Re: How do transistors work?

Reply:

>How do transistors work?

That is a pretty big question! There are many kinds of transistors, with different characteristics and, furthermore, these transistors usually have different operating regimes, depending upon whether you want to operate them in analog or digital ways!

To keep focused, I'll go over the basics of operation of one kind of transistor, the MOSFET (for Metal-Oxide- Semiconductor Field Effect Transistor) when operated in a digital way. Since most transistors made in the world are digital MOSFETS, this isn't a bad first choice :-).

If you look at the diagram, and remember that picture, and if you can remember what the acronym MOSFET stands for, you will hopefully have no problem remembering the basic operation of the transistor. The MOS part of the acronym refers to the structure--what makes the whole thing work. This is a sandwich of metal on the top, a slice of insulating oxide, and the silicon underneath. To the sides of this sandwich are the drain and the source. For now it is OK to think of them as just conducting plugs that don't touch each other, but are separated by the silicon area underneath the gate.

The goal of the transistor is to allow the drain to be in electrical contact (meaning current can pass between them) with the source ONLY when the gate "gives it permission," meaning that something happens on the gate to produce a contact between drain and source. The second part of the acronym hints at what that something is. FET stands for field effect transistor, and the field effect is what actually turns the transistor on and off. What happens is this: some charge (say positive charge) is placed onto the gate. This positive charge produces an electric field all around itself. Since the silicon dioxide part of the sandwich is very thin (think of a single leaf of lettuce), the electric field goes down into the silicon. Now you remember that silicon is a semiconductor. This means that it doesn't quiiiite have enough charges in it to conduct electricity well, but it is close. The field produced by the charges on the metal gate of the transistor attract the opposite charge (negative, as we decided earlier, although it works the other way, too).

Now comes the neat part. If you think for a moment, it would be really nice if this whole circuit were isolated from the rest of the world. That way, we could put another transistor just like this one right next to it! As we have it now, the negative charges in the semiconductor will all race up close to the gate, forming a conductive bridge between the drain and source, just what we wanted! However, since the charges came from all over the semiconductor, they'll be able to conduct a bit of current away from our device. How can we get rid of that?

Here's what the transistor makers do: they make the original silicon semiconductor with an excess of free positive charges. Then, when the gate gets positive charge, these positive charges are driven away, and negative charges come in underneath the gate to form that conductive bridge. Now, there are negative charges close in and positive charges a bit farther out. These form another electric field that means there is a region between them in the semiconductor that doesn't have any charges at all! This means it will insulate between the current flowing in the transistor and the rest of the silicon.

The transistor is turned on or off by applying this charge to the gate. It is also pretty easy to make the other sort of transistor-- the one where a negative charge to the gate make the transistor turn on. You just reverse all the positives and negatives in the story.

Hope that helped you understand a bit better! If you would like more information about how transistors of many kinds operate, try an electronics textbook, like Horowitz and Hill, in the local library.

-Greg Billock