Solid-solubility limit

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 [1]. 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 [1]. 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.[1]

Solid solubility and electrically active impurity-concentration limits in silicon[1].