A diffusion furnace and method for processing semiconductor wafers standing on edge wherein the furnace has vertically adjacent electrical resistance heating elements wired in parallel and disposed above the wafers. This arrangement enables the heat input to the furnace by the heating elements to be varied. In a preferred embodiment, the heating elements in the upper section of the tube are connected in one circuit and the heating elements in the lower section of the tube are connected in a second circuit and each circuit is controlled in response to the temperature in that section.