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.