The breakdown voltage of GaN/Si high electron mobility transistors (HEMTs) for power electronics has shown to be improved by removing the silicon substrate. The drawback to this approach is the increase in the device's thermal resistance which limits the power dissipation that the device can achieve before severe degradation. This study shows the ability to improve the thermal dissipation of these devices by depositing Copper (Cu) below Aluminum Nitride (AlN) filled etched back GaN-on-Si HEMTs. The device's channel temperature is measured via Raman thermometry. The device's transient thermal dynamics is investigated via transient thermoreflectance imaging and the temperature profile across the gate metal is monitored. In addition to the device's thermal properties, residual stress analysis of the GaN channel is performed via photoluminescence. A notable decrease in the tensile residual stress is observed with the removal of the substrate and the addition of the AlN and Cu layers. Overall, the backside copper is shown to decrease the gate temperature of the etched backed AlN filled devices while maintaining a high breakdown voltage.