Changes

==Function==

==Function==

There are two basic types of transistors, field effect transistors, which use an electric field to control how conductive a channel of semiconductor is, and bipolar junction transistors which work by modifying the level of electric charge in certain semiconductors.  

Modern transistors are so small that designers make heavy use of [[quantum theory]].  However, this isn't important for everyday use of the devices and a simple explanation of transistors is given below.  There are two basic types of transistor: field-effect transistors (FETs) and bipolar junction transistors (BJTs). 

 

 

A field effect transistor contains a thin strip of [[semiconductor]] called the "channel".  A pair of metal contacts are put at either end of the channel, allowing it to be connected into a [[circuit]].  These contacts are called the "source" and the "drain".  The rest of the channel is coated with an [[insulator]] to stop any unwanted currents from entering or leaving.

 

Finally, a third metal contact called the "gate" is placed on top of the insulator.  When a battery is connected between the gate and source, the insulator stops current getting through.  Instead, the electrons in the channel are all pushed away from the gate.  In other words, they can only move through a very thin part of the channel, making it difficult for current to flow between the source and drain.  The [[resistance]] of the channel therefore depends on the [[voltage]] at the gate.  If a big enough voltage is placed on the gate, the electrons can be pushed completely out of the channel and no current can flow at all.

 

The first transistors were made of [[germanium]], but more modern devices are made of [[silicon]].  Some modern devices use [[silicon-germanium]] alloys or [[gallium arsenide]].

==History==

==History==