Impurity Defects and Semiconduction

Conversely, doping Ge with As introduces an extra electron that cannot be accommodated in the tetracovalent network (valence band), and this creates a narrow band of occupied donor levels, just below the conduction band in energy. The Fermi level is now located between the donor band and the conduction band, and electrons in the donor band can be readily excited thermally into the conduction band (Fig. 5.5). Thus, a negative or n-type semiconductor is created. Semiconductors can exhibit electrical conductivities in the range 10 to 10 S m, as compared to 10 to 10 S m for metals. 

If we place n- and p-type semiconducting crystals in contact (a p-n junction), we create a device that conducts electricity preferentially in one direction this is the basis of action of the semiconductor diodes used in the electronics industry, although specially refined silicon (Section 17.8.2) is usually employed rather than Ge. Transistors and electronic chips are designed using similar basic principles—typically with n-p-n or p-n-p junctions. We consider chemical aspects of electronic devices in more detail in Chapter 19. 

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