These two layers are called emitter and collector, and the third terminal is called the base.
When the base current is applied, the emitter–base junction is forward biased, and the collector–base junction is reverse biased.
This allows the current to flow from the emitter to the collector. The amount of current flowing from the collector to the emitter is controlled by the base current.
This current flow is known as amplification, because the small base current is amplified to control a much larger collector current. BJTs can be used in a variety of applications, such as amplifiers, switches, oscillators, and other digital circuits. They are also commonly used as transistors in power supplies.
BJTs are more efficient than other types of transistors, and they are more reliable and can handle higher power levels. BJTs are widely used in consumer electronics, such as computers, televisions, radios, and other electronic devices.