What Is 2N3904

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Last updated: April 15, 2026

Quick Answer: The 2N3904 is a general-purpose NPN bipolar junction transistor introduced by Motorola in 1966, commonly used in amplification and switching circuits with a maximum collector current of 200 mA and a power dissipation of 625 mW.

Key Facts

Introduced by Motorola in 1966
NPN bipolar junction transistor (BJT)
Maximum collector current: 200 mA
Power dissipation: 625 mW at 25°C
Commonly used in amplifiers, switches, and digital logic circuits

Overview

The 2N3904 is one of the most widely used NPN bipolar junction transistors (BJTs) in electronics, known for its reliability and versatility in low-power applications. It operates efficiently in both amplification and switching roles, making it a staple in consumer electronics, hobbyist projects, and industrial circuits.

Originally developed by Motorola in the mid-1960s, the 2N3904 has become a standard due to its consistent performance and widespread availability. Its compact TO-92 package allows for easy integration into printed circuit boards, and its low cost supports mass production.

Transistor Type: The 2N3904 is an NPN bipolar junction transistor, meaning it uses electron flow from emitter to collector controlled by base current.
Collector Current: It can handle a continuous collector current of up to 200 mA, making it suitable for driving small loads like LEDs and relays.
Power Dissipation: The device has a maximum power dissipation of 625 mW at an ambient temperature of 25°C, limited by its TO-92 package.
Operating Temperature: It functions reliably within a temperature range of -55°C to +150°C, ideal for both commercial and industrial environments.
Gain (hFE): The DC current gain ranges from 100 to 300 depending on load conditions, allowing for effective signal amplification in audio and digital circuits.

How It Works

The 2N3904 operates by controlling the flow of current between the collector and emitter terminals using a small base current, enabling amplification or switching functions in circuits.

Base-Emitter Junction: When a small forward voltage (typically 0.7 V) is applied to the base-emitter junction, it allows current to flow from collector to emitter.
Collector-Base Junction: The collector-base junction remains reverse-biased during normal operation, ensuring the transistor operates in the active or saturation region.
Current Amplification: A base current of 1–10 µA can control a collector current up to 200 mA, providing high current gain for signal boosting.
Switching Mode: In digital circuits, the 2N3904 acts as a switch, turning fully on (saturation) or off (cutoff) based on input at the base terminal.
Breakdown Voltage: The collector-emitter breakdown voltage is rated at 40 V, limiting its use in high-voltage applications but ensuring safety in low-voltage designs.
Frequency Response: It has a transition frequency (f_T) of 300 MHz, allowing effective use in audio and intermediate-frequency amplification stages.

Comparison at a Glance

Below is a comparison of the 2N3904 with similar transistors commonly used in electronics.

Transistor	Type	Max I_C	V_CEO	Power	Package
2N3904	NPN	200 mA	40 V	625 mW	TO-92
2N2222A	NPN	800 mA	40 V	500 mW	TO-92/TO-39
BC547	NPN	100 mA	45 V	500 mW	TO-92
BC337	NPN	500 mA	45 V	625 mW	TO-92
2N3906	PNP	200 mA	40 V	625 mW	TO-92

The 2N3904 strikes a balance between current handling and efficiency, making it ideal for low-power signal applications. While the 2N2222A offers higher current capacity, the 2N3904 remains preferred for compact, low-noise designs. The BC547 is functionally similar but varies slightly in gain and availability by region. The 2N3906 is its PNP counterpart, used in complementary circuit designs. This comparison highlights why the 2N3904 remains a go-to choice for basic transistor needs.

Why It Matters

The 2N3904 plays a crucial role in modern electronics by enabling simple, cost-effective circuit designs for both analog and digital applications. Its widespread adoption has made it a reference point in education and prototyping.

Educational Use: It is commonly used in electronics courses to teach transistor fundamentals due to its predictable behavior and low cost.
Hobbyist Projects: Found in Arduino shields, LED drivers, and sensor interfaces, it enables makers to build functional circuits quickly.
Industrial Applications: Used in relay drivers, signal conditioning modules, and power control circuits across automation systems.
Legacy Support: Its decades-long production ensures compatibility with existing designs and replacement parts.
Design Flexibility: Engineers use it in oscillators, amplifiers, and logic gates due to its reliable switching and gain characteristics.
Global Availability: Manufactured by multiple companies including ON Semiconductor, Fairchild, and STMicroelectronics, ensuring supply stability.

Despite newer surface-mount alternatives, the 2N3904 endures due to its proven performance and ease of use. It remains a foundational component in electronics, symbolizing the transition from vacuum tubes to solid-state technology.