If the dopant added to an intrinsic semiconductor is a pentavalent impurity such as phosphorous, Arsenic, Antimony the resulting semiconductor is termed as N-type conductor. The added impurity atoms will displace some of silicon atoms in the crystal lattice. Four of the valence electrons of dopant will occupy covalent bonds with silicon; fifth electron will be nominally free and can be used as carrier of current. The energy required to detach this fifth electron is of the order of 0.01eV for Germanium and 0.05eV for Silicon.
This can be understood alternatively as follows, due to replacement of some of the silicon atoms with pentavalent atoms from the crystal the energy band structure of the crystal lattice gets altered. The consequence of this alteration is the introduction of new allowable energy levels in the band structure in the forbidden gap close to conduction band. The energy required to excite the electrons occupying this new energy level to conduction band is of the order of 0.01 eV for Germanium and 0.05 for Silicon.