Which of the following describes an NPN transistor?

An NPN transistor is made up of one p-type semiconductor material sandwiched between two n-type materials. In this configuration, the p-type material acts as the base, while the two n-type materials serve as the collector and emitter. This arrangement is critical for the functioning of the transistor as it allows for the control of current flow. In an NPN transistor, when a small input current flows from the base to the emitter, it enables a larger current to flow from the collector to the emitter, which is the fundamental operation of the device.

The other options suggest different structures that do not correctly represent the NPN transistor. Two p-type materials or only n-type materials do not align with the NPN construction, and the arrangement of terminals in a series does not accurately describe their configuration or operational principles. Understanding the correct composition of the NPN transistor—two n-type materials with a p-type junction—is essential for grasping its behavior and applications in electronic circuits.