At a given temperature, there is an upper limit to the amount of an impurity, which can be absorbed by silicon. This quantity is called the solid-solubility limit for the impurity and is indicated by solid lines in the figure below for boron, phosphorus, antimony, and arsenic at normal diffusion temperature . For example, the solid-solubility limit for boron is approximately 3.3X1020/cm3 at 1100°C and 1.2X1021/cm3 for phosphorus at the same temperature . Surface-concentrations achieved through solid-solubility limited diffusions are quiet high and are useful for emitter and collector regions in bipolar transistors and also for, source and drain regions in MOSFETs.
Solid solubility and electrically active impurity-concentration limits in silicon.