Flexible (and stretchable, ibid.) electronics provide a ubiquitous platform for many applications that cannot be addressed by its rigid chip-based counterparts. Flexible electronics can be divided into two large domains based on the major active material used: organic flexible electronics and inorganic flexible electronics. Over the past 15 years, the latter, particularly single-crystalline semiconductor materials-based inorganic flexible electronics, has addressed a wide range of applications, including those that have been properly addressed by the organic flexible electronics, thus demonstrating the enormous potential of the new platform. On this platform, the majority of research efforts have been focused on sensors, energy, etc. where biomedical applications have occupied the lion’s share.

Microwave electronics, prevailed by the monolithic microwave integrated circuits (MMIC)-based rigid electronics, have advanced over the last decades and became the backbone of wireless communications. Nonetheless, flexible microwave electronics can command many new wireless applications and wireless sensing applications where its rigid counterparts fail. Single-crystalline semiconductor materials-based inorganic flexible electronics, owing to its superior frequency properties, encompass everything needed for flexible microwave electronics. Since 2005, our group has focused on microwave flexible electronics from the ground level, by creating basic microwave components in the flexible/stretchable forms, such as transistors, inductors, capacitors, etc. A collection of active devices and passive components working from 1 GHz to over 100 GHz have been subsequently demonstrated. However, assembling a working microwave amplifier circuit, the basic circuit building block in a wireless system, using these individual parts has never been demonstrated.

Meanwhile, our society has witnessed the production of a plethora of electronic waste as personal electronics are continuously upgraded. With the explosion of new applications produced for the Internet of Things, flexible electronics are expected to be produced at a much higher rate than traditional rigid electronics, leading to the generation of more electronic waste. Therefore, it is compulsory to consider the environmental impact as we further advance microwave flexible electronics. To this front, investigation of environment-friendly materials for their use in flexible electronics is a promising research direction to pursue.

Cellulose nanofibril (CNF), a nanoscale material derived from wood that has been explored for many new applications over the past few years, can be easily made into plastic-like flexible substrates suitable for microwave applications. These substrates are biodegradable or simply burned. If they can be used as electronic chip substrates, the cost of the substrate in the overall electronic chip becomes negligible. From a microwave performance point of view, today’s best microwave amplification semiconductor device is the AlGaN/GaN HEMT. However, its fabrication is a costly process due to the methods used for material production and expensive substrates. Consequently, employing the minimum amount of AlGaN/GaN materials to create high-performance microwave flexible electronics on the CNF substrate is economically, technologically, and environmentally significant.

In our recent publication, “Heterogeneously integrated flexible microwave amplifiers on a cellulose nanofibril substrate”, we described a flexible AlGaN/GaN HEMT microwave amplifier on a CNF substrate by employing a heterogeneous integration strategy. In our amplifier, high performance was achieved with the majority of the area and weight of the circuit chip occupied by CNF.  As a result, the amplifier chip can be easily disposed of or rapidly incinerated.

The work was accomplished in collaboration with engineers from Wisconsin Institute For Discovery and from Forest Products Laboratory under Forest Service of the Department of Agriculture, located in Madison, Wisconsin.