# Number of electrons per unit volume

Why is the number of electrons per unit volume , n fixed in a conductor regardless of the temperature while its not in semiconductors ??

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I think you mean number of free/conduction electrons.
In a semiconductor, electrons can promoted to the conduction band by thermal energy; i.e. by raising the temperature of the material. In a conductor, the number of free/conduction electrons is more or less at saturation, and heating the material does not produce any more.

Of course the total number of electrons in the material is not affected by temperature.

I think you mean number of free/conduction electrons.
In a semiconductor, electrons can promoted to the conduction band by thermal energy; i.e. by raising the temperature of the material. In a conductor, the number of free/conduction electrons is more or less at saturation, and heating the material does not produce any more.

Of course the total number of electrons in the material is not affected by temperature.
thanks so free electrons and electrons are different ? I thought they are the same .

They are only "different" in that in any material some are free to take part in conduction and others are not.
Some are situated deep in the lower energy levels of the atom where they are strongly bound, while others are in the outer levels where they are weakly bound.