A microwave transistor package is provided having a thermal and electrical conductive refractory type substrate. A ground plane plate is provided, such plate having an open-ended compartment therein, the open end of such compartment being disposed adjacent an upper surface portion of the ground plane plate. A bottom surface of such plate is disposed on an upper, inner surface region of the substrate. The thermal coefficient of expansion of the ground plane plate is substantially greater than the thermal coefficient of expansion of the substrate. An electrical insulator is disposed within the compartment, such insulator having a relatively high thermal transfer characteristic and having upper and lower conductive layers with side wall portions disposed adjacent side wall portions of the compartment. A first apertured ceramic insulating layer having conductive metallization regions disposed on the upper surface of the ceramic insulator extending from the apertured edge of the ceramic insulator to the outer edge of the ceramic insulator is provided. The lower surface of the ceramic insulator is disposed over the substrate and about peripheral portions of the inner surface region of the substrate.