Apparatus comprising: a first compound semiconductor composition layer doped to have a first charge carrier polarity; a second compound semiconductor composition layer doped to have a second charge carrier polarity and located on the first layer; a third compound semiconductor composition layer doped to have the first charge carrier polarity and located on the second layer; a base electrode on the second layer; and a spacer ring interposed between and defining a charge carrier access path distance between the base electrode and the third layer, the path distance being within a range of between about 200 Å and about 1000 Å. Techniques for making apparatus. Apparatus is useful as a heterobipolar transistor, particularly for high frequency applications.