Gallium arsenide, one kind of semiconductor material, is made by using epitaxial technology. Epitaxial wafer is typically 4-6 inches in diameter, much smaller than a 12-inch silicon wafer. MOCVD is the main method used for Epitaxy now. Epitaxial wafers need a special equipment, which require tungsten and molybdenum materials.
The silicon-on-sapphire process in RF CMOS process is commonly referred to as Ultra CMOS in this industry. Sapphire is essentially an ideal insulator with high parasitic capacitance insertion loss and low isolation under substrate. Ultra CMOS can make a very large RFFET. As for normal substrate with thickness of 150 ~ 225μm, there is almost no parasitic capacitance. Sapphire crystal furnace is inseparable from the heating element, Heating zone and insulation layer equipment, which are made of tungsten and molybdenum materials.