What are three terminal devices made of semiconductor material that can function as switches or signal amplifiers?

Transistors are indeed three-terminal devices made from semiconductor materials and are widely used in electronic circuits as both switches and signal amplifiers. In their role as switches, transistors can control the flow of electrical current in a circuit, allowing for the on-off functionality essential in digital computing and control systems. When used as amplifiers, they take a small input signal and produce a significantly larger output, which is crucial for audio applications, radio frequency transmissions, and various other signal processing tasks.

The other options listed, while important components in electronic circuits, do not serve both of these functions. Capacitors are used for storing electrical energy temporarily and filtering signals, not as switches or amplifiers. Resistors are used to limit current flow and divide voltages, rather than amplify signals or act as switches. Diodes allow current to flow in one direction but do not have the capability to amplify signals or act as switches in the way that transistors do. Thus, transistors stand out as the correct answer due to their unique ability to perform both switching and amplification tasks in electronic applications.