Trap-Induced Instability in β -Ga 2 O 3 Solar-Blind Phototransistors Under Electro-Optical Stress

The trap-assisted instability in$\beta $-Ga2O3solar-blind phototransistors under long-term electro-optical stress is systematically investigated. Under the negative bias stress (NBS) in the dark, threshold voltage (${V}_{\text {th}}\text {)}$first shifts slightly negative at −3 V and then positive to 0.05 V at −5 V, accompanied by a reduced hysteresis ($\Delta {V}_{\text {hysteresis}}$). This transition is attributed to a crossover from border-trap-dominated detrapping to field-assisted capture by bulk traps in the$\beta $-Ga2O3buffer layer. Although the responsivity (${R}\text {)}$remains nearly constant, detectivity (${D}^{\ast }\text {)}$decreases by almost one order of magnitude due to leakage-induced enhancement of the pinch-off current (${I}_{\text {pinch}-\text {off}}$). Under simultaneous ultraviolet (UV) illumination, the${V}_{\text {th}}$shift enlarges to 0.3 V, and the${I}_{\text {pinch}-\text {off}}$degradation ratio increases from 136% to 298%, indicating strong photo-assisted charge buildup in the buffer. In this regime, both$R$and${D}^{\ast }$degrade by nearly an order of magnitude as photo-generated carriers activate abundant deep traps and accelerate electron capture. A trap-assisted physical model incorporating border and buffer traps reproduces the observed parameter shifts and current degradation. These results reveal that photo-induced activation of buffer-layer traps is the primary origin of electro-optical instability, highlighting buffer-layer defect engineering as a key route to enhance the long-term reliability of$\beta $-Ga2O3solar-blind phototransistors.