Semiconductor Component

Types of Transistors

Transistors are available in many different technologies and configurations. Although all transistors are used to control electrical current, they differ considerably in their construction, control method, switching characteristics, power capability and applications. The most common families include bipolar junction transistors (BJTs), field-effect transistors (FETs), MOSFETs and IGBTs.

1. Bipolar Junction Transistors (BJTs)

A Bipolar Junction Transistor (BJT) is a three-terminal semiconductor device that uses both electrons and holes for current conduction. The three terminals are the base, collector and emitter.

BJTs are commonly used for signal amplification, switching, current amplification and analog circuits.

There are two main types of BJTs:

  • NPN transistor
  • PNP transistor

NPN Transistors

NPN transistors are widely used in electronic circuits because they are particularly convenient for low-side switching and many common amplifier configurations.

PNP Transistors

PNP transistors perform the complementary function of NPN transistors and are frequently used in complementary amplifier stages and high-side switching circuits.

2. Field-Effect Transistors (FETs)

A Field-Effect Transistor (FET) controls current using an electric field rather than the base-current mechanism used by a BJT.

The main terminals of a FET are:

  • Gate
  • Drain
  • Source

FETs have a high input impedance because the gate requires very little steady-state current.

Important FET families include MOSFETs and JFETs.

3. MOSFETs

The Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is one of the most widely used transistor types in modern electronics.

MOSFETs are particularly important in power electronics, switching power supplies, motor controllers, digital electronics and load-switching applications.

The two major polarity types are:

  • N-channel MOSFET
  • P-channel MOSFET

N-Channel MOSFET

N-channel MOSFETs are extremely common in switching applications. They generally provide low conduction resistance and are widely used as low-side switches.

P-Channel MOSFET

P-channel MOSFETs are commonly used for high-side switching and applications where a relatively simple gate-drive arrangement is desirable.

4. JFETs

A Junction Field-Effect Transistor (JFET) controls current through a semiconductor channel using an electric field produced by its gate junction.

JFETs are available as:

  • N-channel JFETs
  • P-channel JFETs

They are commonly used in low-noise amplifiers, analog circuits, signal processing and voltage-controlled applications.

5. IGBTs

An Insulated-Gate Bipolar Transistor (IGBT) combines an insulated-gate control structure similar to a MOSFET with a bipolar conduction mechanism.

IGBTs are particularly useful for high-power switching applications where their voltage and current capabilities are advantageous.

Typical applications include:

  • Motor drives
  • Inverters
  • Industrial power supplies
  • Welding machines
  • Uninterruptible power supplies
  • Solar inverters
  • Electric vehicle power systems

6. Darlington Transistors

A Darlington transistor consists of two BJTs connected so that the first transistor drives the second.

This arrangement provides a very high effective current gain, allowing a relatively small base current to control a larger collector current.

Darlington transistors are used in:

  • Relay drivers
  • Motor drivers
  • Solenoid control
  • High-current switching
  • Power amplification

The main disadvantage is that a Darlington pair generally has a higher effective base-emitter voltage and can have slower switching behavior than a comparable single transistor.

7. Phototransistors

A phototransistor is a transistor that responds to light. Light falling on the semiconductor produces a current that is amplified by the transistor action.

Phototransistors are commonly used in:

  • Optical sensors
  • Object detection
  • Optical interrupters
  • Encoders
  • Light detection circuits
  • Optocouplers

8. Unijunction Transistors

A Unijunction Transistor (UJT) is a specialized semiconductor device that behaves differently from conventional BJTs and FETs.

UJTs have historically been used in triggering, timing and relaxation oscillator circuits.

Although they are much less common in modern designs, they can still be encountered in older electronic equipment.

9. Programmable Unijunction Transistors

A Programmable Unijunction Transistor (PUT) is a three-terminal semiconductor device whose triggering characteristics can be controlled using external components.

PUTs have been used in timing, triggering and oscillator circuits.

10. RF Transistors

RF transistors are designed to operate efficiently at radio frequencies. They are optimized for characteristics such as high-frequency gain, switching speed, low parasitic capacitance and appropriate power handling.

RF transistor technologies include specialized BJTs, FETs and other high-frequency semiconductor structures.

Typical applications include:

  • Radio transmitters
  • RF amplifiers
  • Wireless communication
  • Oscillators
  • Receivers
  • High-frequency signal processing

11. Power Transistors

Power transistors are designed to handle significantly higher voltage, current and power than typical small-signal transistors.

Power transistors can be based on BJT, MOSFET, IGBT and other semiconductor technologies.

They are commonly found in:

  • Audio power amplifiers
  • Switch-mode power supplies
  • DC-DC converters
  • Motor controllers
  • Inverters
  • Automotive electronics

12. Small-Signal Transistors

Small-signal transistors are designed primarily for low-power amplification and switching.

They are commonly used in:

  • Audio preamplifiers
  • Sensor circuits
  • Oscillators
  • Signal amplifiers
  • Logic interfaces
  • Voltage amplifiers

13. High-Frequency Transistors

High-frequency transistors are optimized to operate at frequencies where ordinary general-purpose transistors may no longer provide adequate performance.

Important characteristics include transition frequency, parasitic capacitance, switching speed and gain at the intended operating frequency.

14. Darlington vs Conventional BJT

Characteristic Single BJT Darlington
Number of transistor junctions One transistor Two BJTs
Current gain Moderate to high Very high
Base-emitter voltage Typically around one junction drop Higher because two junctions are involved
Switching speed Generally faster Generally slower
Typical application General amplification and switching High-gain current switching

15. BJT vs FET vs IGBT

Feature BJT MOSFET IGBT
Control Base current Gate voltage Gate voltage
Input impedance Lower Very high Very high
Switching speed Moderate High Moderate to high
Typical use Amplification and switching Fast switching and power conversion High-power switching

How to Choose the Right Transistor

The correct transistor depends on the requirements of the circuit. Important parameters include:

  • Maximum voltage
  • Maximum current
  • Power dissipation
  • Switching speed
  • Gain
  • Frequency response
  • Thermal characteristics
  • Package type
  • Pin configuration
  • Safe Operating Area

For switching applications, MOSFETs and IGBTs are often preferred depending on the voltage, current and switching-frequency requirements. BJTs remain important for many analog and amplification applications.

Key Points

  • BJTs include NPN and PNP types.
  • FETs include MOSFETs and JFETs.
  • MOSFETs are widely used for fast and efficient switching.
  • IGBTs are commonly used in high-power switching applications.
  • Darlington transistors provide very high current gain.
  • Phototransistors detect and amplify light-generated current.
  • Power transistors are designed for higher electrical and thermal stress.
  • RF transistors are optimized for high-frequency operation.
  • The correct transistor should always be selected using its datasheet.

Continue Learning About Transistors

After understanding the different transistor families, the next step is to learn their circuit symbols and how each type is represented in electronic schematics.

Next → Transistor Symbols