This paper will review the properties of thin silicon films deposited on sapphire (SOS) and magnesium aluminate spinel by the pyrolysis of silane in the temperature range WO-1200'C.Variations of carrier mobility, free-carrier concentration, minority carrier lifetime, crystalline perfection, and surface quality will be discussed as a function of substrate crystal and growth parameters. M O S transistors exhibiting field-effect mobility close to that obtained with bulk silicon have been fabricated and a complementary M O S transistor memory cell bas been constructed with a WRITE-READ delay of 6 ns. The standby power for the cell is typically 10 p W . Other C M O S circuits display a pair-delay of 1.5-2.0 no. All-epitaxial bipolar transistors with a current gain of 5 and f r of 350 M H z have been made in which all layers are sequentially deposited during one hightemperature operation. Recent improvements in bipolar fabrication techniques have lead t o current gains as high as 25 at 1 0 mA. I. INIXODUCTION E PITAXIAL SILICON layers are routinely deposited [l ] on silicon during the manufacture of monolithic integrated circuits. However, the deposition of highquality single-crystal silicon on an insulator is a much more difficult problem and places rigid demands on the insulator. Very few insulators fulfill all the necessary conditions and allow the growth of high-mobility silicon.The selection of sapphire as a film substrate was a natural one because of its mechanical and chemical properties and because a lattice plane could be selected that was a close match to that of silicon. Single-crystal films grown from SiCl, were first announced in 1963, and later described in a letter published in 1964 [2]. Mueller and Robinson described singlecrystal silicon grown from SiH, and the first MOS transistors made from it in 1964 [3].The orientation relationship of the substrate and the silicon films have been studied extensively [4], [5]. Although the lattice match between the silicon film and the sapphire substrate is not good on any sapphire plane, the long range order in the aluminum-oxygen sites in the substrate appears to allow epitaxy to take place. Two of the more frequently studied orientation relationships have been (1 1 1 ) and (100) silicon grown on (0001) and (1i02) sapphire, respectively.Spinel (various ratios of MgO : A1,0,) is closely related to sapphire but has a cubic structure and consequently provides a simpler crystallographic match to silicon. A con-