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

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 ~4.28×1
⁰¹³
cm
^-2
, which was about two times larger than that located in Al
₂
O
₃
/GaN interface (~1.81×10
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 Vth of 0.35 V, maximum transconductance (G
_m,max
) of 96.5 mS/mm, maximum drain current (I
_d,max
) of 516 mA/mm at Vg=8 V, and f
_T
/f
_max
of 19/12.5 GHz. While the FR normally-off MISHEMT exhibiting a Vth of 1.3 V, G
_m,max
of 75.6 mS/mm, Id,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.