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Free electrons and holes are the charge carriers-not only in intrinsic semiconductors(these are the purest form of semiconductors-typically as pure as can be made available with the present technology) but also in extrinsic semiconductors(doped semiconductors).
In intrinsic semiconductors,electron-hole pairs are created due to the natural processes like-
- absorption of heat energy from the surroundings
- absorption of energy from photons.
this absorbed energy results in breakdown ofcovalant bonds in intrinsic semiconductors as a result of which electron-hole pairs are created.
It is this electron hole pair which is responsible for carrying the current through the intrinsic semiconductor when a potential difference is applied across it.
In extrinsic semiconductor the case is slightly different-
here, we have-majority charge carriers and minority charge carriers.
in an n-type semiconductor-
majority charge carriers are the electrons contributed by the pentavalent impurities while the minority charge carriers are the holes which are generated as electron-hole pairs due to natural processes discussed above.
in p-type semiconductor-
majority charge carriers are the holes contributed by trivalent impurities
whereas the minority charge carriers are the electronswhich are generated as electron-hole pairs due to natural processes discussed above.
these are the majority charge carriers which contribute heavily in the flow of current through the extrinsic semiconductors than the minority charge carriers.
I suggest you to please go through mass action law and law of electrical neutrality of semiconductors for better understanding.
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