Abstract: To minimize contamination of gas flow lines and reactor surfaces from high impurity concentrations present in the CVD reactor, control of the dopant gas supply is located closely adjacent to the reactor input port and the dopant gas supply line is separately vented. First and second dopant gas supplies and a diluent gas supply are connected to branch lines which converge to form the dopant supply line. A solenoid valve is situated in the main dopant supply line as close to the input port as possible. A vent line is connected to the dopant supply line, prior to the solenoid valve. The etchant and silicon gas supplies are each connected to the reactor input by a separate supply line. The etchant and silicon gas supply lines are vented separately from the dopant gas supply line.