The most commonly used BJT (Bipolar Junction Transistor) is the **NPN transistor**. This is because NPN transistors are easier to use in many electronic circuits compared to PNP transistors. In an NPN transistor, the current flows from the collector to the emitter when the base is provided with a small current, making it more efficient for most applications.
In terms of general usage:
- NPN transistors are more common in **digital circuits** and **switching applications**.
- They are also more commonly used in **amplification circuits** for audio and radio-frequency applications.
To sum up, **NPN BJTs** are preferred due to their higher electron mobility, which makes them faster and more efficient in many circuits.