Electronics underpins all avionic systems, and semiconductor devices such as diodes, transistors and integrated circuits are the basic building blocks of these systems. This chapter provides you with an introduction to the theory and operation of semiconductors and their application in basic electronic circuits. As with Chapter 1, if you are in any doubt as to whether or not you should work through this chapter you can always turn to Section 2.6 on page 62 and see how you get on with the multiple choice questions at the end of the chapter.