Transistors are among the most essential components in modern electronics. Whether you’re working on a simple hobby circuit or designing complex systems, understanding transistors is key.
This guide covers what transistors are, how they work, and introduces common types like the 2N3904, 2N2222, and BC547, as well as powerful devices like the 2N3055. You’ll also learn about NPN and PNP transistors, the Darlington transistor, and the bipolar junction transistor (BJT) family.
Table of Contents
- What Is a Transistor?
- Types of Transistors You Should Know
- Symbols and Standards for Transistors
- How Do Bipolar Junction Transistors Work?
- Common Transistor Packages
- Tips for Working with Transistors
- Did You Know About Transistors?
- Frequently Asked Questions About Transistors
- Conclusion
What Is a Transistor?
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It has three terminals: the collector, base, and emitter. Small currents at the base terminal control larger currents flowing from collector to emitter. This switching and amplification ability makes transistors fundamental to analog and digital circuits.
There are two main types:
- Bipolar Junction Transistor (BJT): Includes NPN and PNP transistors.
- Field-Effect Transistor (FET): Operates via voltage control, not current. Includes the MOSFET type.
The following image shows part of the datasheet for a 2N3904 NPN transistor from onsemi — a semiconductor manufacturing company. Datasheets give all the technical information about components. In this example, the top of the first page of the datasheet shows the NPN transistor symbol. In addition it shows the physical packages that this type of transistor is available in, as well as the pin numbering.
Types of Transistors You Should Know
These are the two polarities of BJTs:
- NPN transistor: Allows current to flow when a positive voltage is applied to the base.
- PNP transistor: Conducts when the base is at a lower potential than the emitter.
NPNs are generally more common due to easier circuit design with positive voltage supplies.
Common NPN Transistors
Here are several widely used NPN devices:
| Transistor | Max Collector Current | Max Voltage | Common Uses |
|---|---|---|---|
| 2N3904 | 200 mA | 40V | Low-power general-purpose circuits |
| 2N2222 | 800 mA | 40V | Switching, amplifiers |
| BC547 | 100 mA | 45V | Audio and signal applications |
| 2N3055 | 15 A | 60V | Power amplification, motor drivers |
Each of these transistors can be used in either switching or amplification configurations depending on the biasing setup.
Darlington Transistor and Darlington Pair
A Darlington transistor is a configuration of two BJTs that act as a single high-gain transistor. This arrangement, known as a Darlington pair, offers very high current gain but comes with a higher base-emitter voltage drop (around 1.2V).
Applications include:
- Power regulators
- Audio amplifiers
- Relay drivers
Symbols and Standards for Transistors
Understanding transistor symbols is essential for reading and designing circuit schematics. While the function remains the same across regions, schematic symbols can vary slightly between standards. The most common systems are the American (ANSI) and European (IEC) standards.
NPN and PNP Transistor Symbols
Both NPN and PNP transistors are types of bipolar junction transistors (BJTs). Their symbols differ in the direction of the emitter arrow:
- NPN Transistor
- ANSI Symbol: The arrow on the emitter points outward.
- IEC Symbol: Often drawn similarly, but with different graphical style (more block-like).
- PNP Transistor
- ANSI Symbol: The arrow on the emitter points inward.
- IEC Symbol: Again, similar form, but uses the IEC block conventions.
Tip: The arrow always indicates the direction of conventional current flow in the emitter leg.
In the following image, the symbol for a NPN transistor is shown on the left and PNP on the right.
Common NPN Transistors: 2N3904, 2N2222, BC547, and 2N3055
All of these are NPN BJTs, and share the same core symbol structure:
- Three terminals: Collector (C), Base (B), and Emitter (E)
- Emitter arrow points outward
You can label the symbol accordingly for each:
- 2N3904 – low current, general-purpose
- 2N2222 – medium current
- BC547 – common in European circuits
- 2N3055 – power transistor (symbol may be larger, and include heatsink indicators in some diagrams)
PNP Transistor Symbols
Although not as common in beginner circuits, PNP transistors are used where negative control voltages are needed. Symbol features:
- Emitter arrow points inward
- Same terminal labels: C, B, and E
PNP variants of the transistors mentioned (e.g., 2N2907, the PNP counterpart to 2N2222) would use this symbol style. Part of the 2N2907 datasheet is shown in the following image.
Darlington Transistor and Darlington Pair Symbols
A Darlington transistor symbol represents two BJTs connected together:
- ANSI Standard: Often shown as two NPN or two PNP transistors joined together, sharing a single base and emitter.
- Simplified ANSI Version: A single transistor symbol with a double-circle or additional arc to indicate Darlington configuration.
- IEC Standard: May use a simplified block with internal transistor indications.
Common labeling includes:
- Input (Base)
- Output (Collector)
- Emitter
This symbol is used for Darlington pairs built from discrete components, or packaged devices such as TIP120 (NPN Darlington). The following image shows a representation of an NPN Darlington transistor.
Circle in Transistor Symbols
- The circle around a transistor symbol is not an official part of either the IEC or ANSI (American) standard.
- Instead, it has traditionally been used as a drawing convention to indicate that the transistor is a discrete component (i.e. not part of an integrated circuit).
Standards Breakdown
| Feature | ANSI (American) | IEC (European/International) |
|---|---|---|
| Circle around symbol | Optional, often omitted | Rarely used, mostly omitted |
| Symbol style | More detailed, with arrow and C/B/E lines | More stylized/block-based |
| Arrow for current direction | Always present on emitter | Always present on emitter |
When Is the Circle Used?
- Often seen in older American schematics, especially textbooks and educational materials.
- Used to distinguish discrete transistors from those inside ICs or logic gates.
- In modern professional schematics, both ANSI and IEC standards typically omit the circle.
How Do Bipolar Junction Transistors Work?
BJTs operate by controlling the current between the collector and emitter with a smaller current at the base. Here’s a simplified explanation:
- In an NPN transistor: When a small current flows into the base, it allows a much larger current to flow from the collector to emitter.
- In a PNP transistor: A small current flowing out of the base enables current flow from emitter to collector.
The amplification factor is called beta (β), typically ranging from 20 to 1000 depending on the transistor type.
An NPN transistor represented in a block diagram as physical layers of N-type and P-type materials is shown in the following image.
Common Transistor Packages
Transistors come in a variety of packages, which affect how they are mounted, cooled, and used in a circuit. The package type influences a transistor’s power handling, size, and thermal performance.
Whether you’re working with a 2N3904, 2N2222, or a power transistor like the 2N3055, it’s important to recognize the most common transistor packages. Refer to each transistor’s datasheet for more information on which package or packages it is available in.
Through-Hole Transistor Packages
These are used in breadboards and perf boards, especially in educational or prototyping environments:
- TO-92
- Used for: Small-signal transistors like 2N3904, 2N2222, and BC547
- Features: Compact, plastic-encapsulated; usually three in-line leads
- Notes: Ideal for low-power, general-purpose applications
- TO-18
- Used for: Some metal-can versions of the 2N2222
- Features: Metal casing; better thermal dissipation than TO-92
- Notes: Common in older or industrial designs
- TO-220
- Used for: Medium-power transistors and voltage regulators
- Features: Plastic body with metal tab for heat sinking
- Notes: Mountable to heatsinks with thermal compound
- TO-3
- Used for: High-power transistors like the 2N3055
- Features: Large metal case with two pins and a metal tab base
- Notes: Designed for heavy heat dissipation; often bolted to heatsinks
A BD135 NPN transistor is shown in the image below. This transistor is housed in a TO-225 package, which is smaller than a TO-220 package and has no tab.
Surface-Mount Transistor Packages
Surface-mount technology (SMT) is common in modern electronics manufacturing:
- SOT-23
- Used for: Small-signal transistors in compact circuits
- Features: Tiny 3-pin package; soldered directly to PCB
- Notes: Equivalent to TO-92 in function but takes up less space
- DPAK / D2PAK
- Used for: Power transistors in SMT form
- Features: Flat body with large tab for surface heatsinking
- Notes: Common in power control and automotive applications
Summary of Common Transistor Packages
| Package | Type | Common Transistors | Power Level | Mounting Style |
|---|---|---|---|---|
| TO-92 | Through-hole | 2N3904, BC547 | Low | Vertical |
| TO-18 | Through-hole | 2N2222 (metal can) | Low-Medium | Vertical |
| TO-220 | Through-hole | TIP series | Medium | Heatsinkable |
| TO-3 | Through-hole | 2N3055 | High | Heatsinkable |
| SOT-23 | SMT | Various | Low | Surface mount |
| DPAK | SMT | Power transistors | Medium | Surface mount |
Tip: Always match the transistor’s thermal and current ratings to the package. For high-power use (like with the 2N3055), proper heatsinking is critical.
This knowledge helps when sourcing parts, reading datasheets, and designing reliable circuits.
Tips for Working with Transistors
- Always check the datasheet for pin configuration and ratings.
- Use a base resistor to limit base current and prevent damage.
- For switching applications, ensure the transistor is fully saturated to reduce heat.
- When using a 2N3055, ensure proper heat sinking due to high power dissipation.
- Use Darlington pairs for high gain needs, but note the increased voltage drop.
Did You Know About Transistors?
- The first transistor was invented in 1947 at Bell Labs by John Bardeen, Walter Brattain, and William Shockley.
- Before transistors, vacuum tubes were used—bulky, fragile, and power-hungry.
- 2N2222 is one of the oldest still-in-use transistors and is beloved in education for its robustness.
- The Darlington transistor was patented in 1953 by Sidney Darlington to achieve higher current gain.
- Over 10 billion transistors can fit on a modern microprocessor chip today.
Frequently Asked Questions About Transistors
What is the difference between an NPN and a PNP transistor?
An NPN transistor turns on when the base is positive relative to the emitter. A PNP transistor activates when the base is more negative than the emitter. NPNs are used more frequently in modern circuits.
Can I use the 2N3904 and 2N2222 interchangeably?
Not always. Although both are NPN transistors, 2N2222 can handle significantly more current (up to 800 mA) compared to the 2N3904 (200 mA). Use according to your current requirements.
What does a Darlington pair do?
A Darlington pair combines two transistors to amplify current more effectively. It’s ideal when very high gain is needed, but it does result in a higher voltage drop between the base and emitter.
How do I test a transistor?
Use a digital multimeter in diode test mode:
- Measure from base to emitter and base to collector.
- For NPN: both should show a diode-like drop (~0.6–0.7V).
- For PNP: reverse the leads.
See our transistor test guide for more details and diagrams.
Why is the 2N3055 used in power applications?
The 2N3055 can handle high currents (up to 15 A) and voltages (up to 60 V), making it perfect for power amplifiers, motor drivers, and voltage regulators—provided it’s properly cooled.
Conclusion
Transistors are at the heart of nearly every electronic device. From small-signal NPN transistors like the BC547 to power giants like the 2N3055, their versatility is unmatched.
Understanding how a bipolar junction transistor works, knowing the differences between NPN and PNP, and learning about advanced configurations like the Darlington transistor prepares you to tackle real-world electronic projects with confidence. Keep this guide as a foundation as you explore the many uses of transistors in your own circuits.
You may also be interested in the history of the transistor.