Electronics Components: Look inside a Transistor
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.
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.