Semiconductor as a Charge Container
A bulk semiconductor may be viewed as a container of electrical charge. It contains both fixed charge and mobile charge. The fixed charge is the ionized impurity atoms, fixed in the lattice, replacing host (such as Si) atoms. They are fixed, i.e., immobile. The mobile charge is the electrons and holes. The mobile charge can move in and out of the semiconductor, while the fixed charge does not move at all.
Iin order to get connected to the external world, the semiconductor piece in any device must be contacted by two or more junctions of Ohmic contacts, PN junctions, or MS Schottky junctions. Therefore, the charge supply property of each of these junctions is importnat.
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