Fabrication and Characterization of Flexible AlGaN/GaN HEMTs on Kapton Tape

In this work, we successfully used wet etching to remove the Si substrate and transferred AlGaN/GaN high-electron mobility transistor (HEMT) film with a gate length of ${2}~\mu \text{m}$ to the flexible Kapton tape. The transferred area of AlGaN/GaN epitaxial thin film reached $1.2\times1.2$ cm2. The flexible HEMTs still exhibit excellent electrical characteristics with the maximum drain current density ( ${J}_{\text {DS,max}}$ ) of 290 mA/mm and the maximum transconductance ( ${g}_{\text {m,max}}$ ) of 108 mS/mm. Furthermore, we analyzed the physical mechanisms when devices were under bending. Under bending, the tensile strain enhances the piezoelectric field within the AlGaN layer, consequently increasing the 2-D electron density. In the condition that flexible HEMTs are bent with a curvature radius of 0.3 cm, ${J}_{\text {DS,max}}$ still reaches 346 mA/mm and ${g}_{\text {m,max}}$ achieves 125 mS/mm. Besides, the main reason for the decrease in dc characteristics is verified by self-heating. As for the RF characteristics, the Si substrate removal can effectively decrease the output parasitic capacitances. Although ${g}_{\text {m,max}}$ becomes half after the transfer processes, the cutoff frequency and the maximum oscillation frequency of flexible HEMTs still reaches 3.8 and 6.5 GHz, respectively.