We present a new type of nanometer-scale semiconductor nonlinear device, called self-switching device (SSD). The device was realized by simply etching insulating grooves into a semiconductor, between which a narrow channel with a broken symmetry was formed. Because of the asymmetry in the channel boundary, an applied voltage V not only changes the potential proÿle along the channel direction, but also either widens or narrows the eeective channel width depending on the sign of V . This results in a diode-like current–voltage characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually 0 to more than 10 V by simply adjusting the channel width. Furthermore, only one lithography step was needed to fabricate SSDs. We used two diierent material systems, InGaAs–InP and InGaAs–InAlAs, to realize SSDs and the results at room temperature were compared. We also show that by adding a third terminal to an SSD as a gate, the turn-on voltage of the device could be tuned by the gate bias and the device functions either as a tunable diode or as a transistor.