Millions of transistors are found in integrated circuits across every electronic device. Here, we focus specifically on bipolar junction transistors.

There are two main types of transistor: bipolar junction transistors (BJTs) and field effect transistors (FETs). BJTs are made of doped materials and can be configured as NPN and PNP. A transistor is an active device with three terminals, and these three terminals are known as the Emitter (E), the Base (B), and the Collector (C) (Figure.1). The Base is responsible for controlling the transistor while the Collector is the positive lead, and Emitter is the negative lead.

The transistor symbol indicates the three terminals. (Courtesy of Quora)

The semiconductor physics of BJTs will not be discussed here, but it is worth mentioning that a BJT is fabricated with three separately doped regions with two junctions. The PNP transistor has one N region between two P regions (Figure. 2) while the NPN transistor has one P region between two N regions (Figure. 3). The junctions between N and P regions are similar to the junctions in diodes and they can be forward-biased or reverse-biased as well. BJTs can operate in different modes depending on the junction bias

A PNP transistor has a layer of N-doped semiconductor between two layers of P-doped material (Courtesy of Wikibooks)

An NPN transistor has a layer of P-doped semiconductor between two N-doped layers (Courtesy of Wikibooks)

• Cutoff: BJT operates in this zone in switching operations. In cutoff, the transistor is inactive.
• Active: BJT operates in this zone for amplifier circuits because the transistor can act as a fairly linear amplifier.
• Saturation: BJT operates in this zone in switching operations. The transistor appears as a near short circuit between the collector and emitter terminals.
• Reverse Active: Like active mode, the current is proportional to the base current, but flows in reverse. This mode is rarely used.

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. In an NPN transistor, the current flows from the collector (C) to the Emitter (E) (Figure. 4). In a PNP transistor, however, the current flows from the emitter to the collector (Figure. 5).

         Figure 4                        Figure 5

The arrow shows the direction of the current and how it is always on the emitter

The NPN transistor always has an arrow pointing out.

It is clear that the current directions and voltage polarities in PNPs and NPNs are always opposite to each other. NPN transistors require a power supply with positive polarity with respect to common terminals, but PNP transistors require a negative power supply.

PNPs and NPNs work pretty much alike, but their modes are different because of the current polarities. For example, to put an NPN into saturation mode, VB should be higher than VC and VE. 

Transistor Testing