Short constant-source diffusion is often followed by limited-source diffusion, resulting in a "two-step" diffusion process. 
The constant-source diffusion step is used to de a known dose in a shallow layer on the surface of the silicon and is called the predeposition step. The fixed dose approximates an impulse and serves as the impurity source for the second diffusion step.
The second diffusion is called the drive-in step and is used to move the diffusion front to the desired depth. If the Dt product for the drive-in step is much greater than the Dt product for the predeposition step, a Gaussian distribution closely approximates the resulting impurity profile and is given by the following expression.