The 2N3904 is the default general-purpose NPN bipolar transistor, in a TO-92 through-hole package. It switches and amplifies small signals up to about 200 mA and 40 V, with a current gain (hFE) of roughly 100 to 300 and a transition frequency near 300 MHz, so it works as a switch, a small-signal amplifier, or a logic-level buffer. The TO-92 leads are E, B, C in order (pin 1 = emitter). Its PNP complement is the 2N3906.

Pinout

Pin	Name	Type	Function
1	E	passive	Emitter of the NPN transistor. VEBO breakdown 6.0 V. In TO-92 (viewed from the flat/front, leads down), lead 1 is Emitter.
2	B	input	Base of the NPN transistor. Controls collector current with hFE typ 100-300 at IC=10 mA, VCE=1.0 V. VBE(sat) ≤ 0.85 V at IC=10 mA, IB=1.0 mA.
3	C	passive	Collector of the NPN transistor. VCEO breakdown 40 V, VCBO breakdown 60 V, IC continuous max 200 mA. VCE(sat) ≤ 0.2 V at IC=10 mA, IB=1.0 mA.

Key specifications

Parameter	Value
Type	NPN bipolar transistor
Collector-emitter (VCEO)	40 V
Collector current (IC)	200 mA
DC gain (hFE)	100-300
Transition freq (fT)	300 MHz
Power dissipation	625 mW
Pinout	E-B-C (pin 1 = emitter)
Package	TO-92

Ordering variants

2N3904BU (TO-92)
2N3904TA (TO-92, ammo pack)

CAD (KiCad official)

Symbol: Transistor_BJT:2N3904
Footprint: Package_TO_SOT_THT:TO-92_Inline
3D model: TO-92 (KiCad packages3D)

Provenance

Data extracted from the manufacturer datasheet, not a distributor listing:

Datasheet: onsemi datasheet (7 pp, sha256 39b20929e3d2d854, retrieved 2026-08-17, tier: manufacturer)
Extractor: ds2sf (Claude). Every pin and dimension is cited to a datasheet page; it refuses to fabricate.
CAD: KiCad official libraries (CC-BY-SA 4.0).

Datasheet PDF