Pd-Functionalized Graphene-Gated GaN HEMTs for High-Sensitive Gas Detection and Optoelectronic Recovery

High-performance monitoring of nitrogen dioxide (NO2) is critically limited by the trade-off between sensitivity and recovery kinetics. Commercial sensing platforms predominantly rely on electrochemical or optical principles, which are inherently constrained by bulky form factors and slow response speeds, limiting their utility for compact, real-time monitoring. While solid-state sensors offer a compact alternative, conventional designs utilizing thick metal gates suffer from electrostatic screening and rely on thermal heating for desorption, resulting in high power consumption and slow response time. Here, we report a hybrid-dimensional heterostructure that integrates palladium (Pd) nano-islands and a graphene gate onto an AlGaN/GaN high electron mobility transistor (HEMT). This architecture circumvents screening effects, allowing surface adsorption events to efficiently modulate the underlying two-dimensional electron gas. We demonstrate that visible-light illumination induces rapid desorption of NO2, enabling room temperature operation with a recovery time of approximately 7 s. The device exhibits a current modulation exceeding 3000% at 10 ppm NO2, a performance supported by density functional theory (DFT) and technology computer-aided design (TCAD) simulations of the interface electrostatics. These results establish a pathway for a low-power, high-responsivity gas sensing system.