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Without going into detail, I would like to introduce the concept of mobility. Mobility, with the symbol μ, is basically a measure of how easy it is for electrons and holes to move: the opposite of resistance. Look at Figure 2.10. An electron in the conduction band without any impediments, like a car on the freeway, can move quite easily. The hole has more problems moving. Electrons in the valence band have to hop from one atom to an adjacent one that has empty space at the left. The next electron in line can jump to the empty space that the previous electron left behind, and so on. To an external viewer, a positive charge is moving to the right. You can intuitively see that electron motion is easier than hole motion.

Figure 2.10 Electrons in the conduction band are free to move, while those in the valance band have to hop to the closest atom that is missing an electron, which means the mobility of electrons in the conduction band is higher than that of holes (electrons in the valence band).

The mobility of electrons in silicon is μ_n = 1400 cm²/(V × s), while the mobility of holes is only μ_p = 450 cm²/(V × s). For the same applied voltage, the electrons in silicon are more than three times faster than the holes. This makes a lot of sense, and it is true for all semiconductors.