Bipolar junction transistors are so-called because they are created using two different kinds of silicon semiconductors to combine positive and negative charges (thus bipolar). Those two different kinds of silicon charge carriers are physically sandwiched together in varying configurations, forming a type of junction.
A bipolar junction transistor comprises three terminals or pins, known respectively as a base, a collector, and an emitter. A simple definition of collector in BJT is that it is the part of the transistor which outputs the amplified current. As with any transistor, the core working concept of a BJT is that a small amount of current flowing between the base region and collector region causes a larger current to flow between the collector and emitter regions.
In this regard, a transistor is essentially a type of amplifier - so, perhaps unsurprisingly, one of their most common applications is driving modern audio technologies. Since bipolar devices can be manufactured to handle large currents, they are particularly effective when used as high-power amplifiers in electronic audio I/O setups, as well as in other demanding technologies such as wireless transmitters.
Like all transistors, BJTs can function both linearly and non-linearly, depending on the range of the base-emitter junction current. Outside of a given model’s specified current range, the linear relationship between base-emitter current and collector-emitter current (also known as the current gain) will cease to be linear. In other words, the BJT can be driven into cutoff (off) and saturation (on) modes, as well as its standard active (amplification) mode.
In this mode, the transistor can function as an amplifier for the current flowing from base pin to collector. It can proportionally increase that current flow from the collector pin to the emitter.
This is among the most versatile and powerful modes that a transistor can operate in and is arguably the most common application for transistors used in circuits.
On or Saturation Mode
In this mode, the transistor effectively acts as a short circuit between the common collector and the common emitter. Current flowing between the two is essentially unrestricted as the transistor operates as a closed or complete circuit.
Off or Cutoff Mode
This mode is the opposite of saturation. The transistor essentially resembles a broken or open circuit; no collector current is allowed to flow, and so there is no emitter current output.
When driven into either cutoff or saturation mode, the BJT effectively functions more like a binary (on/off) circuit switch. In addition to amplification, this is another of the most powerful and versatile uses of transistors.