Why the donor electrons from the dopant atoms always approach the space below the conduction band?

Question

Why the donor electrons from the dopant atoms always approach the space below the conduction band?

Answer 1

They don't always - it depends on the doping concentration.  But the donors are choosen such that their energy is higher than the Fermi energy for the host material.  That allows the electrons in the dopant to be in the forbidden band.  Further, the dopants have lower ionization potentials than the host, so their electrons' energies lie close to the host's conduction band edge.