When a piece of pure silicon is doped with aluminium, then

When pure silicon, a Group IV element with four valence electrons, is doped with aluminium, it becomes a p-type semiconductor [1]. Aluminium is a trivalent element (Group III) with only three valence electrons [1]. When it replaces a silicon atom in the crystal lattice, it can only form three covalent bonds, leaving one vacancy or 'hole'. This hole acts as a positive charge carrier, and aluminium is thus termed an 'acceptor impurity' because it can accept an electron from the valence band to fill this vacancy [1]. The introduction of these holes significantly increases the conductivity of the material compared to pure silicon [1]. Therefore, doping silicon with aluminium creates a p-type semiconductor where holes are the majority charge carriers.

Sources

1. [1] https://www.ucl.ac.uk/~ucapikr/Solid_State_Physics/Section%209.pdf