A method and apparatus for depositing III-V material is provided. The apparatus includes a reactor partially enclosed by a selectively permeable membrane 12. A means is provided for generating source vapors, such as a vapor-phase halide of a group III element (IUPAC group 13) within the reactor volume 10, and an additional means is also provided for introducing a vapor-phase hydride of a group V element (IUPAC group 15) into the volume 10. The reaction of the group III halide and the group V hydride on a temperature-controlled substrate 18 within the reactor volume 10 produces crystalline III-V material and hydrogen gas. The hydrogen is preferentially removed from the reactor through the selectively permeable membrane 12, thus avoiding pressure buildup and reaction imbalance. Other gases within the reactor are unable to pass through the selectively permeable membrane.