This invention presents a nanoscale vacuum tube or vacuum transistor fabricated entirely using current silicon integrated circuit manufacturing techniques. Vacuum is better for electron transport than any semiconductor since there is no electron scattering. In addition, vacuum devices are immune to radiation. Nevertheless, vacuum devices lost out to silicon devices due to ease of large-scale manufacturing, robustness, versatility, and low cost. Here, the best of vacuum and silicon technologies are combined to produce nanoscale vacuum transistors that are amenable to large wafer fabrication and are inexpensive, while providing exceptional performance.
The devices are fabricated with electrodes or alternatively called emitter and collector with a separation distance d, achieved using plasma ashing. To date, a distance of 50 nm has been achieved with a potential to reduce this down to 10 nm. At distances below 50 nm, the mean free path of electrons is larger and therefore, the vacuum requirement becomes relaxed so that near-atmospheric pressures can be maintained. Nevertheless, a moderate vacuum is recommended in order to make the devices robust. Silicon is degeneratively doped with phosphorous ion implantation to a doping value of 1020 cm–3, and the ions are activated by rapid thermal annealing at T = 1,000 °C for 10 s. Two different emitter shapes, sharp/concave and rounded/convex, can be used here with the reduced separation. The vacuum channel transistor relies on thermionic emission (Fowler-Nordheim) and on tunneling. When the gate voltage is less than turn-on voltage, a few electrons are emitted over the barrier with thermal energy. When the gate voltage is greater than the turn-on voltage, the vacuum energy level curve bends downward and allows electron tunneling through the narrow, nominal barrier. The turn-on voltage is defined as the voltage required to provide an electrical field of 1 V/μm adjacent to the tip. To date, a drive voltage of 2 V has been achieved, which is remarkable for vacuum devices. The nanoscale vacuum transistor has tremendous potential for THz electronics and space electronics.