How small can be our transistor? It is reported that TSMC has 1.0 nm transistor in their lab. The latest AMD cpu zen3 is 7nm, while Intel cpu mostly still at 14nm.
The single-atom transistor would be it. Supposedly the Karlsruhe version can operate at room temperature rather than cryogenic. They arguably recruit electron (quantum) tunneling rather than being inhibited by such. If the device was still referred to as a "transistor", there might versions in the future that can manipulate photons, but will only be more efficient and actually or probably be larger. Picotechnology that descends to the subatomic level is still pretty speculative.
Nature provides biological transistors at nano-scale. See; "Is consciousness to be found in quantum processes in microtubules" in Alternative Theories.
