In sub-90 nm CMOS processes, mobility is incerased by creating channel stress - tensile for n-channel and compressive for p-channel. It's interesting that electrons and holes need opposite types of stress. I was trying to figure out this behavior intuitively.

Electrons are in the conduction band and their mobility is limited by atomic scattering. So further apart the atoms less the scattering. Now why doesn't this same logic hold for holes? Because holes are in the valence band, more or less orbiting the silicon atoms. The valence band is formed because of Si-Si covalent bonds whereby valence electrons can tunnel from atom to atom, forming propagating wave functions in the periodic potential of the lattice. Ergo, the closer the atoms the easier the atom-hopping.

So, electrons and holes are indeed very different beasts.

M.G.Rajan
www.eecalc.com

Hi Rajan,
Good concept.
Usually how the tensile and compressive stress are formed?
By varying the doping concentration?

Hi  Geoffrey, Thanks for the explanation