If the dopant added to an intrinsic semiconductor is a trivalent impurity such as Boron, Indium, Gallium the resulting semiconductor is termed as P-type conductor. The added impurity atoms will displace some of silicon atoms in the crystal lattice. Three of the valence electrons of dopant will occupy covalent bonds with silicon; vacancy in the fourth bond constitutes a hole which effectively acts as positive charge carrier and can accept an electron.
Due to replacement of some of the silicon atoms with trivalent atoms from the crystal the energy band structure of the crystal lattice gets altered. The consequence of this alteration is the introduction of new allowable energy levels in the band structure in the forbidden gap close to valence band. The energy required to excite the electrons from valence band to this new energy level is of the order of 0.01 eV for Ge and 0.05 for Si generating holes in valence band.