A new model is provided for the CVD growth of silicon germanium from silicon-containing and germanium-containing precursors. According to the new model, the germanium concentration x is related to the gas phase ratio according to the equation [x/(1−x)]2=mPGe/PSi, and m=Ae−E/(RT), where PSi is the partial pressure of the silicon-containing precursor, PGe is the partial pressure of the germanium-containing precursor, A is a constant, R is the universal gas constant, and T is the temperature. Methods and apparatuses are described for controlling CVD process parameters, associated with a series of reactions at constant or varied temperature, to achieve targeted germanium concentrations in silicon germanium films deposited onto semiconductor substrates. In particular, the new model can be used to calculate the resultant germanium concentration for selected precursor flow rates. The new model can also be used to control a precursor injection apparatus to achieve a desired germanium concentration.