Transistor Types, Applications, and Symbols Explained

Transistors are essential components in modern electronics, used for switching, amplifying, and signal processing. From BJTs to FETs and UJTs, each type of transistor has distinct characteristics and applications. This guide explores the various types of transistors, discussing their symbols, key features, and practical uses to help you select the right component for your electronic needs.

What is a Transistor?

Introduction:

• Transistors are manufactured as discrete components and incorporated into Integrated Circuits (ICs). ICs can contain thousands of transistors on a tiny piece of silicon.
• The transistor is a solid-state equivalent of a triode valve, which was used in radios and early computers.
• It’s made of three layers of semiconductor materials, commonly silicon or germanium.
• It acts as both an insulator and a conductor.

• They are three-terminal devices.

• They are active components, unlike passive components like resistors, capacitors, inductors, and diodes.

• Transistors can be used as a switch or as an amplifier. As a switch, it operates in either the cutoff or saturation region. As an amplifier, small changes in the base current cause the collector current to mimic the input base signal with a higher amplitude.

• Transistors use the same identification codes as diodes, but the American system uses the letters “2N” followed by a serial number. The conventional system uses two letters followed by a serial number. The first letter indicates the semiconductor material (A=germanium, B=silicon), and the second letter indicates its use (C=Audio frequency amplifier, F = radio frequency amplifier, S = switching transistor).

  • Example: BC108 is a Silicon Audio Frequency Amplifier.

Transistor Types with Their Applications

Here are some common transistor applications:

• Amplification: To amplify current or voltage.
• Switching: As a switching device in analog or digital circuits.
• Regulation: As a regulator for voltage, current, or power.
• Building Blocks: As building blocks of ICs.
• Buffering: For buffering or impedance matching between mismatched circuits.

Let’s explore different types of transistors and their applications.

BJT Transistor

• BJT stands for Bipolar Junction Transistor.
• Its operation depends on the flow of both majority and minority carriers.
• It consists of two p-n junctions within the same crystal.
• A thin slice of lightly doped p-type or n-type semiconductor (the base - B) is sandwiched between two thicker, heavily doped materials of the opposite type (the collector - C and emitter - E).
• There are two possible configurations: NPN and PNP.
• In NPN transistors, current flows from “C” to “E” terminal when base is driven with sufficient bias voltage . In PNP transistors, current flows from “E” to “C” terminal when base is driven with sufficient bias voltage.

Applications:

BJT transistors are used as oscillators, switches, and amplifiers. They are also used in logic circuits. You can find them in mobile phones, TVs, radio transmitters, industrial controls, etc.

FET Transistor

• FET stands for Field Effect Transistor.
• The current flow is due to majority carriers only (either electrons or holes).
• Unlike BJTs, which are current-controlled amplifying devices, FETs are voltage-controlled devices. In FETs, the output current is controlled by an input voltage.
• In FETs, a narrow channel of doped semiconductor connects two metal contacts called the drain (D) and source (S). The voltage applied to the third terminal, the gate (G), located between “S” and “D”, determines the current that flows from “S” to “D”.

Applications:

FET transistors have very high input impedance. They are widely used as input amplifiers in oscilloscopes and voltmeters, and other test and measurement equipment. They are also used as voltage-variable resistors in Op-Amps, tone controls in FM and TV receivers, and in digital switching logic circuits. Other applications include low-noise amplifiers, buffer amplifiers, cascode amplifiers, analog switches, choppers, multiplexers, current limiters, and phase-shift oscillators.

MOSFET Transistor

• In MOSFETs, both positive and negative voltages can be applied to the gate terminal. This is not possible in JUGFETs (Junction Gate FETs). When a positive gate voltage is applied to a JUGFET, it becomes forward-biased, causing the input resistance to drop and a large current to flow, potentially damaging the device.
• MOSFETs operate in two modes: depletion and enhancement.

Applications:

MOSFETs are used in switching and amplifier circuits, as inverters, and in digital circuits. They’re also used as high-frequency amplifiers.

UJT Transistor

• The figure below depicts the structure and symbol of a UJT.
• It consists of an n-type silicon bar with connections at both ends (“B1” and “B2”).
• A PN junction is constructed along the bar between the two bases, between the P-type Emitter and N-type Bar. This lead is referred to as the “Emitter Lead-E”.
• UJT is short for Unijunction Transistor.
• It is a 3-terminal switching device made of semiconductor materials.

Applications:

UJTs are used as triggering devices for SCR/Triac devices. UJT transistor applications include sawtooth generators, oscillators, phase control circuits, and timing circuits.

Conclusion

Knowing the different types of transistors and their applications is vital for designing effective circuits. Each transistor type, whether a BJT or FET, serves a specific function, and understanding these differences allows you to optimize performance in electronic devices. This knowledge equips you to make the best choices in your electronic designs.