When an electron is removed from a covalent bond, it leaves a vacancy behind. An electron from a neighboring atom can move into this vacancy, leaving the neighbor with the vacancy. In this way the vacancy, called a hole, can travel through the material and serve as an additional current carrier. It’s like describing the motion of a bubble in a liquid. In a pure, or intrinsic, semiconductor, valence-band holes and conduction-band electrons are always present in equal numbers. When an electric field is applied, they move in opposite directions (Fig. 44-24). Thus a hole in the valence band behaves like a positively charged particle, even though the moving charges in that band are electrons. The conductivity that we just described for a pure semiconductor is called intrinsic conductivity. Another kind of conductivity, to be discussed in the next subsection, is due to impurities.
The parking-garage analogy that we mentioned in Section 44-5 helps to picture conduction in an intrinsic semiconductor. The valence band at absolute zero is like a completely filled floor. No cars (electrons) can move because there is nowhere for them to go. But if one car is moved to the vacant floor above, it can move freely, just as electrons can move freely in the conduction band. Also, the empty space that it leaves permits cars to move on the nearly filled floor, thereby moving the empty space just as holes move in the normally filled valence band.