Unique Monotonic Positive Shifts in Threshold Voltages of Ga₂O₃-on-SiC MOSFETs Under Both Unipolar Positive and Negative Bias Stresses

In this study, the instability of heterogeneous Ga2O3-on-SiC (GaOSiC) MOSFETs under unipolar positive/negative bias stress (UPBS/UNBS) was investigated systematically. By adjusting key parameters of stress voltage waveform, including frequency (f), holding time (${t} _{\text {h}}$), rising time (${t} _{\text {r}}$), and falling time (${t} _{\text {f}}$), a two-phase shift in threshold voltage (${V} _{\text {T}}$) with cycle number (${C} _{n}$) was observed in the UPBS measurement. The UPBS-induced positive${V} _{\text {T}}$shift is primarily attributed to electron trapping by traps at/near the Al2O3/$\beta $-Ga2O3 interface during the${t} _{\text {r}}$transient. At the beginning of UPBS test (first phase of degradation), most trap states are unoccupied and the electron trapping is predominant, leading to similar${V} _{\text {T}}$shifts in different stress conditions. With${C} _{n}$further increasing (second phase of degradation), deeper level traps begin trapping electrons due to the accumulation effect, which has been found to have a strong correlation with${t} _{\text {f}}$. Notably, a monotonic positive shift in${V} _{\text {T}}$was also observed in the UNBS measurement. This${V} _{\text {T}}$shift under UNBS was modeled using a stretched exponential equation. Parameters of this equation were used to characterize the rate and magnitude of${V} _{\text {T}}$degradation under various stress conditions, elucidating the significant role of f and${t} _{\text {r}}$in the${V} _{\text {T}}$degradation of GaOSiC MOSFET. TCAD simulations suggest that the monotonic positive shift in${V} _{\text {T}}$under UNBS is caused by electron trapping at the$\beta $-Ga2O3/SiC interfacial layer, which contains a high density of traps.