NPN

An NPN transistor is a type of bipolar junction transistor (BJT) with alternating N-type and P-type semiconductor regions arranged as N-type emitter, P-type base, and N-type collector. In its normal active operation, a small current injected into the base controls a larger current flowing from the collector to the emitter, making the device a current-controlled current amplifier.

The emitter is heavily doped N-type, the base is thin and lightly doped P-type, and the collector is moderately doped N-type. The base–emitter junction is forward biased while the base–collector junction is reverse biased in active mode. Electrons are injected from the N emitter into the P base and diffuse toward the collector; most electrons traverse the base to reach the collector, while only a small portion recombine in the base. The resulting collector current is proportional to the base current via the current gain beta.

In terms of operation modes, forward-active is the normal region for amplification; saturation occurs when both junctions are forward biased; cutoff when the base-emitter junction is not forward biased. The symbol for an NPN transistor has an arrow on the emitter pointing outward, indicating conventional current flowing from the base into the emitter.

Common circuit configurations include common-emitter, common-base, and common-collector arrangements. NPN transistors are widely used in switching and amplification applications, appearing in analog amplifiers, digital logic, and driver stages. Popular discrete devices include part numbers such as 2N3904 and BC547, among many others, produced using silicon and various manufacturing processes.