Understanding resistance, resistivity and conductivity
Electrical resistance is a measure of an objects opposition to the electron movements that constitute a current flow. It is related to the atomic particle structure of the object but also to its area and length. Resistivity and its inverse conductivity are the part of resistance that is entirely related to particle structure
A voltage across an object provides a surplus of electrons at one pole on the object and a deficit of electrons at another. The surplus electrons produce photon pressures that encourage the least bound outer electrons of the object structure to move away. The deficit produces locations (holes) in the structure that desire electrons and their photon energy gathering capabilities. An energy stable structure structure has a strong hold on its outer electrons and has high resistivity. Energy structures that have little hold on their outer electrons have low resistivity and good conductivity. Silver, copper and aluminium are good conductors.
Resistance is related to structure length because the voltage pressures are trying to set in motion outer electrons that in turn are trying to set in motion other electrons along the length of the object.
A conductor’s area provides a multitude of pathways along which outer electrons can move. For larger areas the voltage pressures on electrons are more spread and the most vulnerable outer electrons move. To get the same current flow through a smaller conductor we have to increase voltage and push harder. Measured resistance is increased. The harder and more concentrated pushes mean the displaced electrons carry more energy and when the displaced electrons engage in new atom homes they release that energy. The structure may burn and be destroyed as for example deliberately happens when a fuse is subjected to a current beyond its rating.
Silicon and germanium as semi conductors
In the periodic table of elements both silicon and germanium atoms have four outer electrons. Their atoms come together in energy saving arrangements in which they share their 4 outer valence electrons covalently with 4 neighbouring atom. Such structures are energy stable and will not easily part with their outer electrons. They have resistivity millions of times that of good conductors yet millions of times lower than insulating structures like rubber, air and quartz. They are semi conductors.
We can upset the energy linked structures by doping them with small quantities of other elements. The doping elements boron, aluminium and gallium have only 3 outer valence electrons and semi conductor material doped with them is termed p type material. Such material desires electrons to improve its energy stability. Doping with phosphorous, arsenic and antimony with their 5 outer valence electrons makes for an n type material willing to part with electrons to improve its energy state. In both cases the level of doping lowers the material making it much nearer that of good conductors.
Diodes and depletion zones.
When we bring together p type and n type materials as in the diagram the electron desiring p type materials take from the n type materials some of their surplus electrons creating a depletion zone. Structures created in that depletion zone are more energy efficient and highly preferred by the structure. They build a resistance to the changes that the doping levels are pushing for and so the established depletion zone width is a function of the doping levels.
Suppose we now apply a voltage across this p n diode junction as in the diagram left. Our voltage is pressuring electrons to move into the n material and encouraging electrons to move out of the p material. However the depletion zone wants to stay as it is and it takes about 0.7 volts of pressure (the forward voltage) to get current to flow in a silicon diode from p to n.
There is a limit to the current a diode can pass. As with our fuse the heat produced by electron photon emissions can destroy a diode. However, unlike the fuse, the level of doping is what creates more electron pathways and increases the diodes current rating.
If we reverse the voltage supply on the above diode we push electrons into the p material and pull electrons out of the n type material. Both actions extend the energy saving depletion zone structures making for a larger and more highly resisting depletion zone seriously limiting any reverse current flow. However if this reverse voltage is higher than the manufacturer’s peak inverse voltage, a breakdown current will flow via limited pathways which is usually destructive of the diode.
Zener diodes have highly doped structures that are designed to breakdown at a specific reverse voltage. The high levels of doping provide many pathways for good conduction at and above this zener voltage and so avoid damage to its structure. The breakdown voltage of zeners can be as low as 2.4 volts and as high as 200 volts. When source voltages are above the zener voltage, current flows. The flow lowers the source voltage and so the diode switches to off which raises the supply voltage. This on/off switching of a zener at the zener voltage makes it a useful device in voltage regulation.
Light receiving photo diodes and light emitting diodes (LED’s).
We explained above how putting a reverse voltage on a diode extended its depletion zone and how the energy stability of that depletion zone offered a high resistance to any further current flow; it acted as a current blocker. Suppose we now make a diode so that photons of light energy can get through to its depletion zone materials and apply to that diode a reverse voltage .
The diode won’t conduct until many photons of light energy land on its depletion zone. Such photons remove electrons from their desired homes and being no longer firmly held by a specific atom they are influenced by the photon pressures of the diode supply voltage. The result is that these electrons drift in the p to n direction between atomic structures and in so doing influence other electrons in the circuit causing a current flow. We have a photo diode in which light causes a current flow.
LED lights generally use aluminium gallium arsenide as the doped semi conductor material. As for a forward biased silicon or germanium diode the depletion zone is overcome and current flows when the forward bias voltage is exceeded. However outer electrons in the doped aluminium gallium arsenide are held onto more strongly than those of doped silicon or germanium. They require increased photon energies to set them in motion and carry higher energies which they release, on engaging with an atom, as the higher energy photons of visible light. By changing the characteristics of such diodes we can get them to emit specific colours of visible light, infra red light or ultra violet light.