The last decade has seen radical changes in our understand- ing of the physical properties of semiconductors. It has been es- tablished that the energy spectrum of electrons is much more complex than had originally been predicted: in many cases, there are several energy bands with different parameters. It has been found that the effective carrier mass, which had been assumed to be constant for a given material, depends on the carrier energy, temperature, pressure, and even the nature and number of de- fects. Our understanding of the mechanism of the motion and scat- tering of carriers,...