A Bipolar Junction Transistor (BJT) is a current-controlled three-terminal semiconductor active device comprising two back-to-back p-n junctions within a single crystal. In an NPN configuration operating in the forward-active regime, a small forward-bias base-emitter current injects electrons into an ultra-thin, lightly-doped base region. Because the base is narrower than the minority carrier diffusion length, the vast majority of electrons diffuse across into the reverse-biased collector junction, yielding a high current amplification factor β.