Once again, the assembly is inserted into a vacuum device and a layer of an insulating material such as Al2 O3, for example, is uniformly evaporated onto the exposed surfaces thereby yielding a structure as shown in FIG. 4F. Upon dissolution of the photoresist material 26 of FIG. 4F, the planar configuration of FIG. 4G having a substrate 12 with a source electrode 14, a drain electrode 16 and insu