Well we the Van der Walls radius of a silicon atom is 220pm, so it is reasonable to assume that any transistor must be at least 6 times that. (3 atoms two for each junction) that's 1.32 nm. Then you probably cannot 'dope' a single atom so that figure is double again to 2.64nm.
If you then consider that any semiconductor must include a certain doping ratio that figure (the minimal number of atoms per junction) becomes larger and larger so we are not that far away and it's reasonable to assume that transistors can only get one magnitude smaller than they are currently at best.
Before you go too far with that, 2 and 3nm transistors have been demonstrated. You still want to say our noses are against the silicon performance wall? If you then consider that any semiconductor must include a certain doping ratio that figure (the minimal number of atoms per junction) becomes larger and larger so we are not that far away and it's reasonable to assume that transistors can only get one magnitude smaller than they are currently at best.
IIRC that weren't 2 to 3 nm transistors but transistors with a resolution of 2 to 3 nm (that's ~6 atoms). It's close to what is physically possible but not the end of the line.
That is I always assumed the nm figure in microelectronics refers to the feature size, alas the average size of an element, the transistor, correct me if I'm wrong. In that case the demonstrated transistor would even be about at the end of the line.
However, current tech approaching it's limit doesn't need to mean progress must stop here. We can for one produce larger chips or even wafer scale designs, stack them for "3D chips" and finally something like "holographic computers" where the properties of the quantum states of individual atoms are used for useful computation. The next limit would be the plank units but that's a long way.
