Sensor Project
Introduction
In this project I am going to test and evaluate a thermistor. I am going to test the thermistor on how the resistance of this component changes under different temperatures. Thermistors are thermally sensitive resistors and have, according to type, a negative (NTC), or positive (PTC) resistance/temperature coefficient. A thermistor is a type of resistor whose resistance changes significantly when its temperature changes.
A negative temperature coefficient (ntc) thermistor has a resistance that decreases with increase of temperature. This is the most common type. This is the type of resistor that I shall be evaluating and will be using in my experiment.
A positive temperature coefficient (ptc) thermistor has a resistance that increases with increase of temperature. The resistance of a semiconductor generally decreases with increase of temperature. Semiconductors are therefore used to manufacture ntc thermistors.
When the temperature of a semiconductor is increased, the number of charge carriers increases as more valence electrons gain sufficient energy to break free from atoms to become conduction electrons. The number of charge carriers increases as the temperature is increased so the resistance of the semiconductor falls. Semiconductors are used to make a wide range of electronic devices including electronic chips, light emitting diodes and solid state lasers. Communications, commerce and entertainment have been revolutionised as a result of semiconductor devices. In electrical terms, materials are classed as either conductors or insulators or semiconductors.
Semiconductors are solid materials with conductivities in between the very high conductivity of metals and the very low conductivities of insulators. There are a variety of types of semiconductor, including metal oxides as well as elements like silicon and germanium. In insulators, essentially all the electrons are tightly bound to atoms or ions, and none are free to move under an external electric field. In effect, these materials do not conduct electricity at all. In metallic conductors, essentially all the atoms are ionised, providing free electrons, which move freely through the ions and can move under an external electric field. These conduction electrons 'glue' the ions together, and provide non-directional bonding which holds the material together. They become shared amongst all the atoms in the material instead of remaining attached to one atom.
Semiconductors differ from both insulators and metallic conductors. Only a small proportion of atoms are ionised, so that although there are conduction electrons they are relatively small in number and the material conducts, but not well. At higher temperatures, more atoms are ionised, and the conductivity rises. This is the picture for intrinsic semiconductors. Doping the semiconductor with atoms of other elements can increase the conductivity by increasing the number of mobile charge carriers. Doping of Group 4 elements such as silicon or germanium is done by adding about 1 in 1 million of atoms either from Group 5 or Group 3. In semi conducting group 4 elements such as silicon and germanium each atom has four outermost electrons in a shell that can hold eight electrons. Each atom shares an electron with a neighbouring atom to form a covalent bond. In this way, each atom is held in a lattice by four neighbouring atoms. Doping Group 4 elements with Group 5 elements produces n-type (negative type) semiconductor. The Group 5 atoms have five electrons in their outer shell, and use four of these to bond with Group 4 atoms, leaving one electron free to conduct. Doping Group 4 elements with Group 3 elements produces p-type (positive type) semiconductor. The Group 3 atoms have three electrons in their outer shell. One of the four possible bonds with neighbouring Group 4 atoms is incomplete. This vacancy behaves like a mobile positive hole, accepting an electron from a nearby atom. The electron fills one hole but leaves behind a new one. When a potential difference is applied, conduction by holes occurs as electrons hop from hole to hole, causing the holes to migrate to the negative end of the semiconductor. Extrinsic semiconductors are used in the manufacture of diodes, transistors, integrated circuits, hall effect devices as well thermistors. At constant temperature, the current through a semiconductor is in proportion to the potential difference so its resistance is constant. Thermal runaway can occur in semiconductor devices if the current causes a heating effect, which then reduces the resistance thus causing even more current and an even greater heating effect until the semiconductor becomes so hot that it becomes permanently damaged. A resistor in series with a semiconductor device may therefore be needed to prevent thermal runaway. The resistance of a metal and certain other materials increases with temperature. Barium titanate is a non-metal used in the manufacture of ptc thermistors as it has a much larger temperature coefficient of resistance than any metal or any other commonly available material.
