Semiconductors have made an enormous impact on 20th century science and technology. This is because components made from semiconductors have very favorable properties such as low energy consumption, compactness, and high reliability, and so they now dominate electronics and radio engineering. Semiconductors are indispensable for space exploration and where the requirements of small size, low weight and low energy consumption are especially stringent. The book uses quantum-mechanical concepts and band theory to present the theory of semiconductors in a comprehensible for. It also describes how basic semiconductor devices (e.g. diodes, transistors, and lasers) operate. The book was written for senior high-school students interested in physics.