Abstract: A long wavelength (6 to 20 .mu.m) phototransistor is described which has n-doped silicon as emitter and collector regions bracketing a base region having a quantum well structure made up of alternating layers of p-doped silicon germanium and undoped silicon, The silicon germanium layer adjacent to the emitter region is thicker and has a higher percentage of germanium in order to provide a quantum well that is wider and deeper than the other quantum wells in the base region thereby resulting in a larger current and optical gain. The silicon barrier layer of the quantum well closest to the collector region is p-doped in order to reduce the leakage current of the base-collector junction.

Abstract: The presence of oval defects on MBE-grown compound semiconductor (e.g., GaAs, InP, or InGaAs) epitaxial layers has proven to be a serious obstacle to the use of such material for the manufacture of integrated circuits (ICs), even though the use of such material potentially could result in ICs having superior performance. One particularly prevalent type of oval defect is generally referred to as .alpha.-type. It has now been discovered that compound semiconductor epitaxial layers that are essentially free of .alpha.-type oval defects can be grown by MBE if first at least a portion of the Ga and/or In metal crucible is coated with an appropriate second metal. The second metal is chosen from the group of metals that are wetted by the first metal and that are less electronegative than the first metal. Aluminum is a currently preferred second metal.