Comparison of CF4 plasma versus Cl2 plasma dry etching for gate-recessed Normally-off GaN-based MISHEMT

Abstract

Partially recessed (PR) and fully recessed (FR) normally-off GaN-based metal insulator semiconductor high electron mobility transistors (MISHEMT) have been fabricated using CF 4 -based plasma and Cl 2 -based plasma etching process, respectively. By analyzing the threshold voltage (V th ) models of both structures, the positive fixed sheet charge density located in Al 2 O 3 /AlGaN for PR MISHEMT was estimated to be $\sim 4.28 \times 10^{13}cm^{-2}$, which was about two times larger than that located in Al 2 O 3 /GaN interface ($\sim 1.81 \times 10^{13}cm^{-2}$) for FR MISHEMT. The CF 4 -based plasma was intentionally utilized to etch the AlGaN barrier as well as introduce negative fluorine charges in Al 2 O 3 /AlGaN interface for compensating the large positive charges brought by the ALD-Al 2 O 3 deposition on AlGaN layer and eventually promoting fabrication of the PR normally-off MISHEMT. The fabricated PR normally-off MISHEMT featuring a V th of 0.35 V, maximum transconductance (${\rm{G}}_{m,max}$) of 96.5 mS/mm, maximum drain current (${\rm{I}}_{d,max}$) of 516 mA/mm at Vg=8 V, and ${\rm{f}}_{T}/{\rm{f}}_{max}$ of 19/12.5 GHz. While the FR normally-off MISHEMT exhibiting a V th of 1.3 V, ${\rm{G}}_{m,max}$ of 75.6 mS/mm, ${\rm{I}}_{d,max}$ of 405 mA/mm and f T /f max of 13/9.6 GHz. The above results indicates that CF 4 based plasma etching process is a promising candidate for the gate recessing process in fabrication of partially recessed Al 2 O 3 /AlGaN/GaN normally-off MISHEMTs for RF power applications.