Three layers of semiconductor material: a layer of p-type material sandwiched between two layers of n-type material. The p-type material is doped with impurities that create an excess of positively charged holes, while the n-type material is doped with impurities that create an excess of negatively charged electrons. This configuration allows the transistor to function as a switch or amplifier.

In a typical npn transistor circuit, the emitter is connected to the negative terminal of the power supply, the base is connected to the control signal, and the collector is connected to the load or output. When a small current flows into the base, it controls a much larger current flowing between the collector and emitter.

The npn transistor operates in three different modes: cutoff, active, and saturation. In cutoff mode, there is no current flowing between the collector and emitter because the base-emitter junction is not forward biased. In active mode, the base-emitter junction is forward biased, allowing a small current to flow from the base to the emitter, resulting in a larger current flowing between the collector and emitter. In saturation mode, the base-emitter junction is heavily forward biased, allowing a large current to flow from the base to the emitter, resulting in maximum current flow between the collector and emitter.

The npn transistor can be used in various applications, such as amplifiers, switches, oscillators, and voltage regulators. It is commonly used in electronic devices and circuits to control and amplify electrical signals.