4. Consider the impurity diffusion of phosphorus into a silicon wafer. If phosphorus is diffused into a thick silicon waler with no previous phosphorus in it at a temperature of 1100℃. If the surface concentr tration of the phosphorus is 1 x atoms/cm³and its eoncentration at ce of 1.2 Jum below the surface is atoms/cm³, how long must be required for the impurity diffusion? The difiusion coeficient for phosphorus diffusing in silicon at 1 100*C is 3.0 x. (10%)