Bipolar Junction Transistor (BJT): Structure, Operation, and Applications

Bipolar junction transistor (BJT) is a semiconductor device used to amplify electrical signals and control current in electronic circuits. Thanks to its design, a small control current can regulate a much larger current flowing through a load. BJTs are commonly found in amplifiers, switches, signal generators, and various other electronic devices. In this article, we'll explore the structure of a bipolar transistor, how it works, and how it differs from a MOSFET.

What Is a Bipolar Junction Transistor and How Is It Built?

A bipolar junction transistor (BJT) consists of three semiconductor regions with alternating conductivity types. Its defining feature is that both electrons and holes participate in charge transport, which is why it's called "bipolar."

The transistor has three terminals:

• Emitter (E) - injects charge carriers into the base region.
• Base (B) - a thin control region that regulates the transistor's operation.
• Collector (C) - collects charge carriers that have traversed the base.

When connected correctly, a small base current allows you to control a much larger current flowing between collector and emitter.

NPN vs. PNP Transistors

Bipolar transistors come in two main types, distinguished by the arrangement of their semiconductor regions and the polarity of their control voltages:

• NPN - Features an N-P-N structure. In typical circuits, it turns on when the base voltage is about 0.6-0.7 V higher than the emitter (for silicon transistors).
• PNP - Built with a P-N-P structure. It turns on when the base voltage is about 0.6-0.7 V lower than the emitter.

Both types perform the same fundamental functions but require different connection polarities. NPN transistors are often used to switch loads on the ground side, while PNP types are preferred for switching on the positive supply side.

How a Bipolar Junction Transistor Works

The operating principle of a bipolar junction transistor is based on controlling the collector current with a small base current. When the required forward voltage is applied between the base and emitter, the transistor starts conducting.

Let's look at the NPN transistor as an example:

• With no base current, the transistor is nearly off, so the collector-emitter current is minimal.
• When a current is applied to the base, the transistor conducts, allowing a much larger current to flow through the collector.
• Increasing the base current further increases the collector current, up to limits set by the circuit and load.

In the active mode, collector current is roughly proportional to the base current, allowing the transistor to amplify weak electrical signals.

Current Gain Coefficient

The ability of a transistor to amplify current is described by the beta (β) coefficient, also denoted as h_FE:

β = I_C / I_B

Where:

• I_C - collector current
• I_B - base current

For example, if the gain is 100 and the base current is 0.1 mA, then the collector current in active mode will be about 10 mA.

Keep in mind that β is not a constant value - it depends on the transistor type, temperature, and operating conditions. In saturation mode, this relationship no longer precisely defines the collector current.

Operating Modes of a Bipolar Junction Transistor

The bipolar transistor can work in several modes, depending on the voltages at its terminals and the amount of base current:

• Cut-off mode: The transistor is nearly off, with very little collector current - used to turn off loads.
• Active mode: Collector current is controlled by the base current - suitable for amplifying analog signals.
• Saturation mode: The transistor is fully on, acting as an electronic switch, and the collector current is mostly limited by the external circuit. The collector-emitter voltage becomes small.

For instance, when controlling an LED with an NPN transistor, providing sufficient base current switches the transistor into saturation and turns the LED on. When the base current stops, the transistor enters cut-off and the LED goes off.

A resistor is typically used to limit the base current, and the LED current should be limited by another resistor or a suitable power supply circuit.

How Is a Bipolar Transistor Different from a MOSFET?

The main difference between a bipolar junction transistor and a MOSFET lies in their control method. A BJT is controlled by base current, while a MOSFET is controlled by the gate-to-source voltage.

For a BJT, a continuous base current is needed to keep it conducting. In a MOSFET, the gate is insulated from the channel, so a steady control current is virtually absent - though charging and discharging the gate capacitance is necessary during switching.

For a detailed explanation of field-effect transistor design and features, see the article: How Field-Effect Transistors (MOSFETs) Work: Simple Explanation, Structure, and Applications.

Key Differences: BJT vs. MOSFET

Bipolar junction transistors are often chosen for amplifying analog signals and simple circuits controlling small loads. MOSFETs are typically preferred for efficient switching of large currents, especially in switching power supplies. However, the final choice depends on the specific component characteristics, voltage, switching frequency, and circuit requirements.

Where Are Bipolar Junction Transistors Used?

Bipolar transistors are used in analog and digital electronics wherever signal amplification or electrical load switching is needed. Key application areas include:

• Audio and signal amplifiers: Amplify small input signals, such as from microphones or sensors.
• Electronic switches: Used to turn LEDs, relays, and other loads on and off via control signals.
• Oscillator circuits: Generate periodic electrical signals in generator circuits.
• Signal processing circuits: Used in current mirrors, differential pairs, and other analog circuit blocks.

Despite the widespread adoption of MOSFETs, bipolar junction transistors remain in demand due to their availability, variety of models, and good amplification characteristics.

Conclusion

A bipolar junction transistor allows you to control a large current with a small base current. Its operation depends on the interaction of three regions - emitter, base, and collector. Depending on the mode, it can amplify electrical signals or serve as an electronic switch.

For amplifying weak analog signals and simple control circuits, a BJT is often a practical choice. If you need to switch large currents efficiently with minimal losses, consider a MOSFET, comparing the properties of specific components.