In an NPN transistor, which junction must be forward biased for current to flow?

For current to flow in an NPN transistor, the base-emitter junction must be forward biased. This is essential for enabling the flow of charge carriers.

In an NPN transistor, the emitter is typically heavily doped with electrons (the majority charge carriers), while the base is lightly doped and composed of holes (the majority charge carriers in p-type material). When the base-emitter junction is forward biased, electrons from the emitter are injected into the base. Because the base is thin and lightly doped, most of these electrons will diffuse through the base and reach the collector junction, which is reverse biased. This movement of electrons from the emitter to the collector constitutes the current flow through the transistor.

Moreover, the operation of a transistor relies on this forward biasing of the base-emitter junction to allow the control of a larger current flowing from collector to emitter with a smaller current from base to emitter, effectively functioning as a switch or amplifier. The critical aspect is that the base-emitter junction needs to be forward biased to initiate this process.