Page 34 - Curriculum Visions Dynamic Book

Silicon chips
A silicon chip is a piece of pure silicon that has been treated with other chemicals in a way that gives it special electrical properties.
A piece of silicon is normally a very good insulator, that is, it has a very high resistance to the flow of an electric current.
Materials only conduct electricity when they have mobile electrons that can flow when an electrical voltage is applied. Metals, for example, have many mobile electrons in their structure and so are very good conductors (they have a very low resistance).
In a silicon atom there are no electrons that are able to move and produce an electric current. However, scientists have been able to give silicon a resistance somewhere between an insulator and a conductor. This resulting material is called a semiconductor.
Making semiconducting materials
Silicon can be made semiconducting by combining phosphorus with silicon. As crystals of silicon and phosphorus grow, they bond together to leave one electron spare. The spare electrons are free to move, and therefore to conduct electricity.
Silicon with added phosphorus has a surplus of electrons; it is called a negative type or n-type semiconductor. Silicon with added aluminium, on the other hand, leaves a deficit of electrons. This makes it a positive, or p-type material.
By sandwiching the two materials and applying a voltage across them that attracts electrons from the n-type to the p-type, a current can be made to flow.
If a voltage is applied in the other direction, however, electrons cannot move. This kind of semiconductor is called a junction diode.
A diode
The simplest silicon chip is called a diode. It is a small piece of silicon that has two regions doped with metal. This is normally achieved by doping the whole of the chip with one impurity and then doping a small section with another impurity.
Sandwich made of silicon wafers doped with different substances
The uses of diodes
Junction diodes are widely used in electronics to detect signals in radios and act as switches in computers. But some diodes can also send out light when a voltage is applied across them. These are known as light emitting diodes, or LEDs. Light emitting diodes readily produce
red, yellow and green light. They have an extremely long life and are now used as indictor lights for most electronics. The indicator light on the front of a computer monitor, for example, does not have a bulb behind it but an LED. And the colour of
the light is not caused by a piece of green plastic; the colour you see is the colour of light emitted by the LED.
Some LEDs send out infra-red radiation. These can be used to send information along optical cables.
The n-type region of the chip has an has a shortage of
The p-type region excess of electrons
+–
electrons
Voltage applied
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