The extremely high concentration of the electric field near the drain corner of the gate is the major cause for premature breakdown and local hotspot formation of AlGaN/GaN high electron mobility transistors (HEMTs). This hotspot negatively impacts the device performance, reliability, lifetime, and limits the operability envelope. Metal field plates have been used to mitigate this electric field concentration. However, it has been proven difficult to reach the theoretical breakdown voltage of GaN using conventional field plate structures. In this work, a novel "resistive field plate" structure based on amorphous indium-gallium-zinc-oxide (a-IGZO) was constructed to minimize the electric field concentration by imposing a linear voltage boundary condition on the device surface, between the drain and source electrodes. Standard electrical testing and state-of-the-art optical thermography techniques were used to understand the electro-thermal interactions behind the improvement in electrical and thermal performance of AlGaN/GaN metal-oxide-semiconductor HEMTs (MOS-HEMTs) employing the a-IGZO resistive field plate. The novel field plated design resulted in -25% increase in the device breakdown voltage (VB) and -24% reduction in the channel peak temperature as compared to a control sample employing no field plate structures. Key knowledge obtained from this study provides insight into enhancing the performance and reliability of AlGaN/GaN HEMTs and also suggests a viable solution that may enable the full exploitation of the potential of wide bandgap and ultra-wide bandgap lateral power electronic devices.