The diffusion equation is,

$\large \frac{\partial C}{\partial t}= D\nabla^2C$ .

In constant source diffusion, the concentration is held constant at the surface. The concentration of dopants is,

$\large C(z,t)=C_0\text{erfc}\left(\frac{z}{\sqrt{4Dt}}\right)$.

The concentration falls at the surface and the total number of dopants remains constant.