The book contains a set of information about physical phenomena in the transistor, necessary for engineers involved in the development and operation of electronic equipment. The electrical and thermal parameters of the transistor, its characteristics and equivalent circuits, maximum permissible direct and pulse currents, voltage and power are considered. The possibilities of using the transistor as an amplifier and switch in inverse switching and in micro-modes are shown. Special attention is paid to the relationship between the transistor parameters and recommendations for determining its data that are not in the reference books and specifications.