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, recombination mechanisms, and interaction with electromagnetic radiation has also changed. New applications of semiconducting materials have been discovered and old ones have been extended; these include high-power devices, devices sensitive to infrared radiation, and lasers. The visible evidence of the pro­ gress is in the form of hundreds of publications, some of which re­ port extremely refined and comprehensive investigations of semi­ conducting materials. A scientist concerned with investigations or applications of semiconducting materials or devices cannot ignore these publica­ tions because of the possibility of repeating work already done or of committing serious error. s. On the other hand, a beginner would require years to obtain a thorough understanding of the literature in his own narrow subject, and in many cases this process would be like the chase of a tortoise by Achilles in the paradox of Zeno of Elea (495-435 B. C'>.