An extrinsic semiconductor is electrically neutral at equilibrium. In an n-type semiconductor, the conduction band electrons equal the sum of electrons from donor atoms and those excited from the valence band.
Electrons from donor levels leave behind positive ions, and excitation from the valence band leaves holes, maintaining charge balance.
The charge neutrality condition states that the total negative charge of mobile electrons equals the total positive charge in the crystal.
The charge neutrality condition applied to the n-type semiconductor implies that the total negative charge of mobile electrons is equal to the total positive charge created in the crystal. It means that
