The book contains articles by A.F. Ioffe related to two periods of his activity. The first part included studies of 1907-1915. on radiation, photoeffect and electrons. The second, and larger in volume, part contains about 20 articles written in 1932-1960. These articles are devoted to the general problems of semiconductor physics, their photoelectric properties, the thermal and electrical conductivity of semiconductors, contact phenomena in them and the basics of semiconductor thermal energy.