Radio frequency (RF) devices have been widely used for commercial wireless communication and military electronics. With the rapid development of wireless communication and radar technology, RF transistors for higher frequency and higher power applications are in demand. Meanwhile, the emerging market of wearable devices and internet of things (IoT) require electronic systems including RF parts to be in flexible or bendable form. Conventional semiconductors for flexible electronics like oxide semiconductors, and amorphous/poly Si and organic semiconductors cannot meet the requirement of wireless communication at the GHz range due to their low electron mobility (0.1 ~ 10 m²/V∙s). To fabricate high frequency flexible transistors, single crystal semiconductor nanomembranes have attracted great attention due to their high electron mobility (>100 m²/V∙s). Our group has reported GHz thin film Si MOSFET with an f_T of 1.9 GHz and a maximum oscillation frequency (f_max) of 3.1 GHz by transferring Si nanomembrane from a silicon-on-insulator wafer to a flexible plastic substrate. To further improve the performance of flexible RF Si MOSFETs, different device structures have been investigated. A strained Si nanomembrane channel has been demonstrated to enhance electron mobility. Flexible Si MOSFET with strained channel has shown an f_T of 5.1 GHz and f_max of 15.1 GHz. Gate trench Si MOSFETs using nanoimprint technology has been successfully fabricated. By reducing the gate trench to 100 nm, the transistor shows an f_T of 5 GHz and f_max of 38 GHz. Besides Si, we have also fabricated GaAs-based thin film heterojunction bipolar transistors (HBTs) for flexible RF electronics due to the high electron mobility of GaAs. We have successfully demonstrated thin film GaAs HBTs on biodegradable cellulose nanofibril (CNF) substrates by the transfer printing technique. HBTs on CNF substrate show an f_T of 37.5 GHz and f_max of 6.9 GHz, facilitating use in GHz range applications.

The reported flexible RF transistors based on Si and GaAs thin film have shown the capability of working at the GHz range. However, due to a small bandgap and low breakdown field, they are not suitable for high power applications at high frequency. GaN has been rapidly developed for high-frequency and high-power applications due to its large bandgap, high breakdown field, high electron mobility, and high electron saturation velocity of two dimensional electron gas (2DEG). Several flexible AlGaN/GaN HEMTs have been fabricated by the transfer printing AlGaN/GaN thin film to flexible substrate. Our group have reported flexible RF AlGaN/GaN HEMT on PET substrate with an f_T of 60 GHz and f_max of 115 GHz. A layer of 3.5 µm intrinsic single crystalline GaN thin film was used as the heat spreading layer for the HEMT due to large thermal conductivity of single crystalline GaN. The HEMT can successfully work under 0.5 W, which indicates the potential use of AlGaN/GaN HEMTs in high-frequency and high-power flexible electronics. We further combined flexible RF AlGaN/GaN HEMTs with environmental friendly green flexible substrates to fabricate flexible green RF electronics for high-power applications. We successfully fabricated a 500 µm × 500 µm AlGaN/GaN HEMT on biodegradable CNF substrate using the transfer printing technique. The HEMT can achieve an f_T and f_max of 40 GHz and 79 GHz, respectively, and work under 0.3 W on CNF substrate. To further exploit the application of flexible green RF AlGaN/GaN HEMTs, we fabricated a flexible RF power amplifier based on the HEMT. The RF power amplifier was fabricated by combining the flexible HEMT and impedance matching network consisting of a spiral inductor and metal-insulator-metal (MIM) capacitor on a temporary Si substrate with a PMMA sacrificial layer. After dissolving the PMMA sacrificial layer, the entire RF power amplifier (~10 µm thick) was transfer printed on a CNF substrate. The RF power amplifier on CNF substrate shows a small signal gain of ~6 dB from 5 GHz to 6 GHz. The RF power performance measurement shows it can deliver an RF power of ~10 mW with peak power added efficiency (PAE) of 4% at 5.5 GHz. The recent progress of flexible RF AlGaN/GaN HEMTs and RF power amplifiers will significantly advance the development of flexible electronics for high-frequency and high-power applications.