In semiconductors, the process of diffusing an impurity is called doping. The conduction properties are very sensitive to the amount of impurities For eg. When a phosphorous atom with five outer electrons is substituted for a silicon atom, an extra electron is made available for conduction. Thus, the number of conduction electrons increases due to the introduction of a penta-valent impurity in silicon. The introduction of phosphorus in the proportion of 1 in increases the conductivity by a factor of about . So, the desired properties of semiconductor can be obtained by controlling the amount and distribution of impurity.