The 2N3904 is a common NPN bipolar junction transistor used for general-purpose low-power amplifying or switching applications. The type was registered by Motorola Semiconductor in the mid-1960s, together with the complementary PNP type 2N3906, and represented a significant performance/cost improvement, with the plastic TO-92 case replacing metal cans. It is designed for low current and power, medium voltage, and can operate at moderately high speeds. This transistor is low-cost, widely available and sufficiently robust to be of use by experimenters and electronics hobbyists.^[1] When looking at the flat side with the leads pointed downward, the three wires emerging from the bottom are connected to, from left to right, the emitter, the base and the collector.^[2] Some manufacturers mark "EBC" on the molded part, but all are required to have those connections for a part which is a "2N3904".^[3]

It is a 200 mA, 40 V, 625 mW transistor with a transition frequency of 300 MHz,^[4] with a minimal beta, or current gain, of 100 at a collector current of 10 mA. It is used in a variety of analog amplification and switching applications.

The 2N3904 is used very frequently in hobby electronics projects, including home-made ham radios, code-practice oscillators and as an interfacing device for microcontrollers.