Normally dopants will occupy a lattice site, and the dopant is said to be substitutional. When the dopant is in the substitutional position it is not likely that it will move around. However, when it gets in contact with a point defect, it will be mobile. A point defect in silicon, can be a Si self-interstitial (a silicon atom which does not occupy a lattice site), or a vacancy (a lattice site which does not contain a silicon atom). As illustrated in Fig. 3.37, t here are mainly two possible atomic diffusion mechanisms known as interstitial and vacancy diffusion. The diffusion mechanisms through grain boundaries and surface are also possible. The highest activation energy is for volume diffusion though interstitial and vacancy sites. The grain boundary diffusion requires less energy and surface diffusion requires the least.