  The electrical performance of a tunnel field-effect transistor depends
critically on the band offset at their semiconductor heterojunction interface.
Historically, it has been difficult to experimentally determine how the
electronic bands align at the heterojunction interface. We report here on
experimental methods to ascertain a complete energy band alignment of a
broken-gap tunnel field-effect transistor based on an InAs/GaSb
hetero-junction. By using graphene as an optically transparent electrode in a
traditional internal photoemission measurement, both the electron and hole
barrier heights at the InAs/GaSb interface can be quantified. For a
Al2O3/InAs/GaSb layer structure, the barrier height from the top of InAs and
GaSb valence band to the bottom of Al2O3 conduction band is inferred from
electron emission whereas hole emissions reveal the barrier height from the top
of Al2O3 valence band to the bottom of InAs and GaSb conduction band.
Subsequently, the offset parameter at the broken gap InAs/GaSb interface is
extracted and thus can be used to facilitate the development of predicted model
of electron quantum tunneling efficiency and transistor performance.
