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Bipolar junction transistors (BJTs) are one of the primary types of transistors utilized in modern electronic devices. Commonly being referred to as Bipolar transistors, they are commonly used in a variety of electronic applications including amplifiers, logic circuits, and other applications requiring linear gain. As compared to field-effect transistors (FETs) BJTs have the advantage of more linear voltage “on-state“ gain, meaning that the output signal produced by the transistor is a more accurate representation of the input signal.

A single polarized BJT devices are composed of three different layers of material – the base, the collector and the emitter. The base is a small layer of semiconductor material with just a few parts per million of an impurity being introduced, this impurity is typically a tin or antimony type impurity which acts as a bridge between the two other layers. The collector is the larger external layer which collects any out flowing carriers within the device, which are holes that are missing in the base layer and electrons produced by the emitter layer. Finally, the emitter is the smallest layer of the BJT device which acts as the source of the electrons flowing out of its terminations.

BJT’s are commonly designed in NPN form where the majority of the current is flowing from collector to emitter. This form is suitable for most linear amplifying applications where the signal tends to be bi-directional. Typically the BJT device will be operated by applying an input voltage at its base contact selecting the collector current to flow. With this type of bi-directional operation, the BJT will act in its linear range, usually described as the active region, and output signal that follows the signal being applied at its base.

Another type of BJT device is the PNP type which differs from its NPN counterpart in that the majority of the current flows from emitter to collector. This type of configuration is generally used in applications where bias current inversion is required, such as in active voltage dividers. It is also used in higher power switching applications. As compared to NPN type devices, PNP transistors do not require the need for a voltage to be applied at their bases in order to act as a switch and are easily operated from any applied current.

In terms of their application field, BJT devices are mostly suitable for any linear application requiring voltage amplification. This includes amplifiers, signal conditioners, and any other type of application where limited currents are required. Another major application includes switching applications, like motor speed control, dimmer switches, and light dimmers. In both cases, the BJT is used to either change the shape of the input signal or switch it on or off depending on the output from the BJT device.

For switching applications, BJTs are primarily used in their saturation region, where they conduct a relatively high amount of current, and are either turned on or off depending on the applied signal. As compared to FETs, BJTs have the advantage of a higher operating frequency and a more linear voltage gain, meaning that their “on-state” gain is slightly higher than that of FETs.

The working principle of a BJT is relatively simple in theory, though difficult to comprehend at the atomic level. The base of the BJT is designed to act as a bridge between the collector and emitter layers. Any current that enters the base region travels through a highly doped path and into the collector layer. At the same time, current will be produced in the emitter layer due to the injection of high concentrations of electrons from the base.

This current is then sent from the emitter through a moderately doped path towards the collector. It is this separation of paths and voltage drop between the two regions that produces the current gain characteristic for the BJT. As the voltage between the two regions increases the current gain will increase as well, allowing for linear voltage gain when operated in the active region.

The current being produced by the emitter and injected into the base is also known as the “early current”, while any current flowing through the collector and back around to the base is referred to as the “recovery current”. When operating in the active region, the early current will decrease while the recovery current will increase resulting in an overall increase in the current gain.

The base-emitter junction of a BJT can also be clamped using a negative voltage, which is commonly referred to as the “saturation” region. In this configuration, the device operates like a switch and the current flow is greatly increased. This type of operating region is most commonly used in switching applications where large currents need to be switched on or off.

To summarize, bipolar junction transistors (BJTs) are one of the primary types of transistors used in modern electronics. They are composed of three different layers – base, collector and emitter – and their primary application field is for linear amplification and switching applications. The working principle that governs BJTs is based on the injection of currents into the base layer, as well as the separation of paths between the collector and emitter layers, which result in an increase of voltage gain.

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