Now we have the smallest transistor with a single atom in solid state!

Physicist Professor Thomas Schimmel and his team at Karlsruhe Institute of Technology have developed a single-atom transistor. Basically, it's a current switcher controlled by repositioning of a single atom in the solid state in a gel electrolyte, which also works at room temperature and consumes very little energy.

Source for more info: https://www.nanowerk.com/nanotechnology-news2/newsid=50895.php