What is a transistor? A transistor is a semiconductor device that controls the flow of electric current in circuits. It is the fundamental building block of modern electronics. Whether you’re working on a simple LED circuit or a complex microcontroller project, understanding what a transistor is and how it works is essential.
Table of Contents
- What Does a Transistor Do?
- What Do Transistors Look Like?
- What is a Transistor Part Number?
- What Are Transistors Made Of?
- What is a Transistor of NPN Type?
- What is a Transistor of PNP Type?
- What Is a FET Transistor?
- What Is MOSFET?
- What Is a Transistor? : Tips
- Did You Know About What Is a Transistor?
- Frequently Asked Questions About: What Is a Transistor?
- What is a Transistor : Conclusion
What Does a Transistor Do?
Transistors are used to amplify signals and switch electrical power on and off. In essence, they function as electronic gates. A small input current or voltage at one terminal controls a larger current flowing between two other terminals.
Main Functions of a Transistor:
- Amplification: Boosts electrical signals (common in audio devices).
- Switching: Turns current on or off (used in digital logic circuits).
- Signal Modulation: Used in radios and communication systems.
The following image shows the schematic symbol of an NPN transistor. This is the symbol that represents an NPN type transistor in a circuit diagram. In the image, a small current flowing into the transistor base (B) flows through the transistor to the emitter (E). This causes a larger current to flow from the collector (C) to the emitter (E). Control of this large current by the small current is what allows a transistor to act as an amplifier or a switch.
What Do Transistors Look Like?
Transistors come in different shapes and sizes, depending on their use and power rating. Each different transistor housing or package type has its own name. For example, TO-92 is the package name of a small plastic transistor package. Some common package types include:
| Package Name | Description | Example Use |
|---|---|---|
| TO-92 | Small plastic package | Low-power circuits |
| TO-18 | Metal can package | High-frequency use |
| TO-220 | Larger with metal tab with single mounting hole | Power switching |
| TO-3 | Big metal body with two mounting holes | Higher power switching |
Inside the casing, transistors typically have three pins:
- Emitter (E)
- Base (B)
- Collector (C)
The arrangement of these pins varies by package type and transistor model. Different transistor models can be manufactured in the same package. The package is independent of the transistor model and is chosen by the designer of the transistor according to the transistor use and power rating.
What is a Transistor Part Number?
Each transistor has a specific part number or model number. In the photo below, two different transistors parts are shown. They have the part numbers 2N2222 (the two transistors on the left) and KSP2222 (also available as PN2222) – the two transistors on the right.
The 2N2222 is packaged in a metal can package known as a TO-18 package. The KSP2222 is an equivalent of the 2N2222 but is packaged in a plastic package known as a TO-92 package.
Each transistor type has its own datasheet available from the transistor manufacturer. The datasheet gives all the necessary information about the particular transistor part or model. For example, which transistor pin is emitter, base and collector. Power, voltage, current rating, and more information is found in the datasheet. The following image shows part of a transistor datasheet for a KSP2222 transistor that has the full part name KSP2222A.
An example of a semiconductor manufacturer that manufactures transistors is On Semiconductor (onsemi). This manufacturer has information and links to datasheets, for example see the On Semiconductor page for the KSP2222 transistor which contains a link to the datasheet of the KSP2222 transistor.
What Are Transistors Made Of?
Transistors are made from semiconductor materials, primarily silicon. These materials are doped with impurities to create regions with excess electrons (N-type) and regions with electron holes (P-type). P-N junction theory explains the physics of what transistors are made of and how they work internally at an atomic level.
Types of transistor construction:
- NPN Transistor: Made of a P-type layer sandwiched between two N-type layers.
- PNP Transistor: Made of an N-type layer between two P-type layers.
The following image shows the schematic symbols for transistors used in circuit diagrams. An NPN transistor has the emitter (e) arrow pointing out from the symbol. A PNP transistor has the emitter (e) pin pointing in to the symbol.
What is a Transistor of NPN Type?
What is a NPN Transistor? NPN transistors are one of the most commonly used bipolar junction transistors (BJTs). In an NPN transistor:
- A small current entering the base allows a larger current to flow from the collector to the emitter.
- The arrow on the symbol points outward, indicating the direction of conventional current flow.
NPN transistor examples: 2N2222, BC547, and PN2222.
NPN Transistor Examples
Transistors have three pins called emitter (e), base (b) and collector (c). The way that a 2N2222, BC107, BC108 and BC109 transistor symbol maps to the physical device or TO-18 package is shown in the diagram below. These are all NPN transistors.
Transistors with different part numbers do not necessarily share the same pin order. This is particularly relevant for transistors housed in the TO-92 package.
For example, when viewing the KSP2222 or PN2222 transistor with its flat side (part number facing you) and pins oriented downwards, the pinout from left to right is emitter (e), base (b), collector (c). However, other transistors may feature a different pin arrangement, such as base (b), collector (c), emitter (e) from left to right.
Always consult the device’s datasheet for accurate pin mapping. Pin mapping for the PN2222 and KSP2222 is shown in the following image.
What is a Transistor of PNP Type?
A PNP transistor is the complement of an NPN transistor. It is another type of Bipolar Junction Transistor (BJT), and it also has three terminals or pins: emitter, base, and collector. However, the internal construction and current flow are reversed compared to NPN transistors.
In a PNP transistor:
- The emitter emits holes (positive charge carriers).
- The base is thin and lightly doped with electrons.
- The collector collects holes.
For a PNP transistor to turn on, the base must be at a lower voltage than the emitter—typically by around 0.7V. When turned on, current flows from the emitter to the collector. In the below image, a small current flows from the collector to the base of the PNP transistor. This small current causes a larger current to flow from the emitter through to the collector of the transistor.
The following image shows part of the datasheet for a KSP2907A PNP transistor. This PNP transistor is a complement of the KSP2222A NPN transistor. Compliment means that it is the PNP equivalent of the NPN KSP2222 transistor, in terms of electrical ratings.
Although the above datasheet shows the name FAIRCHILD, it is actually from onsemi. Fairchild semiconductor was bought out by ON Semiconductor (onsemi) in 2016.
Find the KSP2907A page on the onsemi website where there is a link to the KSP2907A datasheet.
Key Differences Between NPN and PNP Transistors
- Current Flow: NPN allows current from collector to emitter; PNP allows current from emitter to collector.
- Base Biasing: NPN needs a positive base; PNP requires a negative base (relative to the emitter).
- Symbol Arrow: The arrow on the symbol for a PNP transistor points inward, indicating the direction of conventional current flow.
Common Uses for PNP Transistors
- High-side switches in power supply circuits
- Complementary pairs in push-pull amplifier stages
- Inverting logic or signal switching
PNP transistors are widely available in the same package types as NPNs, including TO-92 and TO-220, and they often have part numbers beginning with 2N2907, BC557, or similar.
Understanding both PNP and NPN types is essential when learning what transistors do and how to use them in circuits.
What Is a FET Transistor?
A FET (Field Effect Transistor) controls current using an electric field. Unlike BJTs that use current control, FETs are voltage-controlled devices.
FETs have three terminals:
- Source (S)
- Gate (G)
- Drain (D)
What Is MOSFET?
What is a transistor of the MOSFET type? MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor. It’s the most commonly used FET in digital electronics.
MOSFETs are highly efficient and can switch rapidly, making them ideal for power control, switching regulators, and processors.
Types of MOSFETs:
- N-channel MOSFET: Conducts when the gate voltage is positive.
- P-channel MOSFET: Conducts when the gate voltage is negative.
Part of a MOSFET datasheet is shown below. This datasheet shows the schematic symbol for a MOSFET. In this example, the MOSFET part number is 2N7000 and it is housed in a TO-92 package. The onsemi web page for the 2N7000 has a link to the 2N7000 datasheet.
The same MOSFET is also manufactured by Microchip. Microchip also has a web page for the 2N7000 MOSFET where a link to the Microchip 2N7000 datasheet is found.
What Is a Transistor? : Tips
- Always check the transistor’s datasheet to confirm the pinout (pin numbering) and maximum ratings.
- Use a multimeter to test transistor functionality before use.
- Keep BJTs and FETs separate in storage; they have different operating principles.
- Match the transistor’s power rating to your circuit requirements to prevent overheating.
Did You Know About What Is a Transistor?
- The first working transistor was created at Bell Labs in 1947 using germanium.
- Modern CPUs contain billions of transistors packed into a tiny chip.
- Transistors revolutionized electronics by replacing bulky vacuum tubes.
Frequently Asked Questions About: What Is a Transistor?
What is the difference between a BJT and a FET transistor?
A BJT (Bipolar Junction Transistor) is current-controlled, while a FET (Field Effect Transistor) is voltage-controlled. FETs are generally more power-efficient.
What are transistors used for in real life?
Transistors are used in amplifiers, switches, digital logic circuits, voltage regulators, and nearly every electronic device.
How can I tell what type of transistor I have?
Check the part number printed on the transistor body and look up the datasheet. Also, identify the package type (e.g., TO-92, TO-220).
Are all transistors the same inside?
No. While many share similar components, variations in doping, structure, and design result in different electrical characteristics and functions.
Can I replace one transistor with another?
Only if their electrical characteristics match. Always consult datasheets to compare specifications.
What is a Transistor : Conclusion
Understanding what a transistor is can open the door to mastering electronics. From simple switches to powerful amplifiers, transistors play a vital role in every electronic device. Whether you’re learning about what a FET transistor is or exploring what MOSFETs are used for, this foundational knowledge is key for anyone getting started in electronics.