High threshold voltage in GaN MOS-HEMTs modulated by fluorine plasma and gate oxide

AlGaN/GaN high-electron-mobility transistors (HEMTs) have great potential for the next generation of power electronics. In this application, enhancement-mode (E-mode) metal-oxide-semiconductor (MOS) HEMTs with a threshold voltage (V_th) ~ 3 V are highly desirable as they allow simpler circuits and fail-safe operation. However, it has been recently shown that the V_th in GaN MOS-HEMT structures does not typically increase by increasing the gate oxide thickness, due to the presence of positive interface charges and oxide bulk charges [1]. On the other hand, a high V_th was reported in fluorinated MOS-HEMTs (~5 V) [2], although no analysis or explanation have been presented for this behavior. There is, therefore, a need for a systematic study of the V_th in fluorinated MOS-HEMTs. In this work, E-mode fluorinated MOS-HEMTs were realized with a V_th higher than 3 V and a V_th increasing with the gate oxide thickness and fluorine plasma time. A comprehensive analytical model for V_th was proposed for fluorinated MOS-HEMTs for the first time and was verified by experimental data. Moreover, fluoride-induced negative bulk charge inside the oxide layer was demonstrated to significantly contribute to the high V_th. This suggests that the fluorine plasma treatment is a promising technique to facilitate a high V_th for E-mode MOS-HEMTs.