How do p-n junctions make transistors possible?

The p-n junction is the fundamental building block of the transistor. By creating an interface between p-type and n-type materials, these junctions allow for the precise control of electrical current.12

In a bipolar transistor, two closely coupled p-n junctions are used to manipulate charge carriers. The device typically consists of three regions—the emitter, base, and collector—arranged in a p-n-p or n-p-n configuration.3

The transistor functions by biasing these junctions differently:
* Forward-biasing the emitter-base junction allows majority carriers to be injected into the base region.3
* Reverse-biasing the base-collector junction creates a high-resistance barrier that normally blocks current.3

Because the base region is kept very narrow, most of the carriers injected from the emitter can pass through the base to reach the collector. This mechanism allows a small input current or voltage at the base to control a much larger current flowing between the emitter and collector, enabling the transistor to act as an amplifier or a high-speed switch.34