Transistors can be classified into which types?

Transistors are semiconductor devices used to amplify or switch electronic signals and electrical power. They can indeed be classified into two main types based on their construction and operation: NPN and PNP.

In an NPN transistor, the device consists of a layer of p-type semiconductor sandwiched between two n-type layers. When a small current flows into the base (the middle p-type layer), it allows a larger current to flow between the collector and emitter (the two n-type layers). This makes NPN transistors effective for switching and amplification tasks.

Conversely, a PNP transistor has a layer of n-type material between two p-type layers. In this case, when a small current leaves the base, it enables a larger current to flow in the opposite direction, from the emitter to the collector.

Both types are essential for different applications in electronic circuits, and understanding the distinction between them is crucial for anyone studying design technology and electronics. The other options refer to different concepts: AC and DC relate to types of electrical current, anode and cathode terms reference electrodes in an electrochemical context, and series and parallel describe how components are connected in a circuit, none of which pertain to transistor classification.