Abstract: A semiconductor heterostructure device is disclosed which includes a first semiconductor layer having a barrier layer disposed thereon, the barrier layer being formed of a semiconductor material having a wider bandgap than the material of the first semiconductor layer. A carrier transport layer is disposed on the barrier layer, the carrier transport layer being formed of a semiconductor material having a narrower bandgap than the material of the barrier layer. A contact layer is disposed on the carrier transport layer. A negative potential is applied to the contact layer with respect to the first semiconductor layer. In operation, for small voltages, under the indicated bias configuration, electrons supplied to the carrier transport layer by the source of negative potential supply will be blocked at the barrier presented by the larger bandgap barrier layer, and little current will flow.