We have fabricated GaSb/InAs hot‐electron transistors with base widths of 30 to 200 A˚. The barrier height of thermionic emission was evaluated with two techniques; deducing from values of collector current density itself, and from its temperature dependence. The difference between the two results reveals that electrons are emitted from both &Ggr; andLvalleys of the GaSb conduction band. From the measured barrier height, the energy of the ground quantum level formed in InAs base was also deduced. Its dependence on the base width was compared with simple calculation where nonparabolicity of the InAs conduction band was considered. It indicates electron density in the InAs base is enhanced due to electron’s transfer from GaSb to InAs.