An improved chemical vapor deposition reaction chamber having an internal support plate to enable reduced pressure processing. The chamber has a vertical-lateral lenticular cross-section with a wide horizontal dimension and a shorter vertical dimension between bi-convex upper and lower walls. A central horizontal support plate is provided between two lateral side rails of the chamber. A large rounded rectangular aperture is formed in the support plate for positioning a rotatable susceptor on which a wafer is placed. The shaft of the susceptor extends downward through the aperture and through a lower tube depending from the chamber. The support plate segregates the reaction chamber into an upper region and a lower region, with purge gas being introduced through the lower tube into the lower region to prevent unwanted deposition therein. A temperature compensation ring is provided surrounding the susceptor and supported by fingers connected to the support plate. The temperature compensation ring may be circular or may be built out to conform to the rounded rectangular shape of the support plate aperture. The ring may extend farther downstream from the susceptor than upstream. A separate sacrificial quartz plate may be provided between the circular temperature compensation ring and the rounded rectangular aperture. The quartz plate may have a horizontal portion and a vertical lip in close abutment with the aperture to prevent devitrification of the support plate. A gas injector abuts an inlet flange of the chamber and injects process gas into the upper region and purge gas into the lower region. The gas injector includes a plurality of independently controlled channels disposed laterally across the chamber, the channels merging at an outlet of the injector to allow mixing of the adjacent longitudinal edges of the separate flows well before reaching the wafer.