Transistors

What are they and what do they do?

Transistors are 3 layer electronic devices.  The layers are made of materials that either (1) conduct electricity very well, or (2) conduct electricity not so well. 

Here is a drawing I made of a transistor...

The two small zones in this transistor are made of the good conducting matrerial, and the middle zone is the not so good conducting material.  The conducting zones are labeled "n" because they have lots of spare electrons they can give away...
is kinda similar to...where the "+" area tends to hold its electrons and the "-" areas will easily let them pass

But don't forget; the zones are actually neutral... the + and - signs are there only to show you what type of material is where.
The material with many loose electrons is represented with the "-" signs and the material without loose electrons has the "+".
KEY idea: transisitors are not charged, they're just made of materials that either have or don't have loose electrons.  I added a few "+" and "-" here and there to help you remember that the # of + equals # of -

If we apply a voltage to the center wire,

we get a buildup of electrons

This provides a "bridge" for the current to pass across the normally non-conductive p layer.

So in operation, the steps are as follows
You can see that the transistor exists in one of two states: on or off.

If used like this, the transistor is a switch that can be used to turn on/off other devices, including other transistors.
If the voltage on the left terminal is large, then a little current in the middle terminal will allow large current to flow through.
For this reason, the transistor is the main component in signal amplifiers. Your car radio uses transistors to boost incoming radio station signals. 


Here's a site where the transisor is briefly described: http://library.thinkquest.org/16497/components/transistors/index.html
also, intel maintains a transistor site:
http://www.intel.de/education/transworks/shock.htm