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Substrate (also called a wafer) is a solid (usually planar) substance onto which a layer of another substance is applied, and to which that second substance adheres. In solid-state electronics, this term refers to a thin slice of material such as silicon, silicon dioxide, aluminum oxide, sapphire, germanium, gallium arsenide (GaAs), an alloy of silicon and germanium, or indium phosphide (InP). These serve as the foundation upon which electronic devices such as transistors, diodes, and especially integrated circuits (ICs) are deposited.
Note that a substrate in the field of electronics is either a semiconductor or an electrical insulator, depending on the fabrication process that is being used. For the cases in which an insulator such as silicon oxide or aluminum oxide is used as the substrate, what happens next is the following. On top of the oxide, a thin layer of semiconducting material, usually pure silicon. Next, using the standard photographic processes repeatedly, transistors and diodes are fabricated in the semiconductor.
The advantage of this (more costly) fabrication process is that the oxide layer can provide superior insulation between adjacent transistors. This process is especially used for electronics which must withstand ionizing radiation, such as in space exploration missions through the Van Allen Radiation Belts; in military and naval systems which might have to withstand nuclear radiation; and in instrumentation for nuclear reactors.
In the manufacture of ICs, the substrate material is usually formed into or cut out as thin discs called wafers, into which the individual electronic devices (transistors, etc.) are etched, deposited, or otherwise fabricated.