NPN and PNP transistors

There are two main types of transistor: bipolar junction transistors (BJTs) and field effect transistors (FETs). The invention of the transistor in the 1940s was a major turning point in the history of electronics. The bipolar transistor was the first transistor to be invented, when one simply says “transistors,” it sometimes means bipolar transistors.

BJTs are made of doped materials and can be configured as NPN and PNP when they are manufactured. It’s easy to remember that NPN stands for Negative-Positive-Negative and PNP stands for Positive-Negative-Positive. Let’s take a closer look at how NPN and PNP transistors work. There are numerous differences between NPN and PNP transistors, and even though both are bipolar junction transistors, the direction of current flow is the name of the game.

In an NPN transistor, a positive voltage is given to the collector terminal to produce a current flow from the collector to the emitter. In a PNP transistor, a positive voltage is given to the emitter terminal to produce current flow from the emitter to collector. The operation of NPN vs PNP transistors involves the movement of charge carriers, electrons, and holes. The primary difference between NPN and PNP transistors lies in the type of majority charge carriers they use.

Transistors can operate in three different modes: active, cutoff, and saturation. In the active mode, the transistor can amplify signals. If the emitter-base junction is not at least 0.6-0.7 volts (with typical bipolar transistors made from silicon) when observing the terminals of a BJT, the transistor is in the cutoff region.

The choice of PNP vs NPN transistors in a circuit design is not a one-size-fits-all decision. The choice between using an NPN or a PNP transistor depends on the specific requirements of the circuit. NPN transistors are generally preferred for their higher electron mobility, which leads to faster switching times and higher frequency operation. However, PNP transistors are often used in high-side switches and voltage regulators due to their ability to handle high voltages. NPN transistors are more commonly used than PNP transistors in modern electronics.

The structure of a bipolar transistor looks symmetrical, so you might think if the transistor works if C and E are interchanged? The bipolar transistor does not work properly in the wrong way, because the dopant concentrations in the collector and emitter regions are quite different. Therefore, if the emitter and collector terminals are reversed, a bipolar transistor has a much lower hFE and does not function as intended.

There was this question in one Facebook electronics discussion group:

Is it possible to use PNP transistors instead of NPN transistors?

My answer:
Not without changing the circuit. Some simple NPN transistor amplifier circuits could work with similar PNP transistor when you use opposite polarity power and turn around other polarity sensitive components. So take this circuit, put in BC557, turn around all capacitors, turn around the LED, power with -9V