When we add n-type or donor impurities to the semiconductor, the width of the forbidden energy gap in the lattice structure is reduce. Now ‘4’ of the five valance electron of impurity atom will involve in bonding with 4 neighbourhood semiconductor atom, but the fifth one electron will not find any place to occupy. This fifth electron of the impurity atom can be made available as free electron. The energy require to detach the this fifth electron from it parent atom is in the range of .01 ev to .05 ev. So discrete energy level just below conduction level.