A transistor is a solid state device used to amplify a signal or serve as a switch. Transistors are referred to as solid state because they do not contain electro-mechanical components and, thus, are solid and do not move. Transistors were developed as the natural progression from vacuum tube electronics, and were a response to cost, manufacturing complexity, and reliability issues with vacuum tubes. The development of the transistor was one of the primary drivers behind the development of modern micro and digital electronics.