The process whereby an impurity of one type is added to the silicon lattice to counteract the effect of an impurity of the opposite type is known as compensation, and is very important in device fabrication. Thus in the case of n -type Si, it is possible to introduce acceptor impurities so that when N A > N D , the Si becomes p -type , forming a p-n junction. In planar processing, a series of impurity diffusions may be needed; for example, diffusion of p -type impurities into an n -type "substrate", followed by a shallower diffusion of n -type impurities into the p -type layer would be required to make an n-p-n bipolar transistor.