Bipolar junction transistor

Okay, let’s break down the Bipolar Junction Transistor (BJT) and turn this information into a clear educational guide for electrical engineers. Think of this as getting the lowdown from someone who knows their way around these components.

What is a Bipolar Junction Transistor (BJT)?#

Alright, first things first, what exactly is a BJT?

A Bipolar Junction Transistor (BJT) is a type of transistor that uses both electrons (which are negatively charged) and electron holes (which act like positively charged carriers) to conduct electricity. This is where the “bipolar” part of the name comes from – it means “two poles” or two types of charge carriers.

Contrast this with something like a Field-Effect Transistor (FET), which is a unipolar transistor because it only uses one type of charge carrier (either electrons or holes, but not both significantly for the main current).

The cool thing about a BJT is that it’s like a controlled valve for electrical current. A tiny current injected into one of its connections (called a terminal) can control a much, much larger current flowing between the other two terminals. This capability makes BJTs super useful for two main jobs:

Amplification: Making a small signal bigger.
Switching: Turning a current flow completely on or off, like a high-speed switch.

BJTs are built using semiconductor materials, usually silicon these days, though germanium and even some advanced materials like gallium arsenide are used for special cases. The structure involves layers of semiconductor material treated with different impurities, a process called doping, to create regions that have an excess of either electrons (n-type) or holes (p-type).

Doping is the process of adding impurities to a semiconductor material to change its electrical conductivity. Adding specific elements (like phosphorus or arsenic to silicon) creates n-type material with extra free electrons. Adding other elements (like boron to silicon) creates p-type material with extra “holes” (places where electrons are missing, acting like positive charges).

Where these n-type and p-type regions meet, they form p–n junctions. A BJT has two of these junctions connected together in a specific way.

How BJTs are Made (Briefly)#

Creating these layers and junctions can be done in a few ways:

Growing: Changing the doping elements as the crystal is grown.
Alloying: Depositing metal pieces (pellets) onto the semiconductor and heating them so they melt and mix (alloy) with the semiconductor, forming the junction.
Diffusion: Heating the semiconductor crystal in an atmosphere containing the doping impurities, allowing the impurities to spread (diffuse) into the crystal.

The methods like diffusion and later techniques offer better control and performance compared to the very first “point-contact” transistors, leading to predictable and reliable devices. This predictability was a huge deal and quickly made the older point-contact types less common.

Where You’ll Find BJTs#

For a long time, BJTs were the go-to components for building logic circuits in computers, especially mainframe and minicomputers. However, nowadays, most digital computer systems lean towards CMOS (Complementary Metal–Oxide–Semiconductor) technology, which uses FETs because they use less power when switching.

But that doesn’t mean BJTs are gone! They are still incredibly important:

Amplification: Still the champion for many types of signal amplification.
Switching: Used in various switching applications, especially where high current or voltage is needed.
Mixed-Signal Circuits: In integrated circuits that handle both analog and digital signals (often called BiCMOS processes, combining BJT and CMOS on the same chip).
Specialized Uses: High-voltage switches, high-current switches, and amplifiers for very high frequencies (like in radio communication, often using special types like HBTs).
Integrated Circuits (ICs): Even in digital CMOS chips, low-performance BJTs can sometimes be made as “bonus” devices and are used for clever things like generating stable reference voltages (bandgap references) or measuring temperature.

Understanding Current Direction#

In electronics diagrams, we use a convention called conventional current.

Conventional Current: The direction in which a positive charge would move in a circuit. This was established before we fully understood that in most metal wires, the actual charge carriers are negative electrons moving the opposite way.

So, when you see an arrow showing current in a diagram, that’s usually the conventional current direction. Electrons, carrying a negative charge, move in the direction opposite to the conventional current arrow.

Inside a BJT, things are a bit more complex because, as we said, it uses both positive holes and negative electrons as carriers. Holes move in the direction of conventional current, while electrons move against it.

The Arrow on the BJT Symbol#

Look at the symbol for a BJT on a schematic. There’s usually an arrow on one of the terminals (the emitter).

This arrow is located on the emitter terminal.
It points from the P-type region towards the N-type region within the base-emitter junction.
This arrow shows the direction of conventional current flow when the base-emitter junction is forward biased (which is how it works in its main “active” or “on” modes).

This little arrow tells you whether you’re looking at an NPN or a PNP transistor.

How a BJT Works (Function)#

BJTs come in two main flavors, determined by the order of the semiconductor layers:

NPN: Has an N-type layer, a P-type layer, and another N-type layer. The middle P layer is the base.
PNP: Has a P-type layer, an N-type layer, and another P-type layer. The middle N layer is the base.

Each layer has a connection terminal: the emitter (E), the base (B), and the collector (C).

Let’s focus on the NPN type for most explanations, as it’s very common. A PNP works similarly but with opposite voltage polarities and current directions.

In an NPN transistor, you have an N layer (emitter), a P layer (base), and an N layer (collector). This looks a bit like two back-to-back diodes (N-P and P-N), but they share the middle P region (the base).

The Importance of Doping Levels and Size#

The way these layers are doped and their physical size is crucial to how the BJT works:

Emitter: This layer is typically heavily doped. This means it has a very high concentration of majority carriers (electrons in NPN).
Base: This layer is lightly doped and very thin. It has a low concentration of majority carriers (holes in NPN).
Collector: This layer is doped more lightly than the base (maybe ten times lighter) but is usually thicker than the base.

Why these specific doping levels and thicknesses? It’s all about making the charge carriers move efficiently from the emitter through the base to the collector.

The Mechanism: Injection, Diffusion, and Sweep#

The magic happens when you apply voltages to the terminals in a specific way, particularly in the most common mode called the forward-active mode.

Forward-Active Mode: This is the key mode for using a BJT as an amplifier. In an NPN transistor:

The base-emitter (BE) junction is forward biased. This means the P-type base is made more positive than the N-type emitter.

The base-collector (BC) junction is reverse biased. This means the N-type collector is made more positive than the P-type base.

Let’s see what happens in an NPN in forward-active mode:

Injection: Because the base-emitter junction is forward biased, it works like a forward-biased diode. The heavy doping of the emitter means lots of its majority carriers (electrons) are pushed across the BE junction into the base region. These injected electrons become minority carriers in the P-type base (where holes are the majority carriers).
Diffusion: Once in the base, these injected minority carriers (electrons) are now in a region where their concentration is much higher near the emitter side than near the collector side. They spread out through the base region from the high-concentration area towards the low-concentration area. This movement due to concentration difference is called diffusion.
Sweep: Remember the base is thin and lightly doped? This is vital!
- Being thin means the electrons don’t have to travel far across the base.
- Being lightly doped means there aren’t many majority carriers (holes) in the base to recombine with the injected electrons.
- So, most of the injected electrons diffuse across the thin base without recombining with holes.
- When these electrons reach the edge of the base-collector depletion region (which exists because the BC junction is reverse-biased), they are met by a strong electric field. This field exists because the collector is positive relative to the base.
- This electric field sweeps the electrons across the BC junction into the collector region. These electrons then form the collector current (I_C).

A small number of the injected electrons do recombine with holes in the base. To replace these lost holes and maintain the balance in the base region, a small current flows into the base terminal. This is the base current (I_B).

The emitter current (I_E) is the total current leaving the emitter, which is the sum of the base current and the collector current: I_E = I_B + I_C.

Because most of the injected carriers make it across the base to the collector (thanks to the thin, lightly-doped base), the collector current (I_C) is much larger than the base current (I_B). This is the fundamental basis of the BJT’s current gain.

Minority Carrier Device: BJTs are classified as minority-carrier devices because the current flow from emitter to collector is primarily due to the injection and movement of minority carriers (electrons in the P-type base of an NPN, or holes in the N-type base of a PNP) across the base region.

Why Not Just Two Diodes?#

Connecting two separate diodes back-to-back won’t work like a BJT. The middle region of a BJT (the base) is a single crystal, and it’s very thin. This allows minority carriers injected from the emitter to diffuse across the base as minority carriers and be collected. In separate diodes connected with wires, the carriers would become majority carriers in the wire and couldn’t easily cross the second junction. The thin, shared, correctly doped base is the key difference.

Asymmetry in Structure#

Most BJTs are not perfectly symmetrical. Swapping the collector and emitter wouldn’t give you the same performance. This is mainly because the emitter is heavily doped and the collector is lightly doped. This doping asymmetry is designed to maximize the emitter injection efficiency in the forward-active mode (ensuring most carriers crossing the BE junction come from the heavily doped emitter, not the base). It also helps the collector-base junction handle higher reverse voltages before breaking down, which is needed because the BC junction is reverse-biased in active mode.

Some special low-performance BJTs used within integrated circuits (called lateral BJTs) are sometimes designed to be symmetrical, but this is an exception.

Voltage Control or Current Control?#

You might hear people describe BJTs as either voltage-controlled or current-controlled devices. Both views are valid and useful depending on what you’re trying to analyze or design.

Voltage Control View: The collector current (I_C) is fundamentally controlled by the base-emitter voltage (V_BE). The relationship between I_C and V_BE is exponential, similar to a diode. A small change in V_BE causes a large exponential change in the current flowing across the BE junction, and most of this current ends up as collector current.
Current Control View: Because of the transistor’s structure (thin, lightly doped base) and the physics of carrier movement, the collector current (I_C) is approximately proportional to the base current (I_B). The ratio I_C / I_B is the transistor’s current gain (called β). So, a small base current can control a much larger collector current.

In practice:

For simple circuit design and intuition, the current control view (I_C ≈ β * I_B) is often used because it’s simpler (linear approximation).
For more accurate design, especially with temperature effects or precise biasing, the voltage control view (with the exponential I-V relationship) is necessary.
Detailed computer simulations (like SPICE) use complex models that capture the underlying physics, including the charge distribution in the base (charge control view). This view is great for understanding why the transistor behaves dynamically (like how long it takes to turn off).

BJT Characteristics: Alpha (α) and Beta (β)#

These are two key parameters that describe how much current gain a BJT provides.

Alpha (α): This is the common-base current gain. It’s the ratio of the collector current (I_C) to the emitter current (I_E) in the forward-active mode.

α = I_C / I_E Since most carriers from the emitter make it to the collector, I_C is very close to I_E. So, α is usually a value very close to 1, typically between 0.980 and 0.998. It’s slightly less than 1 because of the few carriers lost to recombination in the base (which form the base current).
Beta (β): This is the common-emitter current gain. It’s the ratio of the collector current (I_C) to the base current (I_B) in the forward-active mode.

β = I_C / I_B This is the parameter most commonly referred to as the “gain” of the transistor. It’s typically much larger than 1. For small-signal transistors, β can be 50 or much higher (even several hundred). For power transistors designed to handle large currents, β might be lower, maybe 10 to 50.

Alpha and Beta are directly related. If you know one, you can calculate the other:

α = β / (1 + β)
β = α / (1 - α)

Example: If a transistor has a β of 100, its α is 100 / (1 + 100) = 100 / 101 ≈ 0.99. This confirms that most of the emitter current becomes collector current.

Beta is a very convenient number for circuit designers working with the current-control model. Often, circuits are designed assuming β is high enough that the base current is relatively small compared to the collector and emitter currents. However, β varies from transistor to transistor, with temperature, and with current levels, so robust designs don’t rely on β having an exact value.

Regions of Operation#

A BJT can operate in four main modes, depending on how you bias the two p-n junctions (Base-Emitter and Base-Collector). Thinking about the bias (forward or reverse) is key here.

Forward-Active Region:
- Bias: Base-Emitter (BE) junction is Forward Biased, Base-Collector (BC) junction is Reverse Biased.
- Behavior: This is the amplification region. A small base current (or base-emitter voltage) controls a much larger collector current. The transistor acts like a controlled current source (or voltage-controlled current source). This is where β and α are defined as discussed above.
Reverse-Active Region (or Inverse):
- Bias: Base-Emitter (BE) junction is Reverse Biased, Base-Collector (BC) junction is Forward Biased.
- Behavior: The roles of the emitter and collector are effectively swapped. Current flows from collector to emitter. However, because the transistor structure is optimized for forward-active mode (remember the asymmetric doping?), the current gain (often called β_R or inverse beta) is much, much lower than in forward-active mode (often less than 1). This mode is rarely used intentionally for amplification but might be encountered in certain logic circuits or unusual conditions.
Saturation Region:
- Bias: Both Base-Emitter (BE) and Base-Collector (BC) junctions are Forward Biased.
- Behavior: The transistor is fully “on”. It acts like a closed switch between the collector and emitter, allowing maximum current to flow (limited mainly by the external circuit). The collector-emitter voltage (V_CE) is very low (typically a few tenths of a volt). This is the “on” state when using a BJT as a digital switch.
Cut-off Region:
- Bias: Both Base-Emitter (BE) and Base-Collector (BC) junctions are Reverse Biased.
- Behavior: The transistor is fully “off”. Very little current flows between the collector and emitter (only tiny leakage currents). It acts like an open switch. This is the “off” state when using a BJT as a digital switch.

These regions transition into each other. For very small bias voltages (less than a few hundred millivolts), the lines between cut-off and active, or active and saturation, can be a bit blurry.

Switching Speed: Turn-on and Turn-off#

When you switch a BJT from off (cut-off) to on (saturation), or vice versa, it doesn’t happen instantly. One factor is the time it takes to build up or remove the charge of minority carriers stored in the base region.

When turning on and pushing the transistor into saturation, you inject a lot of minority carriers into the base.
When turning off from saturation, you first have to remove this excess stored charge from the base before the collector current can stop. This takes time and contributes to a storage delay.

This storage delay is particularly important for power transistors and can limit how fast you can switch them. Techniques like using a Baker clamp can prevent the transistor from going too deep into saturation, reducing the stored base charge and speeding up the turn-off time.

Structure Details#

Let’s look a bit closer at the physical layers in an NPN transistor:

Region	Semiconductor Type	Doping Level	Role
Emitter	N-type	Heavily Doped	Injects majority carriers
Base	P-type	Lightly Doped	Thin layer, carriers diffuse through
Collector	N-type	Lightly Doped	Collects carriers from base

In a PNP, the types are reversed (P-type emitter, N-type base, P-type collector), and the carriers are primarily holes moving from emitter to collector through the base.

The collector is often designed to surround the emitter slightly. This helps ensure that most of the carriers injected from the emitter into the base are captured by the collector and don’t go elsewhere.

As mentioned, the asymmetric doping (heavy emitter, light collector) is intentional. Heavy doping in the emitter boosts the injection efficiency. Lighter doping in the collector helps it withstand a higher reverse voltage before breaking down, which is crucial since the collector-base junction is reverse-biased in the common active mode.

Most modern BJTs are made from Silicon. For higher speed, especially in radio frequency applications, Gallium Arsenide (GaAs) is used, often in a type called a Heterojunction Bipolar Transistor (HBT).

Heterojunction Bipolar Transistor (HBT): A BJT built using different semiconductor materials for the different layers (like silicon and silicon-germanium, or aluminum gallium arsenide and gallium arsenide). This difference in materials helps create better performance, especially at very high frequencies (hundreds of GHz), making them excellent for modern wireless communication systems.

HBTs require advanced manufacturing techniques like Epitaxy to grow the layers with precise control over doping and material composition.

Modeling BJT Behavior#

To design circuits using BJTs, engineers use mathematical models that describe how the transistor behaves under different conditions. These models help predict currents and voltages without having to build the physical circuit first.

You can think of a BJT kind of like two diodes sharing a base region.

Large-Signal Models#

These models describe the overall, or DC (Direct Current), behavior of the transistor across its different operating regions. They relate the currents and voltages at the terminals.

Ebers-Moll Model#

This is a classic, relatively simple model for describing the large-signal behavior of a BJT. It gives us equations for the terminal currents based on the junction voltages.

Here are the basic Ebers-Moll equations for the currents (using lowercase i for general currents, V for voltages):

i_C = I_S * [ ( e^(V_BE / V_T) - e^(V_BC / V_T) ) - (1 / β_R) * ( e^(V_BC / V_T) - 1 ) ]
i_B = I_S * [ (1 / β_F) * ( e^(V_BE / V_T) - 1 ) + (1 / β_R) * ( e^(V_BC / V_T) - 1 ) ]
i_E = I_S * [ ( e^(V_BE / V_T) - e^(V_BC / V_T) ) + (1 / β_F) * ( e^(V_BE / V_T) - 1 ) ]

Let’s break down what these symbols mean:

i_C, i_B, i_E: The currents flowing into the Collector, Base, and Emitter terminals, respectively.
V_BE: The voltage between the Base and Emitter terminals.
V_BC: The voltage between the Base and Collector terminals.
V_T: The Thermal Voltage. This is a constant related to temperature, approximately 26 mV (milliVolts) at room temperature (300 Kelvin). It’s calculated as kT/q, where k is Boltzmann’s constant, T is the absolute temperature, and q is the elementary charge. This voltage shows up in the exponential terms, reflecting the fundamental behavior of semiconductor junctions.
I_S: The Reverse Saturation Current. This is a very small current (typically in the range of 10⁻¹⁵ to 10⁻¹² Amperes) that flows when a junction is reverse biased. It’s a measure of the device’s size and material properties.
β_F: The forward common emitter current gain (our familiar Beta, h_FE or h_fe). This applies when the transistor is operating forward-active.
β_R: The reverse common emitter current gain (the inverse beta). This applies when operating reverse-active. It’s usually much smaller than β_F.

In the forward-active region (where V_BE is positive and V_BC is negative, making e^(V_BC/V_T) very small, close to 0), these equations simplify to the approximations we saw earlier:

I_C ≈ I_S * e^(V_BE / V_T) (Collector current depends exponentially on V_BE)
I_E ≈ I_C
I_B ≈ I_C / β_F

This exponential relationship between I_C and V_BE is a key characteristic. At room temperature, increasing V_BE by about 60 mV increases the collector (and emitter) current by a factor of 10.

Base-Width Modulation (Early Effect)#

The Ebers-Moll model is good, but it doesn’t account for everything. One important effect is base-width modulation, also known as the Early effect.

Base-Width Modulation (Early Effect): As the voltage across the collector-base junction (V_CB or V_CE) changes, the width of the depletion region at that junction changes. Since the base is between the collector and emitter, changing the BC depletion region width effectively changes the electrical width of the base region itself.

In forward-active mode, the BC junction is reverse biased. Increasing the reverse bias voltage (making V_CE higher while V_BE is held constant) widens the BC depletion region. This makes the effective base region narrower.

Making the base narrower has two main consequences:

Less Recombination: Minority carriers (electrons in NPN) spend less time in the base, so there’s less chance for them to recombine with majority carriers (holes). This means more injected carriers reach the collector.
Steeper Gradient: The concentration gradient of minority carriers across the base becomes steeper (same number of carriers diffusing across a shorter distance). A steeper gradient leads to a larger diffusion current.

Both of these effects cause the collector current (I_C) to increase slightly as the collector-emitter voltage (V_CE) increases, even if the base-emitter voltage (V_BE) is held constant. This causes the output current-voltage characteristic curves of the transistor to have a slight upward slope in the active region, rather than being perfectly flat. This slope is characterized by the Early Voltage.

Punchthrough#

An extreme version of base-width modulation is punchthrough.

Punchthrough: This occurs when the reverse voltage across the collector-base junction is so high that the BC depletion region expands all the way through the thin base and touches the BE depletion region. At this point, the base effectively disappears, and the transistor loses its ability to control current, acting more like a breakdown path.

This is a breakdown phenomenon and can potentially damage the transistor if the current isn’t limited.

Gummel-Poon Model#

While Ebers-Moll is a good start, more complex models like the Gummel-Poon model are used in circuit simulators (like SPICE) for more accurate analysis.

Gummel-Poon Model: A more detailed model that accounts for effects like the variation of Beta (β) with collector current levels, high-level injection, and other non-ideal behaviors that the simple Ebers-Moll model doesn’t capture. It is based on the distribution of charge within the base region.

These advanced models are crucial for designing high-performance or complex analog circuits.

Small-Signal Models#

Large-signal models describe the DC behavior. But for analyzing how a transistor amplifies a small changing (AC) signal, we use small-signal models. These models represent the transistor’s behavior around a specific DC operating point (like representing the exponential I-V curve as a linear slope at that point for small AC signals).

Hybrid-Pi Model#

Hybrid-Pi Model (also called Giacoletto Model): A very common small-signal model that uses resistors, capacitors, and controlled current sources to represent the transistor’s AC behavior. It’s named for its pi-like structure.

This model is great for understanding concepts like input impedance, output impedance, and voltage/current gain for AC signals. It includes components representing the resistance of the base region (r_pi), the transconductance (g_m - how much the output current changes for a small change in input voltage), and internal capacitances (which become important at higher frequencies).

H-Parameter Model#

H-Parameter Model (Hybrid Equivalent Model): Another small-signal model that uses a specific set of parameters (called h-parameters) to describe the transistor’s behavior as a “two-port network”. It’s closely related to the Hybrid-Pi model but uses input current and output voltage as the independent variables in its equations.

The h-parameters are often given on transistor datasheets, especially for older or lower-frequency devices. For the common-emitter configuration (where the emitter is the common terminal for input and output):

h_ie: Input impedance (like r_pi in the Hybrid-Pi model).
h_re: Reverse voltage transfer ratio (how much the input voltage changes with the output voltage - usually very small).
h_fe: Forward current gain (the small-signal AC beta, β_AC). This is the ratio of a small change in collector current to a small change in base current (ΔI_C / ΔI_B).
h_oe: Output admittance (the inverse of output impedance - usually small).

Note the lowercase subscripts (‘fe’ vs ‘FE’).

h_FE vs h_fe: This is important!

h_FE (or just β) uses CAPITAL letters for the subscripts. This means it’s the DC or large-signal current gain (I_C / I_B) at a specific operating point.

h_fe uses lowercase letters for the subscripts. This means it’s the AC or small-signal current gain (ΔI_C / ΔI_B). Often, these two values are close, especially at moderate current levels, but h_fe can change significantly at high frequencies.

For many basic analyses, a simplified h-parameter model is used where h_re and h_oe are assumed to be zero (meaning no voltage feedback from output to input, and infinite output resistance).

Industry Models#

For cutting-edge circuit design and simulation, even more sophisticated models than Gummel-Poon are used, such as Mextram, HICUM, Modella, and VBIC. These models capture additional effects like self-heating, breakdown characteristics, and quasi-saturation (behavior between active and full saturation) to provide highly accurate simulations.

Applications of BJTs#

Even with the rise of other technologies, BJTs are still essential components in many areas of electrical engineering.

Amplification#

This is a classic use case. The current gain (β) means a small signal applied to the base can control a much larger current through the collector-emitter path, effectively boosting the signal’s strength. The very first transistor application that became hugely popular was in consumer electronics like the transistor radio back in the 1950s. This small, portable device, powered by transistors instead of bulky tubes, really kicked off the era of miniaturized electronics.

There are three main ways to configure a BJT for amplification, depending on which terminal is common to both the input and output:

Common Emitter (CE): Most popular for general voltage and current amplification. Provides significant voltage and current gain, but inverts the input signal.
Common Base (CB): Provides voltage gain but no current gain (α is close to 1). Excellent for high-frequency applications and matching low-impedance sources.
Common Collector (CC): Also known as an Emitter Follower. Provides current gain but no voltage gain (gain is close to 1). Used primarily as a buffer to provide high input impedance and low output impedance, useful for driving low-resistance loads.

Switching#

BJTs are widely used as electronic switches, moving between the cut-off (off) and saturation (on) regions.

Digital Logic: Early digital circuits relied heavily on BJTs (like TTL - Transistor-Transistor Logic, and ECL - Emitter-Coupled Logic). ECL, in particular, is known for being very fast because it avoids saturating the transistors, which reduces storage delay.
Power Switching: High-power BJTs are used to switch motors, lights, and other loads in various control systems.

Combining BJTs with MOSFETs on the same chip (BiCMOS process) lets designers use the strengths of both: BJTs for high speed, high current drive, and precision analog, and MOSFETs for low-power logic gates.

Temperature Sensors#

The voltage across a forward-biased p-n junction (like the base-emitter junction) changes predictably with temperature. By measuring the base-emitter voltage at two different currents, you can accurately determine the temperature. BJTs are commonly used in integrated circuit temperature sensors because this characteristic is reliable and well-understood.

Logarithmic Converters#

Since the collector current is exponentially related to the base-emitter voltage (I_C ≈ I_S * e^(V_BE / V_T)), this relationship can be used to calculate logarithms or anti-logarithms of currents or voltages in analog computing circuits. A simple diode also has an exponential relationship, but the BJT offers more circuit flexibility.

Avalanche Pulse Generators#

Under certain conditions (specifically, with a controlled voltage just below breakdown and then a rapid trigger), a BJT can operate in an avalanche mode. This involves impact ionization in the collector-base depletion region, rapidly flooding the base with carriers and causing a very fast turn-on. This effect can be used to generate extremely sharp electrical pulses. Special “avalanche transistors” are designed specifically for this purpose, often used in high-speed pulse generation or timing circuits.

So, while CMOS dominates digital computing now, the BJT remains a versatile and fundamental component in the electrical engineer’s toolkit, prized for its speed, gain, and specific characteristics in a wide range of applications.