When P and N type silicon are brought together during manufacture, a junction is created where the P type and N type materials meet, and holes close to the junction in the P type silicon are attracted into negatively charged N type material at the other side of the junction. The P type silicon now has a shortage of valence electrons in its structure, which can also be considered to be a surplus of ‘holes’ or positive charge carriers, whereas the N type layer is doped with atoms having five electrons in its valence shell and therefore has a surplus of electrons which are negative charge carriers. These doped versions of silicon are known as ‘extrinsic’ silicon. Arsenic or Phosphorus) to make N type silicon. Boron or Aluminium) to make P type, or five valence electrons (e.g. Undoped or ‘intrinsic’ silicon has a lattice structure of atoms, each having four valence electrons, but P type silicon and N type silicon are doped by adding a relatively very small amount of material having either an atomic structure with three valence electrons (e.g.
Using this process for silicon diodes produces two differently doped layers of silicon, which form a ‘PN junction’. The layered construction used in Silicon Planar methods give a number of advantages such as predictable performance and reliability as well as being advantageous to mass production.Ī simplified planar silicon diode is illustrated in Fig. Modern silicon diodes are generally produced using one of various versions of the planar process, also used for manufacturing transistors and integrated circuits. 2.0.2 that conventional current flows from the positive (anode) terminal to the negative (cathode) terminal although the movement of electrons (electron flow) is in the opposite direction, from cathode to anode. On some circuit diagrams the anode of a diode may also be indicated by the letter ‘a’ and the cathode by the letter ‘k’. 2.0.2, the cathode is shown as a bar and the anode as a triangle. Ideally a diode will pass current when its anode is made more positive than its cathode, but prevent current flow when its anode is more negative than its cathode. It has two terminals, the anode or positive terminal and the cathode or negative terminal. Diode Circuit SymbolsĪ diode is a one-way conductor. Counter-clockwise from red: Yellow and green indicator LEDs, an infra-red photodiode, a 5mm warm white LED and a 10mm high luminosity blue LED. Zener Diodes with glass or black resin encapsulation.ĥ. A point contact diode (with glass encapsulation) and a Schottky diode.Ĥ. Three power rectifiers, (a Bridge rectifier for use with mains (line) voltages, and two mains voltage rectifier diodes).Ģ. 2.0.1 shows a selection of common wire ended diodes as follows:ġ. Early types of semiconductor diodes were made from Selenium and Germanium, but these diode types have been almost totally replaced by more modern silicon designs.įig. Photo diodes also produce electrical current from light.ĭiodes are made from semiconductor materials, mainly silicon, with various compounds (combinations of more than one element) and metals added depending on the function of the diode. LEDs produce light of many colours in a very wide range of equipment from simple indicator lamps to huge and complex video displays. Diodes are also built into many digital integrated circuits to protect them from dangerously large voltage spikes. Signal diodes also have many uses in processing signals in electronic equipment they are used to obtain the audio and video signals from transmitted radio frequency signals (demodulation) and can also be used to shape and modify AC signal waveforms (clipping, limiting and DC restoration). Diodes can also be used to prevent disastrous damage to battery powered equipment when batteries are connected in the wrong polarity.
Zener diodes are used for voltage stabilisation, preventing unwanted variations in DC supplies within a circuit, and to supply accurate reference voltages for many circuits. Rectifier diodes are a vital component in power supplies where they are used to convert AC mains (line) voltage to DC. Many types of diode are used for a wide range of applications. Diodes are one of the simplest, but most useful of all semiconductor devices.