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I was going through this article. There it's mention that

The addition of donor impurities contributes electron energy levels high in the semiconductor band gap so that electrons can be easily excited into the conduction band.

What I don't understand is that how can electron reside in extra electron energy level, when the level itself lie in the bandgap region.

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The energy difference between the donor states in the band gap and the bottom of the conduction band is small compared to $kT$. This means a significant number of electrons are excited into the conduction band by thermal interactions i.e. scattering off lattice vibrations.

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