Step 1: Understanding the Concept:
In an n-type semiconductor, pentavalent impurity atoms (like Phosphorus, Arsenic) are added to a pure semiconductor crystal (like Silicon or Germanium).
These impurity atoms have five valence electrons. Four of them form covalent bonds with neighboring host atoms, while the fifth electron is loosely bound.
Step 2: Key Formula or Approach:
Band theory of solids explains this using energy levels.
The loosely bound fifth electron requires very little energy to become completely free (to enter the conduction band).
Therefore, its allowed energy state must be very close to the conduction band.
Step 3: Detailed Explanation:
The energy level associated with the fifth valence electron of a donor atom is called the donor energy level ($E_D$).
Because this electron needs only a small thermal energy (about $0.01 \text{ eV}$ for Ge and $0.05 \text{ eV}$ for Si) at room temperature to jump into the conduction band, the donor energy level must be located just below the bottom of the conduction band ($E_C$).
It cannot be inside the conduction band itself initially because it is localized to the impurity atom at absolute zero temperature.
It is situated within the forbidden energy gap (band gap), very close to the conduction band minimum.
Step 4: Final Answer:
The free electrons occupy energy levels in the band gap and are close to the conduction band.