Question

what is immobile charges?​

Answer 1

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.

Answer 2

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Immobile charges:

The fixed charge is the ionized impurity atoms, fixed in the lattice, replacing host (such as silicon) atom.