What Is a Transistor? Types and How It Works in Circuits

The transistor is arguably the most important invention of the 20th century. Without it, there would be no smartphones, no computers, no internet, no modern medicine, no space exploration. A modern processor chip contains billions of transistors — each one too small to see with the naked eye — switching at billions of times per second.

Yet at its core, a transistor is a simple device: a semiconductor component that either amplifies electrical signals or acts as an electronic switch. Understanding transistors gives you insight into how all of modern electronics works — and how to use them in your own circuits.

What Is a Transistor?

A transistor is a three-terminal semiconductor device that can amplify or switch electronic signals and electrical power. It consists of three layers of semiconductor material (usually silicon) with three terminals: the Base (or Gate), the Collector (or Drain), and the Emitter (or Source).

The critical property of a transistor is that a small signal at the Base terminal controls a much larger current flowing between the Collector and Emitter. This property — using a small input to control a large output — is the basis of both amplification and switching.

The transistor was invented in December 1947 at Bell Laboratories by William Shockley, John Bardeen, and Walter Brattain — an achievement that earned them the Nobel Prize in Physics in 1956.

Types of Transistors

There are two main families of transistors: Bipolar Junction Transistors (BJTs) and Field Effect Transistors (FETs). Each has distinct operating principles, characteristics, and ideal applications.

Bipolar Junction Transistors (BJTs)

BJTs are current-controlled devices: a small base current controls a much larger collector-emitter current. They are available in two complementary configurations:

NPN Transistor

The most common BJT type. In an NPN transistor, the collector is connected to the positive supply, the emitter connects to ground (via a load), and the base receives the control signal. When sufficient base current flows (above the threshold voltage, typically 0.6–0.7V for silicon), the transistor turns on and current flows from collector to emitter.

The relationship: IC = hFE x IB, where IC is collector current, IB is base current, and hFE (or beta) is the current gain — typically 100–300 for common BJTs like the 2N2222 and BC547.

In practice: if hFE = 200 and you provide 0.1mA base current, the transistor allows up to 20mA collector current. This is how Arduino controls loads beyond its 40mA pin limit — drive the transistor base, and the transistor drives the load.

PNP Transistor

The PNP transistor is the complement of the NPN. The current flows from emitter (connected to positive supply) to collector (connected to load/ground). The base signal pulls the transistor on when it goes LOW — opposite logic to NPN. PNP transistors are often used as high-side switches (controlling the positive supply to a load).

Common BJT Transistors

• BC547/BC548: Small signal NPN, 100mA collector current, general purpose. Very common in low-power switching circuits.

• 2N2222/PN2222: NPN, 600 mA, 40 V. Higher current than BC547, used for moderate loads like relays and small motors.

• TIP120/TIP122 (Darlington): NPN Darlington pair with very high gain (hFE ~1000). Used to drive high-current loads from logic-level signals.

• BD139/BD140: NPN/PNP complementary pair, 1.5A, used in audio amplifier output stages.

Field-Effect Transistors (FETs)

FETs are voltage-controlled devices: the gate voltage controls current flow between drain and source, drawing essentially no current from the gate itself. This makes FETs ideal for battery-powered circuits and digital logic, where minimising control power consumption matters.

JFET (Junction Field Effect Transistor)

JFETs use a reverse-biased PN junction to form the gate. They are voltage-controlled with high input impedance and low noise — useful in analogue front-end circuits and oscillators. Less common in modern designs, largely replaced by MOSFETs.

MOSFET (Metal Oxide Semiconductor FET)

The MOSFET is the dominant transistor in modern electronics — it is the building block of virtually all digital logic chips, CPUs, and power electronic circuits. N-channel enhancement mode MOSFETs are the most common type:

When gate voltage exceeds the threshold voltage (typically 2–4V for logic-level MOSFETs, higher for power MOSFETs), a conducting channel forms between drain and source, allowing large currents to flow with extremely low on-resistance. The gate draws no DC current — it only needs to charge the gate capacitance.

Power MOSFETs (like the IRFZ44N, IRF540, or the common IRLZ34N for logic-level gate drive) can switch tens or hundreds of amperes with milliohm on-resistance, making them essential in motor controllers, power supplies, and EV inverters.

Logic Level vs Standard MOSFETs

Standard power MOSFETs require 10V on the gate for full enhancement. Logic-level MOSFETs are designed to turn fully on at 3.3V or 5V gate voltage — essential for Arduino and microcontroller applications where the gate is driven directly from a logic output. Always check the Vgs(th) (gate threshold voltage) and the on-resistance at your logic-level gate voltage before using a MOSFET.

Transistors as Switches vs. Amplifiers

As a Switch

In digital circuits, transistors operate in two states: fully off (cutoff) and fully on (saturation). As a switch, the transistor either blocks all current (like an open circuit) or passes maximum current with minimum voltage drop (like a closed switch). This binary operation is the basis of all digital logic — billions of transistors switching on and off billions of times per second create the Boolean logic operations that power computers.

As an Amplifier

In analogue circuits, transistors operate in the active (linear) region between cutoff and saturation. A small varying input signal (e.g. from a microphone or sensor) applied to the base modulates the larger collector current proportionally, producing an amplified version of the input at the output. This is the principle behind audio amplifiers, RF amplifiers, and operational amplifiers.

Using a Transistor to Drive a Load with Arduino

A common and practical transistor application for makers: using a NPN BJT (like BC547 or 2N2222) to switch a load that draws more current than an Arduino pin can provide.

1. Connect the load (LED, relay, small motor) between the positive supply and the transistor collector.

2. Connect the transistor emitter to GND.

3. Connect a resistor (typically 1k Ohm for 5V logic, 10k for 3.3V) between the Arduino output pin and the transistor base.

4. Add a flyback diode across inductive loads (relays, motors) to suppress voltage spikes when the load turns off.

5. When the Arduino pin goes HIGH, base current flows, the transistor saturates, and current flows through the load.

This circuit allows a 5V Arduino pin providing 5mA to control a relay drawing 60–100mA — a 10-20x current multiplication that is fundamental to almost all Arduino motor and relay projects.

Common Transistor Packages

• TO-92: Small plastic package for signal transistors (BC547, 2N2222). Three leads: EBC or ECB depending on device.

• TO-220: Medium power plastic package (TIP120, IRFZ44N). Large tab for heatsink attachment.

• SOT-23: Small surface-mount package for signal transistors.

• TO-247: Large power package for high-current MOSFETs and IGBTs.

Conclusion

The transistor is the foundational building block of all modern electronics. In its simplest form — a switch controlled by a small electrical signal — it enables microcontrollers to drive real-world loads, forms the logic gates in every digital chip, and amplifies signals from sensors and microphones.

For a maker working with Arduino and microcontrollers, the BJT as a switch and the logic-level MOSFET as a high-current switch are the two transistor circuits you will use most. Master these, and you will have the tools to interface your microcontroller with almost any real-world device.

Explore transistor-based project circuits on the Circuit Diary Blog, and visit our Home page for more electronics tutorials and project guides.