257 / 2017-11-22 23:47:57

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

AlGaN/GaN MISHEMT,etching,Normally-off,interface charge,CF4-plasma,recessed-gate

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Partially recessed (PR) and fully recessed (FR) normally-off GaN-based metal insulator semiconductor high electron mobility transistors (MISHEMT) have been fabricated using CF4-based plasma and Cl2-based plasma etching process, respectively. By analyzing the threshold voltage (Vth) models of both structures, the positive fixed sheet charge density located in Al2O3/AlGaN for PR MISHEMT was estimated to be ~4.28×1013cm-2, which was about two times larger than that located in Al2O3/GaN interface (~1.81×1013cm-2) for FR MISHEMT. The CF4-based plasma was intentionally utilized to etch the AlGaN barrier as well as introduce negative fluorine charges in Al2O3/AlGaN interface for compensating the large positive charges brought by the ALD-Al2O3 deposition on AlGaN layer and eventually promoting fabrication of the PR normally-off MISHEMT. The fabricated PR normally-off MISHEMT featuring a Vth of 0.35 V, maximum transconductance (Gm,max) of 96.5 mS/mm, maximum drain current (Id,max) of 516 mA/mm at Vg=8 V, and fT/fmax of 19/12.5 GHz. While the FR normally-off MISHEMT exhibiting a Vth of 1.3 V, Gm,max of 75.6 mS/mm, Id,max of 405 mA/mm and fT/fmax of 13/9.6 GHz. The above results indicates that CF4 based plasma etching process is a promising candidate for the gate recessing process in fabrication of partially recessed Al2O3/AlGaN/GaN normally-off MISHEMTs for RF power applications.