Au-Free Normally-Off AlGaN/GaN-on-Si MIS-HEMTs Using Combined Partially Recessed and Fluorinated Trap-Charge Gate Structures

In this letter, partially recessed gate structures in conjunction with negative trap charges by F
^-
plasma treatments both at AlGaN barrier and on gate dielectric surface are employed to realize the normally-OFF operation for AlGaN/GaN Metal-Insulator-Semiconductor High Electron Mobility Transistors in Au-free scheme. A partial gate recessed trench is designed to effectively reduce the 2-D electron gas (2DEG) density and achieve positive threshold voltage (V
_th
) without severe degradation in 2-DEG channel mobility. Furthermore, the fixed trap charges are innovatively placed at the gate AlGaN and Si
₃
N
₄
layers by a two-stage F
^-
plasma treatment to further increase the V
_th
, without mobility degradation. A high V
_th
of 1.9 V and a drain current ~200 mA/mm are achieved in the fabricated device, which also has a lower leakage current and the higher breakdown voltage of 580 V.