The BD139 is a general-purpose NPN bipolar junction transistor (BJT). It has three pins:
- Base (B)
- Collector (C)
- Emitter (E)
There are two companies that manufacture the transistor
- STMicroelectronics
- onsemi
The pinout diagram according to STMicroelectronics is as follows:
![](https://i0.wp.com/www.exasub.com/wp-content/uploads/2023/03/BD139-pinout-ST.jpg?resize=455%2C269)
When looking at the center hole and keeping the metal side down the pins are arranged from right to left as:
- Base (B)
- Collector (C)
- Emitter (E)
It’s important to note that the metal tab is connected to the Collector pin.
The pinout diagram according to ON Semiconductor is as follows:
![](https://i0.wp.com/www.exasub.com/wp-content/uploads/2023/03/BD139-pinout-On-Semi.jpg?resize=275%2C272)
When looking at the flat side of the transistor with the pins facing down and the metal tab facing up, the pins are arranged from left to right as follows:
- Emitter (E)
- Collector (C)
- Base (B)
It’s important to note that the metal tab is connected to the Collector pin.
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