A limited source diffusion is used to define the base of an npn transistor. Estimate the starting concentration of impurity if the base-collector junction lies 1micro-meter below the surface after 2h diffusion at 1100 degrees C. The diffusion constant of Boron at this temperature is 2*10^-17 m^2 s^-1. A background donor concentration of10^22 m^3 may be assumed.