These vacancies are fulfilled by four valence electrons each from four individual neighboring semiconductor atom. Actually, in this way all atoms of a semiconductor crystal make a covalent bond with their nearest most neighborhood atoms. Ideally, all valance electrons in a semiconductor crystal are involved in forming of covalent bond hence, there should not be any free electron in the crystal. But this is not the actual case. At absolute 0o Kelvin there would not be any free electron in the crystal but when the temperature rises from absolute zero to room temperature, numbers of valence electrons in the bonds are thermally excited and come out from the bond and generate a numbers of free electrons in the crystal. These free electrons cause the conductivity of the semiconductor materials at any temperature higher than absolute zero. There is a method of increasing conductivity of semiconductors at any temperature greater than absolute zero. This method is called doping. In this method is pure or intrinsic semiconductor is doped with pentavalent impurities like antimony, arsenic and phosphorus. These impurity atoms replace some of the semiconductor atoms in the crystal and occupy their positions. As the impurity atoms have 5 valance electrons in the outermost orbit 4 of them will create covalent bond with 4 neighboring semiconductor atoms.
One valance electron of impurity atom does not get chance to involve in covalent bonding and becomes more loosely bounded with parent impurity atom. At room temperature, these loosely attached fifth valence electrons of impurity atoms can easily come out from its position due to thermal excitation. Due to this phenomenon there will be a huge number of free electrons but still there are breakdowns of covalent bonds in the crystal due thermal excitation at room temperature. The free electrons in addition to free electrons created due to breakdown of a semiconductor to semiconductor and semiconductor to impurities covalent bonds cause total of free electrons in the crystal. Although whenever a free electron is created during breakdown of a semiconductor to semiconductor covalent bond there is a vacancy created in the broken bond. These vacancies are referred as holes. Each of these holes is considered as positive equivalent of negative electron as it is created due to lack of one electron. Here electrons are main mobile charge carriers. In an n-type semiconductor there will be both free electrons and holes but number of holes are quite smaller than that of electrons because holes are created only due to breakdown of semiconductor to semiconductor covalent bond whereas free electrons are created both due to loosely bounded non-bonded fifth valence electron of impurity atoms and break down of semiconductor to semiconductor covalent bonds.