Overview

The 2N3904 is the quintessential general-purpose NPN Bipolar Junction Transistor (BJT) manufactured by ON Semiconductor, STMicroelectronics, and Fairchild. Included in virtually all beginner electronics kits and breadboard prototyping assortments, it is designed for low-power amplification and medium-speed digital switching applications.

Housed in a 3-pin TO-92 plastic package, the 2N3904 is rated for a collector-emitter breakdown voltage (VCEOV_{CEO}) of 40 Volts40\text{ Volts} and continuous collector current (ICI_C) of 200 mA200\text{ mA}. Its complementary PNP counterpart is the 2N3906.

Quick reference

Transistor TypeGeneral-Purpose NPN Bipolar Junction Transistor (BJT)
PackageTO-92 (Flat face front, leads pointing down) / SOT-23 (MMBT3904)
Pinout (TO-92)Pin 1: Emitter (E), Pin 2: Base (B), Pin 3: Collector (C) — E-B-C
Collector-Emitter Voltage (VCEOV_{CEO})40 V40\text{ V} max
Collector-Base Voltage (VCBOV_{CBO})60 V60\text{ V} max
Continuous Collector Current (ICI_C)200 mA200\text{ mA} continuous
DC Current Gain (hFEh_{FE})100…300100 \dots 300 at IC=10mA,VCE=1.0VI_C = 10\text{mA}, V_{CE} = 1.0\text{V}
Transition Frequency (fTf_T)300 MHz300\text{ MHz} min
Power Dissipation (PDP_D)625 mW625\text{ mW} at TA=25∘CT_A = 25^\circ\text{C}

Pinout (TO-92 Package)

Looking at the flat face of the TO-92 package with leads pointing down:

text
        ┌─────────────┐
        │   2N3904    │  (Flat Package Face)
        └─┬───┬───┬───┘
          1   2   3
          E   B   C
PinNameDescription
1EMITTER (E)Emitter terminal (Ground reference 0 V)
2BASE (B)Base input control terminal (Connect via resistor)
3COLLECTOR (C)Collector terminal (Low-side load connection)

Specifications

ParameterSymbolMinTypMaxUnitConditions
Collector Breakdown VoltageV(BR)CEOV_{(BR)CEO}40——VIC=1.0mA,IB=0I_C = 1.0\text{mA}, I_B = 0
Emitter Breakdown VoltageV(BR)EBOV_{(BR)EBO}6.0——VIE=10 μA,IC=0I_E = 10\ \mu\text{A}, I_C = 0
Collector Saturation VoltsVCE(sat)V_{CE(sat)}—0.20.3VIC=50mA,IB=5.0mAI_C = 50\text{mA}, I_B = 5.0\text{mA}
Base Saturation VoltageVBE(sat)V_{BE(sat)}0.65—0.95VIC=50mA,IB=5.0mAI_C = 50\text{mA}, I_B = 5.0\text{mA}

Low-Side NPN Switch Calculation

To drive a small 50 mA50\text{ mA} relay or indicator LED from a 3.3V3.3\text{V} or 5.0V5.0\text{V} MCU GPIO pin:

RB=VMCU−0.7VIBwhere IB=IC10=5.0 mAR_B = \frac{V_{MCU} - 0.7\text{V}}{I_B} \quad \text{where } I_B = \frac{I_C}{10} = 5.0\text{ mA}

  • For 5.0V5.0\text{V} MCU: RB=4.3V0.005A=860 Ω  ⟹  Use 1 kΩR_B = \frac{4.3\text{V}}{0.005\text{A}} = 860\ \Omega \implies \text{Use } 1\ \text{k}\Omega.
  • For 3.3V3.3\text{V} MCU: RB=2.6V0.005A=520 Ω  ⟹  Use 470 Ω or 560 ΩR_B = \frac{2.6\text{V}}{0.005\text{A}} = 520\ \Omega \implies \text{Use } 470\ \Omega \text{ or } 560\ \Omega.

Common mistakes

  • Confusing 2N3904 (E-B-C) and BC547 (C-B-E) pinouts: Although both are TO-92 NPN transistors, 2N3904 is E-B-C, whereas BC547 is C-B-E. Inserting a BC547 into a circuit designed for 2N3904 reverses the Collector and Emitter pins.
  • Exceeding 200mA collector current: Attempting to drive solenoids or motors drawing >200 mA>200\text{ mA} exceeds the 2N3904 rating. Use a 2N2222 (800mA800\text{mA}) or an N-channel MOSFET instead.

Notes

  • 2N3904 vs 2N3906: 2N3904 is NPN; 2N3906 is the complementary PNP transistor.

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