Proc. 2020 IEEE Int. Electron Devices Meeting (in the press); https://ieee-iedm.org/program/

Vacuum transistors — where electrons travel between the contacts via a vacuum channel instead of a semiconductor channel — are still used in specialized electronics with very high operating voltages, such as X-ray generation. They have, however, been replaced in most applications by high-power semiconductors such as silicon carbide or gallium nitride. This is because vacuum transistors rely on a thermionic electron source, which limits switching speed and scalability. In particular, other components in the device must be spaced out in order to prevent these components getting too hot. Winston Chern and colleagues at the Massachusetts Institute of Technology and Massachusetts General Hospital have now shown that a silicon field emission array can be used as a cold electron source for vacuum transistors.

Credit: IEEE

The field emission array consists of several high-aspect-ratio silicon nanowires that are controlled by a polysilicon gate. The nanowires have tips with diameters of less than 10 nm and can concentrate very high electric fields. The researchers demonstrate that the devices have switching speeds of under 100 ns with 200 mA drive current at 200 V, which is suitable for operation as an X-ray source, while another device is shown to operate at up to 38 kV. However, at these voltages the response times are lowered due to parasitic effects. For high-power and high-frequency applications, further optimization, particularly of high-voltage vacuum packaging, is needed before they can outperform solid-state devices.