2N3904 is a common NPN positive contact transistor used in general-purpose low-power amplification or switching applications
Designed for low current and power, medium voltage and can be operated at moderate high speed.
According to the
2N3904 data sheet, the transistor is a silicon epithelium plane NPN universal amplifier and switch. The useful dynamic range extends to 100 mA as a switch and 100 MHz as an amplifier.