ELECTRONIC DEVICES

Semiconductor apparatus comprises a split-gate semiconductor device (1) whid has a first layer (3) of a first undoped semiconductor material (e.g. GaAs) and a second layer (4) of a second semiconductor material (e.g. AlGaAs) which is doped through at least part of its thickness. The second material has a higher energy band gap than the first material, and the layer form a heterojunction so that electrons from the second layers collect in the first layer to form a two-dimensional electron gas. Drain and source contacts (7,6) are provided on the second layers, and a gate electrode (8) is so configured that on application of a sufficiently large negative bias to the gate electrode a constriction is formed in the first layer through which only a one-dimensional electron gas can pass between the source and drain contacts. The apparatus also includes a bias suply (17) for applying a bias voltage between the source and drain contacts. The source/drain bias is made equal to or greater than Ef/e where Ef is the Fermi energy of electrons in the two-dimensional gas and e is the electron charge. As a result of the large source drain bias, the device exhibits negative resistance, and the apparatus can be used as an oscillator or a detector, operating in the THz range.