Energy bands in solids

THE SECOND WORLD WAR interrupted work in the quantum theory almost completely, and yet it furnished a stimulus that had the most profound effect on postwar work. First, there was the effect of the microwave radar research that was carried on at the Massachusetts Institute of Technology radiation laboratory, Bell Telephone Laboratories and other institutions. Physicists who had been converted into radar engineers during the war went back to their laboratories with a greatly enhanced knowledge of electronics and microwave techniques. This situation resulted in the development of new and powerful experimental techniques of high‐frequency experimentation on solids, leading to the methods of paramagnetic resonance, radiofrequency spectroscopy, cyclotron resonance, and a myriad of other ways of investigating atoms, molecules and solid state. Another outgrowth of the radar work was the increased interest in solid‐state electronics. Radar techniques of the wartime period used silicon crystals as rectifiers much as early radio had used galena crystals. Toward the end of the war remarkable properties of germanium, similar chemically to silicon, were beginning to be appreciated, and an intensive research program to understand these substances was started at Purdue University under Karl Lark‐Horovitz. It was realized that these substances were semiconductors, and those working on them began to recall what had almost been forgotten during the war; namely, the way in which the energyband theory of solids explained semiconductor properties. Bell Telephone Laboratories had the foresight to realize that the electrical properties of semiconductors might have great practical value in more everyday applications than radar and put a team of their best men, including William Shockley, John Bardeen and others, to work.