It depends on how one defines transistor, but if the definition of voltage controlled switch is sufficient then one atom will do. https://en.wikipedia.org/wiki

It depends on how one defines transistor, but if the definition of voltage controlled switch is sufficient then one atom will do. https://en.wikipedia.org/wiki/Single-atom_transistor https://en.wikipedia.org/wiki/Single-atom_transistor This has nothing to do with the mass produced transistors (yet!) those are 10s of nm across even in 3 or 5nm.