More than 50 years have passed since the invention of the transistor in De­ cember 1947. The study of semiconductors was initiated in the 1930s but we had to wait for 30 years (till the 1960s) to understand the physics of semi­ conductors. When the transistor was invented, it was still unclear whether germanium had a direct gap or indirect gap. The author started to study semiconductor physics in 1960 and the physics was very difficult for a begin­ ner to understand. The best textbook of semiconductors at that time was "Electmns and Holes in Semiconductors" by W. Shockley, but it required a detailed knowledge of solid state physics to understand the detail of the book. In that period, junction transistors and Si bipolar transistors were be­ ing produced on a commercial basis, and industrialization of semiconductor technology was progressing very rapidly. Later, semiconductor devices were integrated and applied to computers successfully, resulting in a remarkable demand for semiconductor memories in addition to processors in the late 1970s to 1980s. Now we know that semiconductors play the most important role in information technology as the key devices and we cannot talk about the age of information technology without semiconductor devices. On the other hand, the physical properties of semiconductors such as the electrical and optical properties wcre investigated in detail in the 1950s, lead­ ing to the understanding of the energy band structures.