Elemental semiconductor bulk electronic properties

The properties of semiconductors are extremely sensitive to the presence of impurities at concentrations as low as 1 part in 10 °. For this reason, silicon manufactured for transistors and other devices must be very pure. The deliberate introduction of a very low concentration of certain impurities into the very pure semiconductor, however, alters the properties in a way that has proved invaluable in constructing semiconductor devices. Such semiconductors are known as doped or extrinsic semiconductors. Consider a crystal of silicon containing boron as an impurity. Boron has one fewer valence electron than silicon. Therefore, for every silicon replaced by boron, there is an electron missing from the valence band (Figure 4.10) (i.e., positive holes occur in the valence band and these enable electrons near the top of the band to conduct electricity). Therefore, the doped solid will be a better conductor than pure silicon. A semiconductor like this doped with an element with fewer valence electrons than the bulk of the material is called a p type semiconductor because its conductivity is related to the number of positive holes (or empty electronic energy levels) produced by the impurity. 

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