Review:

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.kpsec.freeuk.com/components/tran.htm • http://www.101science.com/transistor.htm • http://amasci.com/amateur/trshort.html • http://computer.howstuffworks.com/diode1.htm