The 2N3904 is a very common NPN bipolar junction transistor (BJT) used for general-purpose low-power amplification and switching. Here’s a detailed overview:
Key Specifications
Type: NPN transistor
Maximum Collector-Emitter Voltage (V<sub>CE</sub>): 40 V
Maximum Collector-Base Voltage (V<sub>CB</sub>): 60 V
Maximum Emitter-Base Voltage (V<sub>EB</sub>): 6 V
Maximum Collector Current (I<sub>C</sub>): 200 mA
Power Dissipation (P<sub>tot</sub>): 625 mW
Gain (hFE / β): 100 – 300 (depending on the operating current)
Transition Frequency (fT): ~300 MHz
 
Pin Configuration (TO-92 Package)
Looking at the flat side of the transistor with leads pointing downward:
Emitter (E) – Left pin
Base (B) – Middle pin
Collector (C) – Right pin
(Double-check datasheet; some manufacturers may have a different layout.)
 
Applications
Switching small loads (LEDs, relays)
Signal amplification in low-power circuits
Oscillator circuits
Digital logic circuits for interfacing