According to 2N3904 datasheet this transistor is a silicon epitaxial planar NPN general purpose amplifier and switch. The useful dynamic range extends to 100 mA as a switch and to 100 MHz as an amplifier.
2N3904 packages and pin configuration
The TO-92 package is suitable for through-hole PCB assembly. The 2N3904’s PNP complementary type is 2N3906.