Experimental Demonstration of Nonlinear Photoconductive Gain in N-Doped $β$-Ga$_2$O$_3$ Devices
Photoconductive devices based on ultra-wide-bandgap (UWBG) materials offer a promising pathway toward compact, high-voltage (HV) optoelectronic and optical sensing in harsh environments. In this Letter, we report field-tunable nonlinear photoconductive gain in vertical $β$-Ga$_2$O$_3$ photoconductive devices under sub-bandgap visible-light excitation. The devices were fabricated on a $5.6\,μ\text{m}$-thick nitrogen-doped semi-insulating $β$-Ga$_2$O$_3$ epilayer grown on a conductive Sn-doped substrate and characterized under $445\,\text{nm}$ continuous-wave illumination. A distinct transition from linear to nonlinear photoconductive behavior is observed at a threshold electric field of approximately $0.67\,\text{MV/cm}$, resulting in an approximately $20\times$ enhancement in photocurrent. Complementary TCAD simulations indicate strong electric-field localization and a rapid increase in impact-ionization generation at high bias, suggesting that impact-ionization--assisted carrier multiplication contributes to the observed gain. These results demonstrate a high-field visible-light photoconductive detection mode in $β$-Ga$_2$O$_3$ enabled by defect-assisted transport, providing a pathway toward field-tunable gain photodetectors operating without deep-ultraviolet (DUV) excitation.