Answer by Lars Hadidi for What limits CPU speed?

Practically, it is definiteley the thermal power which is approximately proportional to the square of the voltage: http://en.wikipedia.org/wiki/Thermal_design_power#Overview Every material has its specific heat capacity which limits the cooling efficiency.
Not considering the technical issues on cooling and transmission delay, you will find the speed of light limiting the distance a signal can travel within our cpu per second. Therefore, the cpu must get samller the faster it operates. Finally, it it is operating beyond a certain frequency, the cpu may become transparent for the electronic wave functions (electrons modeled as wave functions following Schroedinger's equation).
In 2007 some physicists calculted an fundamental limit for operating speeds: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.110502