The number of charge carriers is therefore determined by the properties of the material itself instead of the amount of impurities.
The electrical conductivity of chemically pure semiconductors can still be affected by crystallographic defects of technological origin (like vacancies), some of which can behave similar to dopants.
In an n-type semiconductor, the dopant contributes extra electrons, dramatically increasing the conductivity.
In a p-type semiconductor, the dopant produces extra vacancies or holes, which likewise increase the conductivity.
That is, the electrons which have been freed from their lattice positions into the conduction band can move through the material.