  A three-terminal spectroscopy that probes both subsurface energy barriers and
interband optical transitions in a semiconductor heterostructure is
demonstrated. A metal-base transistor with a unipolar p-type semiconductor
collector embedding InAs/GaAs quantum dots (QDs) is studied. Using
minority/majority carrier injection, ballistic electron emission spectroscopy
and its related hot-carrier scattering spectroscopy measures barrier heights of
a buried AlxGa1-xAs layer in conduction band and valence band respectively, the
band gap of Al0.4Ga0.6As is therefore determined as 2.037 +/- 0.009 eV at 9 K.
Under forward collector bias, interband electroluminescence is induced by the
injection of minority carriers with sub-bandgap kinetic energies. Three
emission peaks from InAs QDs, InAs wetting layer, and GaAs are observed in
concert with minority carrier injection.
