Described is a molecular beam technique for fabricating semiconductor devices from Group III(a)-V(a) compounds. To form planar isolated devices, an amorphous insulative layer is formed on selected portions of a monocrystalline substrate of the Group III(a)-V(a) material which is at least semi-insulating. The amorphous layer may be formed by deposition of an oxide (e.g., SiO.sub.2), anodization of an oxide (e.g., native oxides) or by conversion of a surface layer of the substrate (e.g., by grit blasting). When a molecular beam containing Group III(a) and Group V(a) elements is directed at the surface, which is preheated to a temperature in the range of 450.degree. to 675.degree. C, monocrystalline Group III(a)-V(a) material grows on the exposed substrate whereas polycrystalline Group III(a)-V(a) material is simultaneously formed on the amorphous layer. The polycrystalline and monocrystalline surfaces are substantially coplanar. The polycrystalline material has a resistivity high enough to provide electrical isolation between active devices formed in the monocrystalline material. Examples of such active devices, which are also described, include beam-leaded Schottky barrier mixer diodes which have reduced parasitic capacitance and sealed-junction Schottky barrier IMPATT diodes. To form devices in which isolation is not required, the same procedure is followed except that neither the amorphous layer nor the substrate need be made of high resistivity material.

A silicon-semiconductor type thermal head comprising a substrate of .alpha.-alumina ceramic of single crystalline sapphire a silicon layer of high electrical resistance formed on the upper surface of the substrate and exothermic dots of low electrical resistance silicon integrally formed on the high resistance silicon layer. The silicon semiconductor type thermal head is formed by forming a substrate of .alpha.-alumina ceramic of single crystalline and sapphire, forming a high resistance layer of silicon on the .alpha.-alumina ceramic, forming a layer of low resistance silicon on the high resistance layer of silicon and selectively etching the low and high resistance silicon layers to produce exothermic dots of low resistance silicon, separated from the substrate by high resistance silicon.

A conductivity modulation type semiconductor device comprises a semiconductor anode substrate of a P type having two surfaces, a semiconductor substrate of an N type having two surfaces, the semiconductor substrate having a high impurity layer-like region on one surface thereof and a low concentration drain region on the other surface thereof, a body region of P type formed in the drain region and exposed at one surface of the semiconductor substrate, source regions of an N type formed in the body region and exposed at the other surface of the semiconductor substrate, and a gate layer formed within the isulating layer, which extends between the source and drain regions, on the body region. The other surface of the anode substrate is polished and is intimately joined to the polished surface of the semiconductor substrate to form a junction layer therebetween.