The 2N3904 is one of the most common general-purpose NPN bipolar junction transistors (BJTs). Housed in the TO-92 plastic package, it is designed as a small-signal amplifier and switch, with a useful range to about 100 mA as a switch and 100 MHz as an amplifier. Its PNP complement is the 2N3906.
This page gives the 2N3904 pinout, the key specifications and absolute maximum ratings from the onsemi 2N3904 datasheet, how to use it as a switch and an amplifier, common equivalents, and the package details.
2N3904 Pinout
The 2N3904 in a TO-92 package has three leads. Hold the transistor with the flat face toward you and the leads pointing down; from left to right the pins are:
| Pin | Name | Function |
|---|---|---|
| 1 | Emitter (E) | Current exits here (NPN) |
| 2 | Base (B) | Control terminal |
| 3 | Collector (C) | Current enters here (NPN) |
This E-B-C order is standard for the JEDEC-registered 2N3904 across manufacturers. Do not assume every TO-92 transistor shares it — many European parts (for example the BC547) use a reversed C-B-E layout, and the metal-can 2N2222 differs again. Confirm the flat-face orientation before wiring.
2N3904 Features and Key Specifications
- NPN silicon bipolar junction transistor (general-purpose)
- Collector-emitter voltage (VCEO): 40 V
- Continuous collector current (IC): 200 mA (absolute maximum)
- DC current gain (hFE): 100–300 at IC = 10 mA
- Transition frequency (fT): 300 MHz
- Total power dissipation (PD): 625 mW (TO-92, at 25 °C)
- Package: TO-92 (through-hole); MMBT3904 (SOT-23) and PZT3904 (SOT-223) for surface mount
- Complementary PNP type: 2N3906
2N3904 Absolute Maximum Ratings
These are limits, not operating targets — exceeding any of them risks damaging the device. Design with margin below them. (Source: onsemi 2N3904 datasheet, Rev. 3.)
| Parameter | Symbol | Value |
|---|---|---|
| Collector-emitter voltage | VCEO | 40 V |
| Collector-base voltage | VCBO | 60 V |
| Emitter-base voltage | VEBO | 6.0 V |
| Collector current (continuous) | IC | 200 mA |
| Total device dissipation (TA = 25 °C) | PD | 625 mW (derate 5.0 mW/°C above 25 °C) |
| Junction / storage temperature | TJ, Tstg | −55 to +150 °C |
The VEBO rating of 6 V is easy to overlook: reverse voltage across the base-emitter junction beyond 6 V can degrade or destroy the part. If the base may see a negative or transient overvoltage relative to the emitter, add a clamp diode across the junction.
2N3904 Electrical Characteristics
Key values at TA = 25 °C (onsemi datasheet, Rev. 3):
| Parameter | Symbol | Conditions | Value |
|---|---|---|---|
| DC current gain | hFE | IC = 0.1 mA | 40 (min) |
| IC = 1.0 mA | 70 (min) | ||
| IC = 10 mA | 100–300 | ||
| IC = 50 mA | 60 (min) | ||
| IC = 100 mA | 30 (min) | ||
| Collector-emitter saturation | VCE(sat) | IC = 10 mA, IB = 1 mA | 0.2 V (max) |
| IC = 50 mA, IB = 5 mA | 0.3 V (max) | ||
| Base-emitter on voltage | VBE(on) | IC = 10 mA | 0.65–0.85 V |
| Transition frequency | fT | IC = 10 mA, VCE = 20 V | 300 MHz (min) |
| Output capacitance | Cobo | VCB = 5 V | 4.0 pF (max) |
| Delay / rise time | td / tr | IC = 10 mA | 35 / 35 ns (max) |
| Storage / fall time | ts / tf | IC = 10 mA | 200 / 50 ns (max) |
Two points worth flagging. First, hFE falls sharply at high current — it drops to a minimum of 30 at 100 mA, so a switching design near the current limit needs more base drive than the headline “300” suggests. Second, the current onsemi revision lists fT = 300 MHz, while older onsemi and some STMicroelectronics datasheets quote 250–270 MHz; the difference reflects process and test conditions, not a different part.
How to Use the 2N3904
As a switch
The most common use is a low-side switch: the load sits between the supply and the collector, the emitter goes to ground, and a logic signal drives the base through a resistor. Size the base resistor for saturation: RB ≈ (Vdrive − 0.7) / IB, with IB ≈ IC / 10. For a 5 V signal switching an 80 mA load, IB ≈ 8 mA, so RB ≈ (5 − 0.7) / 0.008 ≈ 540 Ω — pick 470–560 Ω.
Although the absolute maximum IC is 200 mA, onsemi specifies the useful switching range to about 100 mA. Keep continuous loads below that for reliable saturation and thermal margin; for larger loads, step up to a 2N2222 or a Darlington such as the TIP120. The full low-side / high-side drive method is covered in our NPN vs PNP transistor guide.
As an amplifier
In the active region the 2N3904 works as a small-signal amplifier — a common-emitter stage gives voltage gain with signal inversion. Its 300 MHz transition frequency supports audio through RF/IF work to roughly 100 MHz, which is why it appears in preamps, oscillators, and sensor signal conditioning.
2N3904 Applications
- Small-signal audio and sensor amplification
- Low-power load switching (LEDs and loads under ~100 mA)
- Logic-level interfacing and level shifting
- RC and LC oscillators
- General-purpose hobby, prototyping, and microcontroller (Arduino) projects
2N3904 Equivalents and Replacements
Several general-purpose NPN transistors are close electrical matches. Confirm both the current rating and the pinout before substituting — not all share the 2N3904’s E-B-C layout.
| Part | Type | Key difference vs 2N3904 | Pinout |
|---|---|---|---|
| PN2222A | NPN | Higher current (≈600 mA–1 A) | E-B-C (drop-in) |
| 2N4401 | NPN | Higher current (600 mA) | E-B-C |
| BC547 | NPN | Lower current (100 mA); reversed pinout | C-B-E |
| 2N3906 | PNP | Complement, not a substitute (opposite polarity) | E-B-C |
| MMBT3904 | NPN | Same die, SOT-23 surface mount | — |
| PZT3904 | NPN | Same die, SOT-223, higher dissipation | — |
The BC547 is electrically similar but its C-B-E layout is reversed relative to the 2N3904 — swapping it without rerouting will connect collector and emitter backwards. The 2N3906 is the matched PNP complement for push-pull and H-bridge stages, not a drop-in replacement.
2N3904 vs 2N2222
Both are staple general-purpose NPN transistors. The practical difference is current capability and package:
| 2N3904 | 2N2222A | |
|---|---|---|
| Max IC | 200 mA | 800 mA |
| VCEO | 40 V | 40 V |
| Package | TO-92 | TO-18 (metal) / TO-92 as PN2222A |
| Pinout | E-B-C | C-B-E (TO-18) / E-B-C (PN2222A) |
Use the 2N3904 for small-signal and low-current switching; choose the 2N2222 for higher-current loads. The full breakdown, including the metal-can pinout trap, is in our 2N3904 vs 2N2222 differences.
2N3904 Package and Dimensions
The standard package is the TO-92 (onsemi CASE 135AN), approximately 4.83 × 4.76 mm, with three through-hole leads on 2.54 mm spacing; a lead-formed TO-92 variant is also offered. For surface-mount designs, the same die is available as the MMBT3904 (SOT-23) and the PZT3904 (SOT-223), the latter offering higher power dissipation. Pb-free ordering options include bulk bag, fan-fold, and tape-and-reel.
Where to Buy the 2N3904
The 2N3904 is a low-cost, widely second-sourced part, available in TO-92 (through-hole) and SOT-23 (MMBT3904) packages from onsemi and many other manufacturers. Because it is so common, availability is rarely an issue — but pricing and lead time vary by package, volume, and manufacturer. Prudix IC supplies the 2N3904 and its equivalents (PN2222A, 2N4401, BC547); request a quote for current pricing and stock.
Frequently Asked Questions
Is the 2N3904 NPN or PNP? It is an NPN transistor. Its PNP complement is the 2N3906.
What is the maximum current and voltage of the 2N3904? Absolute maximum collector current is 200 mA and collector-emitter voltage (VCEO) is 40 V. The useful switching range is about 100 mA, so design below the maximum.
What is the gain (hFE) of the 2N3904? The DC current gain is 100–300 at IC = 10 mA, but it falls to a minimum of 30 at 100 mA. Use a forced gain of about 10 in switching designs to guarantee saturation.
What can I use to replace a 2N3904? The PN2222A and 2N4401 are higher-current NPN substitutes with the same E-B-C pinout. The BC547 is electrically similar but has a reversed C-B-E pinout. The 2N3906 is the PNP complement, not a substitute.
Is the 2N3904 the same as the 2N2222? No. Both are general-purpose NPN transistors, but the 2N2222 handles more current (800 mA vs 200 mA) and the metal-can version uses a different pinout.
Can the 2N3904 drive a relay or motor? Only small ones within its 100 mA useful range. For typical relays and motors, use a higher-current transistor or a Darlington such as the TIP120, often with a flyback diode across the inductive load.