An integrated power module having a depletion mode device and an enhancement mode device that is configured to prevent an accidental on-state condition for the depletion mode device during a gate signal loss is disclosed. In particular, the disclosed integrated power module is structured to provide improved isolation and thermal conductivity. The structure includes a substrate having a bottom drain pad for the depletion mode device disposed on the substrate and an enhancement mode device footprint-sized cavity that extends through the substrate to the bottom drain pad. A thermally conductive and electrically insulating slug substantially fills the cavity to provide a higher efficient thermal path between the enhancement mode device and the bottom drain pad for the depletion mode device.