Single Bipolar Junction Transistors (BJTs) are fundamental semiconductor devices used to amplify and switch electrical signals in both analog and digital circuits. A BJT consists of three layers of semiconductor material forming two pn junctions, with three terminals: the emitter, base, and collector. Functionally, a BJT operates by using a small base current to control a larger current flowing from the collector to the emitter, providing current amplification. The device can operate in three modes: active mode for amplification, cutoff mode for switching off, and saturation mode for fully on-state switching.