Test how a thermistor reacts to temperature. Also how the results compare to the manufacturers.

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Kieran Gallagher

Physics Coursework

Aim:

        To test how a thermistor reacts to temperature. Also how the results compare to the manufacturers.

Background Knowledge:

Thermistors are inexpensive, easy to obtain temperature sensors.  They are easy to use and can be used in a variety of ways. Circuits with thermistors can have reasonable output voltages but not the millivolt outputs thermocouples have.  Because of these qualities, thermistors are widely used for simple temperature measurements.  They are not normally used for high temperatures, but in the temperature ranges where they work they are widely used.

Thermistors are made from silicon normally so we must understand silicon and its properties.

Carbon, silicon and germanium (germanium, like silicon, is also a semiconductor) have a unique property in their electron structure namely each has four electrons in its outer orbital. This allows them to form crystals. The four electrons form perfect covalent bonds with four neighbouring , creating a lattice. In carbon the crystalline form as . In silicon, the crystalline form is a silvery, metallic-looking substance.

Metals tend to be good conductors of electricity because they usually have "free electrons" that can move easily between atoms, and electricity involves the flow of electrons. While silicon crystals look metallic, they are not, in fact, metals. All of the outer electrons in a silicon crystal are involved in perfect covalent bonds, so they can't move around. A pure silicon crystal is nearly an insulator basically very little electricity will flow through it.

Doping Silicon
          You can change the behaviour of silicon and turn it into a conductor by doping it. In doping, you mix a small amount of an impurity into the silicon crystal.

There are two types of impurities:

N-type - In N-type doping,  or  is added to the silicon in small quantities. Phosphorus and arsenic each have five outer electrons, so they're out of place when they get into the silicon lattice. The fifth electron has nothing to bond to, so it's free to move around. It takes only a very small quantity of the impurity to create enough free electrons to allow an electric current to flow through the silicon. N-type silicon is a good conductor. Electrons have a negative charge, hence the name N-type.

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P-type - In P-type doping,  or  is the dopant. Boron and gallium each have only three outer electrons. When mixed into the silicon lattice, they form "holes" in the lattice where a silicon electron has nothing to bond to. The absence of an electron creates the effect of a positive charge, hence the name P-type. Holes can conduct current. A hole happily accepts an electron from a neighbour, moving the hole over a space. P-type silicon is a good conductor.

A minute amount of either N-type or P-type doping turns a silicon crystal from a good insulator into a ...

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