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 measures of that part of resistance which is entirely related to the particle’s structure and consequently it is fixed for each type of structure.
A voltage across an object, let’s say a wire, provides a surplus of electrons at one end of the wire and a deficit of electrons at the other. The surplus electrons want to better serve proton and neutron energy desires and their movements to do so produce photon pressures that encourage the least bound outer electrons in the atoms of the wire’s structure to move away. At the other end of the wire the electron deficit is one in which there are locations, often referred to as holes, in the structure’s atoms that proton energy desires want electrons, with their photon energy gathering capabilities, to fill.
Whereas the atoms of more energy stable structures strongly hold onto their outer electrons so exhibiting high resistivity, the energy structures that have atoms with poor hold on their outer electrons have low resistivity. Silver, copper, gold and aluminium are good conductors; they have good conductivity.
Resistance is related to structural length because voltage pressures are having to deliver many more atom outer electron displacements and movements along the object (wire) length.
Whilst electron progress along the wire is slow, as they move between the protons they serve, electron movements throughout a circuit occur almost immediately because each each electron move involves changes to its light speed photon emissions, so causing the movements of other electrons.
A larger conductor area makes available more atom outer electrons; there is more choice in the electron movement pathways that constitute the current flow and it makes for reduced resistance. Much reduced areas result in more concentrated electron moves and interacting energy flows and an increase in associated temperature. If too concentrated they destroy the energy links between atomic particles; we deliberately arrange for this to happen in fuses when its rating is exceeded.
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 stability of these 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. Semi conductor material doped with them is termed p type material. Doping with phosphorous, arsenic and antimony with their 5 outer valence electrons makes for an n type material. In both cases the level of doping lowers the resistivity of the semi conductor making it into a good conductor.
Diodes and depletion zones.
The p type material desires electrons as they can make it more energy stable. The n type material has electrons that are not contributing to its stability.
When we bring together p type and n type materials, as in the diagram, the p type materials takes from the n type materials some of its surplus electrons creating an energy stable depletion zone increasingly resisting further change and established a depletion zone width that is related to the doping levels.
A voltage applied across this p n diode junction as in the diagram left will be seeking to move electrons from the conducting n material to the conducting p material. Resisting the motion is the stable energy depletion zone that wants to stay as it is. That resistance is overcome by about 0.7 volts of pressure (the forward voltage) after which electrons flow from n to p. We say a current flows 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 diode voltage supply we encourage electrons to move into the p material and out of the n material. Both actions extend the energy saving depletion zone structures with their 4 outer electrons 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 and that 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. When that happens outer electrons in the depletion zone respond by moving out of their parent atom locations and now are influenced to move by the photon pressures originating with the diode supply voltage. These electrons moving in the p to n direction between atomic structures, we regard as a current flowing, in a direction opposite that indicated by the diode symbol. That’s it for a photo diode.
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.