When a piece of pure silicon is doped with aluminium, it introduces impurities into the silicon lattice structure. These impurities create extra free electrons or "donor" atoms. These extra free electrons increase the number of charge carriers in the silicon, leading to an increase in conductivity.

Option 1 is incorrect because the conductivity of the doped silicon piece will not remain the same. It will increase.

Option 2 is incorrect because the introduction of donor atoms from the aluminium doping makes the doped silicon piece an n-type semiconductor. In an n-type semiconductor, the majority charge carriers are electrons.

Option 4 is incorrect because doping with aluminium increases the number of charge carriers, resulting in a decrease in resistivity, not an increase.

Therefore, option 3 is the correct answer. When a piece of pure silicon is doped with aluminium, it becomes P-type, as it introduces extra free electrons or donor atoms into the silicon lattice structure. In a P-type semiconductor, the majority charge carriers are holes, and the doping with aluminium creates holes in the silicon crystal structure.