Electronics Components: Look inside a Transistor

By Doug Lowe

Have you ever taken apart an electronic gadget just to see all of the neat electronic stuff inside? If so, you have likely seen a number of transistors already. Consider this a guided tour of the transistors inside electronic gizmos.

There are many different kinds of transistors. The most basic kind is called a bipolar transistor. Bipolar transistors are the easiest to understand, and they’re the ones you’re most likely to work with as a hobbyist.

Now let’s peer inside a bipolar transistor to see how it works.

Recall that a diode is the simplest kind of semiconductor, made from a single p-n junction, which is simply a junction of two different types of semiconductors, one that’s missing a few electrons and thus has a positive charge (p-type semiconductor) and the other with a few extra electrons, thus having a negative charge (n-type semiconductor).

By itself, a p-n junction works as a one-way gate for current. In other words, a p-n junction allows current to flow in one direction but not the other. A diode is simply a p-n junction with a lead attached to both ends.

A transistor is like a diode with a third layer of either p-type or n-type semiconductors on one end. Thus, a transistor has three regions rather than two. The interface between each of the regions forms a p-n junction. So another way to think of a transistor is as a semiconductor with two p-n junctions.

NPN and PNP transistors and their schematic symbols.

NPN and PNP transistors and their schematic symbols.

One way to make a transistor is with a p-type semiconductor sandwiched between two n-type semiconductors. This type of transistor is called an NPN transistor because it has three regions: n-type, p-type, and n-type.

The other way to make a transistor is just the opposite, with an n-type semiconductor sandwiched between two p-type semiconductors. This type is called a PNP transistor because its three regions are p-type, n-type, and p-type.

Each of the three regions of semiconductor material in a transistor has a lead attached to it, and each of these leads is given a name:

  • Collector: This lead is attached to the largest of the semiconductor regions. Current flows through the collector to the emitter as controlled by the base.

  • Emitter: Attached to the second largest of the semiconductor regions. When the base voltage allows, current flows through the collector to the emitter.

  • Base: Attached to the middle semiconductor region. This region serves as the gatekeeper that determines how much current is allowed to flow through the collector-emitter circuit. When voltage is applied to the base, current is allowed to flow.

These two current paths are important in a transistor:

  • Collector-emitter: The main current that flows through the transistor. Voltage placed across the collector and emitter is often referred to as Vce, and current flowing through the collector-emitter path is called Ice.

  • Base-emitter: The current path that controls the flow of current through the collector-emitter path. Voltage across the base-emitter path is referred to as VBE and is also sometimes called bias voltage. Current through the base-emitter path is called IBE.

Here are a few additional points to ponder concerning transistors:

  • In an NPN transistor, the emitter is the negative side of the transistor. The collector and base are the positive sides.

  • In a PNP transistor, the emitter is the positive side of the transistor. The collector and base are the negative sides.

  • Most circuits that you can build with an NPN transistor can also be built with a PNP transistor. But if you do, you must remember to flip the power connections.

  • In a schematic diagram, transistors are usually represented by the letter Q.