC-PTHF nanocomposites possess a number of desirable characteristics including ease of fabrication, low cost, and high sensitivity, thereby making them very promising candidates for applications in electronic skins, electronic textiles, and biomedical detectors.

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.

High-sensitivity silicon ultraviolet p+-i-n avalanche photodiode using ultra-shallow boron gradient doping

Photo detection of ultraviolet (UV) light remains a challenge since the penetration depth of UV light is limited to the nanometer scale. Therefore, the doping profile and electric field in the top nanometer range of the photo detection devices become critical. Traditional UV photodetectors usually use a constant doping profile near the semiconductor surface, resulting in a negligible electric field, which limits the photo-generated carrier collection efficiency of the photodetector. Here, we demonstrate, via the use of an optimized gradient boron doping technique, that the carrier collection efficiency and photo responsivity under the UV wavelength region have been enhanced. Furthermore, the ultrathin p+-i-n junction shows an avalanche gain of 2800 and an ultra-low junction capacitance (sub pico-farad), indicating potential applications in the low timing jitter single photon detection area.

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.

Photolithography-Based Nanopatterning Using Re-entrant Photoresist Profile

Photolithography based on optical mask is widely used in academic research laboratories due to its low cost, simple mechanism, and ability to pattern in micron-sized features on a wafer-scale area. Because the resolution is bound by diffraction limits of the light source, nanoscale patterning using photolithography requires short-wavelength light source combined with sophisticated optical elements, adding complexity and cost. In this paper, a novel method of subwavelength patterning process using conventional i-line mercury lamp is introduced, without the use of such advanced optical tools. The method utilizes the re-entrant geometry of image reversal photoresist produced from the developing process, where a secondary mask is generated by isotropically depositing a metal layer to cover the re-entrant profile of the photoresist. Removing the photoresist by applying ultrasonic vibrations in acetone bath uniformly cracks the metal layer at the sidewalls of the re-entrant profile, exposing the substrate with a reduced feature size. The width of the initial mask pattern can be reduced by 400 nm in a controlled manner, regardless of the original width choice. As a result, the method is shown to achieve sub-100 nm scale linear patterns compatible for both subsequent deposition process and dry-etching process. Our approach is applicable to various shapes of the patterns and can be used in electronic device fabrication requiring nanoscale lithography patterning, such as the gate fabrication of AlGaN/GaN high-electron-mobility transistor.