A three-terminal GaAs switch is demonstrated which utilizes the previously reported regenerative switching mechanism shown in a combined planar-doped barrier (PDB) and p-n junction. This latter, two-terminal switch relies on a mechanism by which holes injected into the PDB reduce the net charge in the p plane, and thus leads to a lowering of the barrier for electrons. The resultant increase in thermionic emission of electrons over the barrier in turn leads to more hole injection from the p-n junction. As bias is increased, a point is reached where the internal gain exceeds unity, the PDB collapses, and the device switches to its ‘‘on’’ state. The addition of a third terminal to the n region of the p-n junction allows hole injection within the PDB region to be controlled independently of the total bias across the device. This, in turn, enables the switching threshold voltage to be varied over the full operating range of the device. The device is also demonstrated to function as a reversible, gate-controlled switch. In many respects the characteristics of the switch resemble those of the metal-insulation silicon switch (MIS), but in this case the device is made entirely within the bulk of the semiconductor by MBE and does not rely on a tunneling oxide. The small dimensions in the direction of carrier transport should lead to fast switching speeds, and preliminary measurements have shown a switching-on transition time of 2 ns, and off times of around 10 ns. These times correspond closely to the measured capacitance and resistance of the device and should be capable of substantial improvement in a more optimized structure. The device has possible applications as a high-speed logic element.