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Almost all integrated circuits are fabricated from silicon. The principal exceptions are some very high-speed digital integrated circuits that use gallium arsenide (GaAs) to take advantage of the very high electron mobility in that material. Integrated circuits are fabricated using the same basic processes as for other semiconductor devices. In integrated circuits the vast majority of devices are transistors and diodes, with relatively few passive components such as resistors and capacitors. In many cases no capacitors at all are used, and in some cases there are no resistors either. The active devices contained in an integrated circuit are transistors, including bipolar junction transistors (BJTs), junction field-effect transistors (JFETs), and metal-oxide silicon field-effect transistors (MOSFETs). Digital integrated circuits are made up predominately of MOSFETs, with generally very few other types of components. Some analog integrated circuits use mostly, or even exclusively, BJTs, although many integrated circuits use a mixture of BJTs and field-effect transistors (either JFETs or MOSFETs), and some are even exclusively FETs, with no BJTs at all.
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