When an atom
of  +5 valency element  occupies the position
of an atom in the crystal lattice of Si, four of
its electrons bond with the four silicon
neighbours while the fifth remains very
weakly bound to its parent atom This is
because the four electrons participating in
bonding are seen as part of the effective core
of the atom by the fifth electron As a result
the ionisation energy required to set this
electron free is very small and even at room
temperature it will be free to move in the
lattice of the semiconductor For example, the
energy required is ~ 0