These delocalized electrons increase the conductivity of doped silicon( or germanium).
This type of semiconductor is known as n-type semiconductor because the increase in conductivity is due to the negatively charged electron.
Electron – deficit impurities
Silicon or germanium of group 14 can also be doped with elements of group 13 element like B, Al or Ga which contains only three valence electrons.
Three electrons of B form covalent bonds with three electrons of Si.