Electrostatic effects of self-trapped holes in β-Ga2O3 devices

β-Ga2O3 is an ultra-wide bandgap semiconductor with exceptional properties for power electronics and UV-C optoelectronics, but its behavior under illumination remains poorly understood. In this work, we investigate how optically generated self-trapped holes influence electrostatics and current conduction in β-Ga2O3 devices. Using a vertical Schottky photodiode with a semi-transparent Ni anode, we performed capacitance–voltage (C–V), current–voltage (I–V), and temperature-dependent I–V measurements under dark and above-bandgap illumination. Analysis of photocurrent gain reveals that conventional image-force barrier-lowering models require unrealistically high interfacial electric fields, suggesting the presence of an alternative mechanism. By applying the Fowler–Nordheim tunneling theory, we reconcile measured photocurrents and photo-capacitance results with physically plausible fields and quantify the two-dimensional concentration of self-trapped holes. Our findings demonstrate that illumination-induced charge significantly alters device electrostatics. Understanding this tunneling-based photocurrent gain mechanism is critical for designing β-Ga2O3 devices for UV-C detectors and power electronics.