This book delves into the structure, operating principles, characteristics, parameters, and various applications of four-layer semiconductor devices. It presents a thorough theory of static characteristics and electrical transient processes during device switching. The text also details experimental studies on the properties of these devices and the methods for measuring their electrical characteristics and parameters. Aimed at engineers, researchers, graduate students, and undergraduates specializing in the theory and applications of semiconductor devices, the material is tailored for readers familiar with the mathematics and physics courses typically offered in technical universities.