Abstract: A novel combination of semiconductor heterojunctions provide a quantum-effect device with resonant or enhanced transmission of electrons (or holes) due to tunneling into a quantum well state in the valence (or conduction) band. A particular heterostructure comprising sequentially grown layers of indium arsenide, aluminum antimonide, gallium antimonide, aluminum antimonide and indium arsenide, permits electrons tunneling from the indium arsenide conduction band through the aluminum antimonide barrier into a sub-band level in the valence band quantum well of the gallium antimonide. This particular embodiment produced a current-voltage characteristic with negative differential resistance and a peak-to-valley current ratio of about 20 at room temperature and 88 at liquid nitrogen temperature. The present invention can be used either as a two-contact device such as a diode or a three-contact device such as a transistor.