The invention relates to an injector configured for arrangement within a reaction chamber of a substrate processing apparatus to inject gas in the reaction chamber. The injector may be elongated along a first axis and configured with an internal gas conduction channel extending along the first axis and provided with at least one gas entrance opening and at least one gas exit opening. The injector may have a width extending along a second axis perpendicular to the first axis substantially larger than a depth of the injector extending along a third axis perpendicular to the first and second axis. The wall of the injector may have a varying thickness.