In a semiconductor, the number density of intrinsic charge carries at 27 $^\circ$ C is 1.5 $\times$ 1016/m3. If the semiconductor is doped with impurity atom, the hole density increases to 4.5 $\times$ 1022/m3. The electron density in the doped semiconductor is $\underline{\hspace{2cm}}$$\times$ 109/m3.