(Invited) Flexible and Stretchable Microwave Electronics
Authors: Ma Zhenqiang , Zhang Huilong 
Published: July 2021

he development of microwave electronics has greatly advanced applications including communication, sensing, detection, and so on. The recently emerging markets like wearable devices, biomedical sensors, and foldable large-area microwave systems impose new requirements on microwave electronics, including flexibility, low weight, small size, and even stretchability. The unique mechanical properties of flexible and/or stretchable electronics renders them perfect candidates for applications like biomedical sensors and wearable devices, where the device needs to be attached or wrapped on nonplanar surfaces. Microwave electronics in flexible and/or stretchable form can substantially expand the application of these devices by eliminating lengthy connection wires and implementing modern wireless techniques. Therefore, flexible/stretchable electronics including fundamental components, functional circuits, and complete systems have attracted great interest from both academia and industry.
The development of flexible microwave electronics has focused on fundamental flexible components like transistors and passive components in the past decades. As the heart of modern electronics, conventional semiconductor-based transistors have transformed into the flexible form through various mechanisms including substrate removal and exfoliation of the active layer on the surface. Silicon (Si), the most widely studied semiconductor material, has been extensively investigated for flexible microwave transistors. By employing technologies like strained channel and nanoimprinting, flexible Si thin-film transistors (TFTs) with excellent high-frequency performance were obtained. In addition, to extend the application of flexible transistors toward higher frequency and higher power, compound semiconductor-based flexible microwave transistors attracted increasing attention due to their superior material properties like high electron mobility, wide bandgap, and so on. The flexible GaInP/GaAs heterojunction bipolar transistor (HBT) and flexible AlGaN/GaN high electron mobility transistor (HEMT) have shown higher operating frequency than flexible Si TFTs. Besides superior high-frequency performance, flexible GaN HEMTs also show great potential for high-power flexible microwave electronics due to their wide bandgap and excellent thermal stability. The demonstrated flexible GaN HEMT can sustain a dissipation power of 0.5 W while achieving maximum oscillation frequency of 115 GHz. Besides the flexible transistors, flexible passive components operating at the GHz range, i.e. necessary parts of microwave circuits, are demonstrated as well. The operating frequency of inductors and capacitors have been pushed to beyond 10 GHz with optimization in device design.
As the flexible microwave fundamental components continue to improve, flexible integrated circuits (ICs) consisting of fundamental components are being developed as well. Simple flexible circuits like switches and filters have been demonstrated by integrating Si or Ge diodes and passive components like inductors and capacitors. Flexible rectifiers, the critical component of wireless power transferring, are demonstrated using membrane GaAs Schottky diodes and flexible capacitors, which can harvest electromagnetic energy at WLAN band. Recently, a flexible power amplifier operating at 5.5 GHz with output power around 10 dBm has been reported. A reliable fabrication process flow has been developed to integrate small membrane GaN HEMT and flexible inductors and capacitors on polyimide thin film. Due to the excellent thermal reliability of GaN HEMT and heat spreader based on large coplanar copper ground plane, the amplifier can retain reliable operation with GaN membrane size as small as 0.5 mm by 0.5 mm.
Compared to the rapid advancement of flexible microwave electronics, the development of stretchable microwave components is quite restricted due to the increased fabrication complexity and degraded device performance. The existing stretchable microwave components focus on passive components, especially the interconnect transmission lines. Due to the rigidness and fragility of semiconductor materials, stretchable microwave devices are typically made by connecting compact high-performance rigid/flexible semiconductor devices with stretchable interconnects. We have demonstrated a stretchable transmission line using a twisted structure, which can operate up to 50 GHz with minimized electromagnetic leakage. Recently, we successfully fabricated stretchable inductors operating at the GHz range. By connecting polyimide encapsulated compact spiral inductors with serpentine interconnects, the stretchable inductors inherit the high performance of a spiral inductor as well as the stretchability of serpentine lines.
Although several key issues, including effective thermal management and enhanced reliability of the devices, require further research, flexible and stretchable microwave electronics have shown great promise in reshaping the existing electronics.