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Figure 1.17 shows the portion of the periodic table that contains the elements used in semiconductors, showing how many electrons are in the upper shells. All the lower shells are full. Silicon, for example, has 14 electrons, so the electrons fill up bands 1s (2), 2s (2), 2p (6), 3s (2), and 2p (the last 2). If you superimpose these numbers into Figure 1.16, you see that the two lowest energy levels are full, but energy level n = 3 (s + p) has four electrons, with the possibility of accepting another four (in the 2p level) to complete its orbit.

Consider another element that is used a great deal in semiconductors: antimony, Sb. It has 51 electrons. By looking again at Figure 1.16, we find the last occupied level is 5p, with three electrons. All the lower levels are full. Thus, the last occupied energy level, level 5, has five electrons – two in 5s and three in 5p levels – which gives it a chemical valence of 5. We will use these numbers in the next chapter to explain the difference between insulators, conductors, and semiconductors.

Figure 1.17 Portion of the periodic table emphasizing elements used in semiconductors.