A Bipolar Junction Transistor (BJT) is a type of transistor used in electronic circuits for amplification, switching, and signal processing. It consists of three layers of semiconductor material: an emitter, a base, and a collector. These layers can be either N-type (negatively charged carriers) or P-type (positively charged carriers) depending on the type of BJT: NPN or PNP.
BJTs have three modes of operation:
Active Mode (or Forward-Active Mode): In this mode, the BJT operates as an amplifier. A small current flowing from the base to the emitter (in NPN) or from the emitter to the base (in PNP) controls a much larger current flowing from the collector to the emitter (in NPN) or from the emitter to the collector (in PNP). The transistor is in active mode when the base-emitter junction is forward-biased, and the base-collector junction is reverse-biased. This allows for current amplification and voltage amplification in the circuit.
Cut-off Mode: In this mode, the BJT is turned off, and there is no significant current flow between the collector and emitter. Both the base-emitter and base-collector junctions are reverse-biased. This is achieved by keeping the base-emitter junction in reverse bias and the base-collector junction in forward bias. The transistor is effectively acting as an open switch.
Saturation Mode: In this mode, the BJT is turned on completely, and there is maximum current flow between the collector and emitter. Both the base-emitter and base-collector junctions are forward-biased. This mode is achieved by keeping the base-emitter junction in forward bias and the base-collector junction in reverse bias. The transistor is acting as a closed switch.
It's important to note that the BJT's behavior is heavily dependent on the amount of current flowing into the base terminal. Small changes in the base current can result in much larger changes in the collector current, which makes BJTs useful for amplification and signal processing applications.
The BJT's modes of operation are crucial for understanding its behavior in various circuit configurations and applications. It's worth mentioning that while BJTs are still widely used, they have been partially replaced in some applications by other types of transistors like MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors), which offer different characteristics and advantages.