The link in a sibling comment https://spectrum.ieee.org/semiconductors/devices/introducing-the-vacuum-transistor-a-device-made-of-nothing https://spectrum.ieee.

The link in a sibling comment https://spectrum.ieee.org/semiconductors/devices/introducing-the-vacuum-transistor-a-device-made-of-nothing https://spectrum.ieee.org/semiconductors/devices/introducing... suggests that helium could be used, and that things could be designed such that "the electrons will never acquire enough energy to ionize helium."