1. Review: Most semiconductors are made of a poor conductor that has had impurities (atoms of another material) added to it. The process of adding impurities is called doping. There are two types of impurities: N-type doping (negative charge) P-type doping (absence of electrons create the effect of a positive charge )

2. Transistors and diodes have a lot in common. A transistor is created by using three layers rather than the two layers used in a diode. You can create either an NPN or a PNP sandwich. A transistor can act as a switch or an amplifier. A transistor looks like two diodes back-to- back.

3. You'd think that no current could flow through a transistor because back-to-back diodes would block current both ways. And this is true. However, when you apply a small current to the center layer of the sandwich, a much larger current can flow through the sandwich as a whole. This gives a transistor its switching behavior. A small current can turn a larger current on and off. Transistors explained

4. Types of transistors There are two types of standard transistors, NPN and PNP, with different circuit symbols. Most transistors used today are NPN because this is the easiest type to make from silicon. The leads are labelled base (B), collector (C) and emitter (E).

5. If a negative voltage is applied to the collector of the transistor, then ordinarily no current flows BUT there are now additional holes at the junction to travel toward point 2 and electrons can travel to point 1, so that a current can flow. It can be shown that most of the current flows between points 1 and 2. In fact the amplitude (magnitude) of the collector current in a transistor is determined mainly by the emitter current which in turn is determined by current flowing into the base of the transistor. Consider the base to be a bit like a tap or faucet handle.

6. Function Transistors amplify current, for example they can be used to amplify the small output current from a logic chip so that it can operate a lamp, relay or other high current device. In many circuits a resistor is used to convert the changing current to a changing voltage, so the transistor is being used to amplify voltage. A transistor may be used as a switch (either fully on with maximum current, or fully off with no current) and as an amplifier (always partly on).

7. Its Use in computers Transistors are basic components in all of today's electronics. They are just simple switches that we can use to turn things on and off. Even though they are simple, they are the most important electrical component. For example, transistors are almost the only components used to build a Pentium processor. A single Pentium chip has about 3.5 million transistors.

8. Sources http://www.electronics- tutorials.com/basics/transistors.htm http://www.electronics- tutorials.com/basics/transistors.htm http://www.kpsec.freeuk.com/components/tran.htm http://www.101science.com/transistor.htm http://amasci.com/amateur/trshort.html http://computer.howstuffworks.com/diode1.htm