A guide to bipolar junction transistors

The term transistor is derived from the words “transfer and “resistor.” The first semiconductor transistor was invented to replace the triode as an amplifier device, a vacuum tube with three electrodes. The transistor was smaller, lighter, and cheaper than a vacuum tube triode. It also proved easy to construct and had no heat losses like in a triode.

A little history:

The first transistor was a point-contact transistor developed by John Bardeen, Walter Brattain, and William Shockley in 1947 at Bells Lab.
In 1948, the first bipolar junction transistor was developed by William Shockley.
In 1951, the first commercial transistor radios were introduced.
In 1954, the first integrated circuit was developed by Jack Kilby at Texas Instruments using transistors.

Since then, transistors have been a building block of electronics, providing an important part of digital circuits to power electronics.

Bipolar junction transistors (BJT) are the most common type. A BJT is similar to two semiconductor diodes combined into a three-terminal device, and is also made up of semiconductor materials, such as silicon and germanium. A transistor is typically used to amplify or switch electronic signals.

A BJT has three terminals:

The base is the control terminal
The collector and the emitter are the input/output terminals

Like its name suggests, a BJT is a bipolar electronic device. This means both electrons and holes participate as charge carriers. The term “junction” indicates two PN junctions in a bipolar junction transistor. The term “transistor” refers to its function as a signal amplifier. A small current applied to its base controls the much larger current between its emitter and collector.

BJT is a fundamental electronic device, which is widely used in electronic circuits for amplification, switching, and signal processing. It’s versatile and is found in multiple applications, including digital integrated circuits, oscillators, signal processors, and power electronics.

In this article, we’ll discuss the basic concepts of a bipolar junction transistor and explore its practical uses in circuits.

What is a BJT?
A bipolar junction transistor (BJT) is a three-layer semiconductor device consisting of two p-type layers and one n-type layer, or two n-type layers and one p-type layer.

A transistor consisting of two n-type layers and a p-type layer sandwiched between them is called an NPN transistor.
A transistor comprising two p-type materials and an n-type layer sandwiched between them is called a PNP transistor.

These transistors are named based on the arrangement of their semiconductor layers and the majority of charge carriers involved in their operation.

The symbols for NPN and PNP transistors are shown below.

BJT construction
A transistor is similar to a triode and is used for amplifying signals. In a transistor, the emitter layer is heavily doped, while the base and collector are lightly doped.

The emitter and collector have widths much greater than the base. The ratio of the base’s width to the transistor’s total width is typically 150:1. The doping level of the base layer is also 10 times less than the collector and emitter. Due to lower doping levels, the base has limited free charge carriers, resulting in higher resistance.

How a transistor works
Like a semiconductor diode, a transistor also requires proper biasing to function. Biasing refers to setting the DC operating conditions of the transistor in a way that it operates in a desired region.

Like a semiconductor diode, the output characteristics of a transistor are divided into distinct regions. The proper biasing for the NPN transistor is shown below.

To understand how a transistor operates, think of it as two PN junctions — one between emitter and base, and the other between base and collector.

Suppose one assumes that the base-collector junction is open-ended (due to the negative potential at the N-type emitter and the positive potential at the P-type base). In that case, the emitter-base PN junction is forward-biased.

In an N-type emitter, a heavily doped transistor region, electrons are the majority charged carriers. Because of the forward biasing at the emitter-base junction, a large number of electrons flow from the emitter to the base, while a small number of holes pass through the base to the emitter.

Next, let’s assume that the emitter-base junction is open-ended. In that case, there’s a negative potential applied at the P-type base and a positive potential applied at the N-type collector. As a result, the base-collector PN junction is reverse-biased.

In a reverse-biased condition, current from the minority charge carriers the flow from the base to the collector. In the P-type base, the electrons are the minority-charged carriers, and in the N-type collector, the holes are the minority-charged carriers.

However, many electrons are also injected from the forward biasing of the emitter-base junction. Because of the high resistance of the base region, only a negligible amount of current passes through the base terminal. Most electrons injected in the base region diffuse into the N-type collector region.

When applying Kirchhoff’s current law to the transistor, we get the following equation.

I_E = I_B + I_C

The above current relationship remains applicable whenever a transistor is connected in a circuit. The collector current has two components:

I_C= I_Cmajority + I_CMinority

The base current is nearly negligible in terms of the order of microamperes or nano amperes. The emitter and collector currents are of the order of milli amperes.

As the base current is nearly negligible when the emitter-base junction is forward-biased and the base-collector junction is reverse-biased, the emitter current and collector current are almost equal.

I_E=̃ I_C

The collector current, due to the minority carriers (i.e. the holes in the N-type collector) is negligible because of the high resistance of the base region.

So, we can say the emitter current and the current passing through the collector from the majority carriers (i.e. electrons) are equivalent in the case of the NPN transistor.

I_E=̃ I_CMajority

So, for any voltage between the base-collector junction, the current passing through the collector is equivalent to the current at the emitter. The DC biasing at the transistor’s base controls the current flow from the emitter to the collector. A PNP transistor operates the same way, except that the roles played by the electrons and holes are interchanged.

In an NPN transistor, the current between the emitter and the collector is largely from the flow of the electrons from the emitter to the collector. This is why the direction of conventional current is indicated by an arrow pointing out from the emitter.

In a PNP transistor, the current between the emitter and the collector is largely because of the flow of the holes from the emitter to the collector. This is why, the direction of conventional current is indicated by an arrow pointing in from the emitter.

VI characteristics
As a three-terminal device, BJT has one input and one output side. One of the terminals remains common between the input and the output sides. This terminal is also closest to, or at, ground potential.

The VI characteristics of a BJT are then divided into two sets of characteristics: the input or driving point parameters and output characteristics.

Now, suppose the transistor is connected so that the base terminal is shared between the input and output sides. This is a common-base configuration of the transistor. The input signal is applied at the emitter, and the collector receives the output signal.

As the input side must be forward-biased for any current to flow between the emitter and the base, the input VI characteristics are similar to the forward-bias characteristic curve of a diode. The input characteristics of BJT in a common-base configuration is shown below.

As seen from the input characteristics curve for common-base configuration, if a collector-base voltage is constant, the emitter current (input signal) rises exponentially after a certain base-emitter voltage.

If the collector-base voltage increases, the exponential rise begins early and the emitter current rises to a higher value for maximum permissible voltage at the collector-base junction.

The output characteristics curve for the common-base configuration is shown below.

The collector current (output signal) has three distinct regions of operation compared to the collector-base voltage. When the emitter-base and collector-base junction are reverse-biased, the emitter current and collector current are zero.
There’s a negligible current flow at the collector due to the minority carriers, which can be ignored. This is the cut-off region of the transistor. In this region, the transistor is off. There’s no base current or collector current.

When the emitter-base and collector-base junction are forward-biased, the collector current increases sharply for a slight change in collector-base voltage. This is the saturation region of the transistor. In this region, the transistor is fully on and a maximum collector of current flows. This region of operation is useful for using a transistor as a switch or for building logic gates.

When an emitter-base junction is forward-biased, and the collector-base junction is reverse-biased, the collector current approximates to the emitter current, and there’s no effect or change in the collector-base voltage on the collector current. This is the active region of the transistor. In this region, the transistor operates as a linear amplifier. A small base current controls a larger collector current, allowing signal amplification.

An NPN transistor is formed by sandwiching a P-type region between two N-type regions. The electrons are the majority carrier.

To operate an NPN transistor in an active region, a positive voltage is applied to the collector (VCC), and a more positive potential is applied to the base terminal with respect to the emitter terminal (VBE).

In the active region, electrons flow from the emitter to the collector in an NPN transistor, resulting in a conventional current flowing from the collector to the emitter when a small current flows from the base to the emitter.

A PNP transistor is formed by sandwiching an N-type region between two P-type regions. The holes are the majority carrier. To operate a PNP transistor in the active region, a negative voltage is applied to the collector (VCC) and more negative potential is applied to the base terminal with respect to the emitter terminal (VBE).

In the active region, the holes flow from the emitter to the collector in a PNP transistor, resulting in a conventional current flowing from the emitter to the collector when a small current flows from the emitter to the base.

NPN transistors are easier to manufacture and typically exhibit better performance characteristics, such as higher current gain (β). They’re often used in low-side switching applications, amplification circuits, and as active components in digital logic gates.

PNP transistors are less common than NPN BJTs but are essential for certain circuit configurations. They’re used when the circuit design necessitates a different polarity or when the power supply voltage is negative with respect to the ground reference. PNP transistors are often used in high-side switching applications, motor control, and as active components in complementary digital logic gates.

In some applications, NPN and PNP BJTs are used together in a complementary pair configuration. This pairing allows for both positive and negative signal amplification and switching, making them suitable for applications like push-pull amplifiers and complementary symmetry power amplifiers.

Three BJT configurations
BJT has an input and an output side when connected in a circuit. One of the transistor terminals remains common between the input and output.

There are three possible configurations for connecting a transistor in a circuit.

1. Common-base (CB) configuration: the base of the transistor is common between the input and output sides of the transistor. The input is applied at the emitter terminal, and the output is connected to the collector terminal. This configuration is not as common as the CE configuration, but it’s used in some specialized applications, such as high-frequency amplifiers.

In this configuration, the output signal is in phase with the output signal. CB offers current gain as a small change in emitter current results in a larger change in collector current. The voltage gain remains less than ‘1,’ so the output voltage is smaller than the input voltage. The configuration features a low input impedance and high output impedance. That is why it’s used for impedance matching rather than voltage amplification.

2. Common-emitter (CE) configuration: the emitter is the common terminal between the input and output sides. The input is applied to the base, and the collector obtains the output signal. This is the most common configuration for BJT.

In this configuration, the output signal is 180 degrees out of phase from the input signal. CE offers high voltage gain and a moderate current amplification. The input impedance is moderate, while the output impedance is relatively high. Since the common-emitter configuration offers voltage and power amplification, it’s used in audio amplifiers and signal-processing circuits.

3. Common-collector (CC) configuration: the collector is the common terminal between the input and output sides. The input is applied at the base terminal and the output signal is obtained from the collector terminal. This configuration is also not as common as the CE configuration.

In the CC configuration, the output is in phase with the input signal and closely follows the input signal with only a slight voltage drop. However, this configuration still offers high current gain and is commonly used in current amplifiers. The configuration features a high input impedance and low output impedance.

Transistor biasing
Biasing a transistor refers to the process of setting the DC operating conditions of the transistor so that it operates in a set operating region, active, saturation, or cutoff. For amplification, the transistor must operate in the active region. To act as a switch in a circuit, the transistor must operate in the cutoff and saturation regions.

Biasing involves setting the transistor’s quiescent (Q-point) or operating point. This point defines the DC voltage and current levels when no AC signal is applied. The Q-point is essential for ensuring that the transistor operates within its linear, active region for amplification or switching applications.

When biased to operate in the active region, biasing maintains the transistor’s stability and prevents it from entering saturation or cutoff when subjected to varying environmental conditions or signal variations. It ensures the transistor remains in the linear region required for amplification.

The process typically involves selecting appropriate resistor values and DC voltage sources in the transistor circuit to set the base current for the transistor and establish the collector current at the desired levels. It’s important to consider several factors, including the transistor characteristics, temperature variations, and the desired output characteristics.

There are several biasing techniques used on bipolar junction transistors, such as fixed bias, emitter bias, voltage-divider bias, emitter-follower, and collector feedback.

A circuit
When a transistor is connected in a circuit, proper biasing is the first thing that must be addressed. If the transistor is used for amplification or switching high currents, an adequate heat sink must be used to prevent any thermal damage.

In switching applications, the BJT may also require protection from voltage spikes and reverse currents using flyback diodes or other techniques.

Applications
BJTs are used in a range of applications in electronics. Some of the common applications are as follows.

Amplification: BJTs are commonly used for current or power amplification. They can amplify weak input signals, making them stronger and suitable for further processing or transmission. Typical amplifier applications for BJTs include audio amplifiers, RF (radio frequency) amplifiers, and instrumentation amplifiers.

Switching: BJTs are excellent electronic switches. When properly biased and driven into saturation (ON state) or cut-off (OFF state), they can control the flow of high-current loads. This makes BJTs ideal for applications like digital logic gates, power switches, and pulse-width modulation (PWM) controllers.

Voltage amplification: BJTs are used to build operational amplifier (op-amp) circuits, which provide high voltage gain and are essential in various analog signal processing applications, such as amplifiers, filters, and instrumentation.

Audio applications: BJTs have an excellent ability to amplify audio signals accurately. This is why they’re often used in audio applications like microphone preamplifiers, amplifiers, and equalizers.

RF applications: BJTs, such as RF amplifiers and mixers, are commonly used in RF circuits. They process and amplify high-frequency signals for wireless communication and broadcasting.

Signal processing: BJTs are used in signal processing circuits such as filters and signal modulators. They help shape, filter, or modify signals in applications like audio processing, communication systems, and equalization circuits.

Digital logic gates: Although FETs are more commonly used in digital logic circuits today, BJTs have been historically used for building logic gates like NOT, AND, OR, and NAND gates.

Voltage regulation: BJTs can help maintain a stable output voltage regardless of variations in the input voltage or load conditions. This is why BJTs are used in voltage regulator circuits.

Current sources and sinks: BJTs can be used to create precise current sources or current sinks. These are useful in biasing other transistors or providing stable reference currents for sensor and measurement circuits.

Light sensing: Phototransistors are a specialized type of BJT, which are used in light sensors, optical switches, and detectors to convert light into electrical signals.

Temperature sensing: BJTs are used as temperature sensors in temperature-compensation circuits.

Oscillation: BJTs are vital components in oscillator circuits, which generate periodic waveforms. Oscillators are used in radios, signal generators, and clocks to produce precise and stable frequency signals.

Motor control: BJTs drive motors and control their speed and direction in motor control circuits. Such transistor circuits are used in robotics, electric vehicles, and industrial automation applications.

Bipolar junction transistor is a versatile electronic device. Engineers and designers rely on BJTs to perform various functions, from signal amplification to power control, making them essential in many electronic devices and systems.

Commercial BJTs
Bipolar junction transistors are versatile electronic devices used in many electronic applications. There are many types of commercial BJTs available in the market.

Engineers and circuit designers choose the appropriate type based on the specific requirements of their circuits and applications, considering factors like voltage, current, frequency, power handling capabilities, and specific circuit needs.

Some of the most common types of commercial BJT are as follows.

1. Small Signal NPN/PNP BJTs: general-purpose transistors used for small-signal amplification and switching applications. Examples include 2N3904 (NPN), BC547 (NPN), 2N3906 (PNP), and BC557 (PNP) transistors.

2. High-frequency BJTs: operate at high frequencies and are often used in radio frequency (RF) amplifiers and other high-frequency applications. Examples include 2N2222 (NPN), 2N3866 (NPN), 2N2369 (NPN), 2N2907 (PNP), and 2N5179 (PNP) transistors.

3. High-frequency RF BJTs: optimized for extremely high-frequency applications, such as microwave amplifiers and communication systems. Examples include 2N3866 (NPN), 2N5109 (NPN), 2N2219A (NPN), 2SC3355 (NPN), 2N2907A (PNP), and 2SC2999 (PNP) transistors.

4. Power BJTs: capable of handling higher current and power levels. They’re used in power amplifiers, voltage regulators, and high-current switching applications. Examples are 2N3055 (NPN), MJE13009 (NPN), BD139 (NPN), TIP31 (NPN), MJ2955 (PNP), 2N6107 (PNP), 2SA1943 (PNP), and TIP32 (PNP) power transistors.

5. High-voltage BJTs: withstand higher voltage levels and are used in applications requiring high voltage amplification or switching. Examples include 2N3773 (NPN) and MJ10012 (PNP) transistors.

6. Low-power BJTs: operate at low power levels, making them suitable for battery-powered or energy-efficient applications. Examples are 2N3904 (NPN), 2N2222 (NPN), PN2222 (NPN), 2N4401 (NPN), BC547 (NPN), BC109 (NPN), 2N3906 (PNP), 2N2907 (PNP), PN2907 (PNP), 2N4403 (PNP), BC557 (PNP), and BC179 (PNP) transistors.

7. High-gain BJTs: have extremely high current gain (β) and are used in applications where signal amplification is critical, like high-frequency RF circuits. Some examples include 2N3904 (NPN) and BC547 (NPN) transistors.

8. Darlington BJTs: consist of two BJTs connected in series, providing very high current gain (β), and are used in applications requiring high current amplification, such as power drivers. Examples include the TIP120 (NPN), TIP122 (PNP), and TIP125 (PNP) Darlington transistors.

9. Switching BJTs: designed for fast switching applications, often used in digital circuits and pulse-width modulation (PWM) applications. Examples include 2N2369 (NPN), 2N3906 (NPN), 2N4401 (NPN), 2SC945 (NPN), and 2SC945 (PNP) transistors.

10. Low-noise BJTs: engineered to have minimal noise characteristics and used in sensitive electronic circuits, such as low-noise amplifiers for radio receivers. Examples are 2N2222A (NPN), 2N4403 (NPN), 2N5109 (NPN), BC549C (NPN), BC550C (NPN), 2N3906 (PNP), BC560C (PNP), 2N4401 (PNP), and BC560C (PNP) transistors.

11. Audio BJTs: designed for audio amplification applications, like audio amplifiers and preamplifiers. They feature low noise and low distortion characteristics. Examples are 2N3904 (NPN), 2N4401 (NPN), 2N2222A (NPN), KSC1845 (NPN), BC546B (NPN), BC547C (NPN), 2N3906 (PNP), 2N4403 (PNP), 2N2907A (PNP), KSA992 (PNP), BC556B (PNP), and BC560C (PNP) transistors.

12. Phototransistors: sensitive to light and commonly used in optoelectronic applications, such as light detectors and optical switches. Examples are TEPT5700 (NPN), PT334-6C (NPN), PT204-6B (NPN), BPW34 (NPN), TSHA4400 (PNP), TIL194 (PNP), and PT138 (PNP) transistors.

Temperature-compensated BJTs: provide stable performance over a wide temperature range and are used in applications where temperature variations are a concern.

Surface-mount (SMD) BJTs: available in compact surface-mount packages, making them suitable for modern PCB assembly techniques.