Bipolar Junction Transistors: What Are They & What Do They Do

January 31, 2022

To say that many of the lightweight and inexpensive electronics that we use daily are taken for granted is a bit of an understatement. Much of what we use and enjoy, however, could not have been possible without the development and implementation of the bipolar junction transistor (BJT). Invented in 1947 by William Shockley, BJTs were integral components during the infancy of modern computing technologies, from computer memory to microprocessors and more. 

Let’s dive into what BJTs are, what they’re used for, and how they’ve changed the world of electronics.

What Is a Bipolar Junction Transistor? 

Unlike unipolar transistors that use only one kind of charge carrier, a BJT transistor uses both electrons and the lack of electrons – known as an electron hole – to carry a charge. Holes in conductive material are left when an electron leaves its current state for a higher one. These holes can move through a material similarly to electrons and behave much like positively charged particles. When a small current is applied to one of the terminals of a BJT, it effectively allows the transistor to control a much larger current between the emitter and collector, which in turn allows the current to be amplified or switched. Put more simply, think of a bipolar junction transistor as a regulator of current. 

Even though BJTs contain three terminals (the base, the emitter, and the collector), the term “bipolar” is used to describe this type of transistor because it uses two different types of semiconductor material (one that is positively charged, and one of course, that is negatively charged, but more on that later). While BJTs generally contain silicon (which replaced germanium as the transistor material of choice in the 1960s due to its superior thermal stability) as its primary material, impurities can be added through a process known as “doping” to get the various layers of the transistor to behave as required. 


Basic Applications of BJTs

Though the original technology is itself almost 70 years old, bipolar transistors are still widely used to amplify and switch signals. In digital circuitry, they’re used to amplify radio frequencies and switch heavy currents. 

When dealing with high-speed digital logic, BJTs are often paired with metal-oxide semiconductor field effect transistors (mercifully known as MOSFET transistors). These transistors are vital to using radio frequencies and are integral components of high-end microchips.

Perhaps more interestingly to those who have an affinity for theoretical physics and space exploration, BJTs combined with MOSFETs make it possible for Shockley’s transistors to be used in particle accelerators. These accelerators unlock  the mysteries of the universe, in the relatively clean energy production afforded by nuclear reactors, and through the many satellites in orbit around Earth and beyond.

Back on Earth, bipolar junction transistors form the basis for many commercially available electronic amplifiers and temperature gauges used by many different trades. These types of transistors are also used in the compression of signals so that non-BJT circuits can handle them. 

Additionally, BJTs are the most-used transistor type used in the following circuits:

Logic Circuits

Logic circuits are those used to perform logic-based operations in computing. There are two basic types of logic circuits: combinational circuits and state circuits.

Amplifier Circuits

As the name suggests, amplifier circuits are used to increase a signal so that the output signal is greater than the input signal, along with a similar waveform

Oscillation Circuits

This type of circuit produces a period or oscillating signal that is used to convert DC current from an AC current.

Multi-Vibrator Circuits

This type of circuit is used in two-state devices such as timers. They generate pulse signals and use passive elements like resistors and capacitors to determine the output state. 

Wave Shaping Circuits

This type of circuit is used to alter the shape of a waveform in order to ensure the voltage does not exceed a predetermined amount. This has no impact on the remaining portion of the waveform. 

Detection and Demodulation Circuits

These types of circuit are used to extract the original signal from a modulated waveform. They recover the intelligence, or message, that was left on the radio carrier wave at the transmitter. The signal output may be in the form of analog audio, images, or binary data. 

Types of Bipolar Junction Transistors

Macro shot of four Transistors

Bipolar junction transistors are constructed using “layers.” These layers can be in either an NPN or PNP configuration. In an NPN layering (negative-positive-negative), the transistor will “switch” on when a current is flowing through the base terminal. Alternatively, if a BJT consists of PNP layering (positive-negative-positive), the transistor will only switch on when there’s no current flowing through the base.

As mentioned above, bipolar junction transistors can also amplify current. In an NPN layering configuration, you can amplify the current by applying a small current to the positively charged terminal (in this case, the base terminal). The electrons, attracted to the polarity of the base terminal, will travel from the emitter to the collector which amplifies the current between the two N-type layers. 

BJTs: Then and Now

Understanding how these transistors function is imperative if you’re interested in modern electronics and if you want to pursue a career in this field. In many respects, the world of technology is constantly evolving. With things changing at lightning speed, it is somewhat comforting to know that the components used to herald in the age of computing and computers 70 years ago remain a vital component to this day. 

If you’re interested in learning more about bipolar junction transistors, you may want to consider joining an Electromechanical Technician Training Certificate. This online program can prepare you for an exciting career in electromechanical systems.


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