Atom size is about 0.1 nm. Processor having about 10^9 transistors and is about 1 cm^2 size. If one transistor is about 50 nm length and wide then in 0.01 m^2 must fit in 10^16 / 500^2 = 4*10^10 transistors. Why going inprocessor going in about 10-100 times less transistors? Perhaps wires between transistors taking many space or transistors in wide are bigger than in length (50 nm * 200 nm, etc).

So what smallest trinsistor size is possible or what will be transistor length in 2020 years? Also I read that in milions silicon atoms must be 1 atom which making pnp, npn. So then transistor can't be bigger than 10 nm *10 nm *10 nm =1000 nm^3 size.

If transistor size decreasing quadraticaly then with 10 nm chip manufacturing will be possible fit in 10^12 transistors in 1 cm^2 (would be 100 times faster than now) . And if manufacturing will be 0.1 nm then in 1 cm^2 will be possible fit in 10^16 transistors (would be milions times faster than now). But transistor size like one atoms is imposible so supose tranzistor 1 nm * 1 nm= 1nm^2. In 1 cm^2 can fit in 10^16 / 10^2 = 10^14 transistors (then it would be about 10000 times fater than now).

It won't be silicon anymore by then, photoresist etching process does not have enough resolution. Probably grown nanotubes or nanowires.

And from what little I know, the reason the transistor density is lower than it could be is because of having to much heat. If we can decrease the amount of power to run a chip, or cool it very effectively, chips could be stacked, vastly increasing the number of transistors.

Oxide and dielectric defects will be a big leakage source all around. Going beyond 45 nm is pretty theoretical if you ask me.