Silicon is known as a four-valence semiconductor because it has four electrons available to make bonds in its outermost electron shell. Although silicon is the most commonly used semiconductor, there is another that requires some mention. The element gallium (chemical symbol Ga) has three electrons available in its outermost shell and the element arsenic (chemical symbol As) has five. A crystalline structure of gallium arsenide (GaAs) is known as a III-V valence semiconductor and can be doped with impurities in a similar manner to silicon.
In a number of respects, GaAs is preferable to silicon, not the least of which is that GaAs transistors can switch approximately eight times faster than their silicon equivalents. However, GaAs is hard to work with, which results in GaAs transistors being more expensive than their silicon cousins.
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