The BD139 is a popular NPN bipolar junction transistor (BJT) with the following specifications:

• Type: NPN Bipolar Junction Transistor (BJT)
• Collector-Emitter Voltage (Vceo): 80V
• Collector-Base Voltage (Vcbo): 80V
• Emitter-Base Voltage (Vebo): 5V
• Collector Current (Ic): 1.5A (continuous)
• Power Dissipation (Ptot): 12.5W (with proper heat sinking)
• Gain Bandwidth Product (fT): 100MHz (typical)
• DC Current Gain (hFE): 40 to 160 (typical)
• Package Type: TO-220

The BD139 is used in various applications including amplifier stages, switching circuits, and low to medium power linear applications. It is favored for its high current handling and low saturation voltage characteristics.