An n-type semiconductor is created by adding impurities with more electrons than the host semiconductor, such as phosphorus in silicon. This results in additional electrons in the conduction band, shifting the Fermi level closer to the conduction band. Let's analyze the band structures:

1. Structure I shows the Fermi level close to the conduction band, which is typical for n-type semiconductors as the majority carriers are electrons.
2. Structure II displays the Fermi level near the middle, which is characteristic of intrinsic semiconductors.
3. Structure III depicts the Fermi level closer to the valence band, which is typical for p-type semiconductors where holes are the majority carriers.
4. Structure IV is incorrect for an n-type semiconductor because it suggests high density of states without specific placement of the Fermi level.

Based on these observations, the correct answer is option I, as it correctly represents the band structure of an n-type semiconductor with the Fermi level near the conduction band.