In present high-power monolithic microwave integrated circuits (MMIC), the heat generated causes increased reliability problems and lower performance. One approach to these problems is to reduce the thickness of the substrate to improve heat conduction away from the MMIC. But typically the substrate is a large wafer, e.g. three inches in diameter, and with reduction in thickness quickly becomes marginally thin and fragile. In addition, the reduction in thickness reduces the impedance of the substrate relative to the RF transmission lines on the substrate and interferes with the microwave transmission line impedance matching design considerations.
A second problem relates to the ground inductance created when "vias" or other conductors are used to connect the source electrode on one side of the substrate to the ground plane on the other side. This causes series feedback, resulting in instability and degradation of RF performance. In addition, with the FET on the opposite side of the substrate from ground, there is a significant thermal resistance which can result in the FET operating at as much as 100.degree. C. above the temperature of the heat sink. One attempt to avoid these problems in discrete FETs used a "flip chip" approach in which the discrete FET was flipped upside down and the drain, source and gate were connected to terminals on the top of raised pedestals. This introduced another set of problems because the connection to the pedestals had to be done blindly, which resulted in misregistration and collection of unseen debris in the pedestal area, which deleteriously affected the performance of the FET. For these reasons this approach never won wide acceptance. Further, since it was not applicable to complex integrated circuits it could not be used to address the problem in the integrated circuit technology.