In a semiconductor, the number density of intrinsic charge carriers at \( 27^\circ \text{C} \) is \( 1.5 \times 10^{16} \, /\text{m}^{3} \). If the semiconductor is doped with impurity atom, the hole density increases to \( 4.5 \times 10^{22} \, /\text{m}^{3} \). The electron density in the doped semiconductor is ______ \( \times 10^{9} \, /\text{m}^{3} \).
