How Diodes Work - The Learning Circuit

the following program is brought to you by element14 the electronic design community where you can connect and collaborate with top engineers from around the world join now at element14.com hi I'm Karen and this is the learning circuit where we learn about basic electronics today we're going to learn about how diodes work here are what most common diodes look like they have two axial leads that come out of the ends some are clear with reddish orange on the inside and a black stripe on one ends some are black with a silver stripe on one ends the stripe indicates which end is the cathode or the negative end this is what the circuit symbol for a diode looks like the line on the symbol matches up with the line on the diode itself this helps you make sure the diode gets put into the circuit the right way a diode is an electrical component that allows current to flow in one direction but not the other let's take a look at the inner workings of a diode to understand how this works we'll start by looking at PN junctions a PN Junction how does it function previously we've talked about how electricity works by electrons passing from one atom to another if you're not familiar with this look back at the science of electricity episode electrons in an atom live outside the core of the atom in valence shells each valence shell can hold a maximum number of electrons before they must occupy the next valence shell diodes are made with a semiconductor material today most diodes are made with silicon but sometimes other materials are used such as germanium selenium or gallium arsenide semiconductors typically have four valence electrons in their outer shell silicon being a semi conductor also has four outer valence electrons however this outer shell can hold up to eight electrons atoms are most stable when their outer valence shell is full silicon is most stable with eight electrons in its outer valence shell it has four but is most stable with eight so how does it get more electrons here we have a bunch of silicon atoms each has four electrons in its outer valence shell each of these electrons will bond with an electron from another silicon atom these electrons become shared and add to the total of the outer valence shell of both atoms so this silicon atom now has one two three four five six seven eight electrons in its outer valence shell now all of these atoms will do this this forms a very strong structure called a crystal or a lattice to make diodes do what they do this structure is manipulated through a process called doping where another element is added to the silicon silicon diodes have two elements added to them making two regions the p-type region with positive charge carriers and the n-type region with negative charge carriers in the p-type region the silicon is doped with an element like boron or aluminum these elements have only three instead of four electrons in their outer valence shells the missing electron creates a hole in the crystal structure instead of having the very stable eight electrons now some atoms only have seven but they still want that eighth electron to make them stable again these holes are positive charge carriers in the n-type region the silicon is doped with an element like antimony or phosphorous these elements have five electrons in their outer valence shell one more than what the silicon crystal structure needs so four of those electrons pair with the silicon atoms while the fifth becomes a free electron free to go wherever the current takes it these free electrons are negative charge carriers so we have these two regions in the diode one doped with positive charge carriers and one doped with negative charge carriers where these two regions meet is referred to as the junction near the junction the positive charges and the negative charges having opposing charges are drawn to each other like magnets the free electrons in the n-type region migrate over and fill the holes in the p-type region because of the charged particles moving around the area near the junction in the p-type region becomes slightly negatively charged while the area near the junction in the n-type region becomes slightly positively charged this area is known as the depletion zone free electrons in the n-type region will continue migrating into the holes in the p-type region causing the charges within the depletion zone to increase these charges will eventually grow strong enough to begin repelling the charges in their region the n-type side gains enough positive charge in its depletion zone to repel the negative charges in the region and the same in the p-type side where the depletion zone gets enough of a negative charge to repel the positive charge in the region eventually the depletion zone becomes charged enough to stop electron migration in a silicon diode this happens at around 0.7 volts now that we know what's happening inside a diode when it's not connected to anything let's see what happens when we hook it up to a power supply in a circuit [Music] let's let the diode do its job and hook it up backwards with the negative terminal connected to the anode and the positive terminal connected to the cathode since the diode is meant to only allow current to flow through it in one direction it should stop electricity flowing through it in this direction let's take a look inside the diode to see what's happening like when we compared electrical charges to magnets the opposing charges attract the atoms in the positively doped p-type region are attracted to the negative charge at the anode they're drawn together while the atoms in the negatively doped n-type region are attracted to the positive charge at the cathode and those are drawn together the charges in both regions move away from the junction widening the depletion zone it's like their two friends who are mad at each other and stop sharing their snacks at lunch time no sharing of electron fruit snacks today with the charges this far apart electron flow is impossible okay not completely impossible there's an exception diodes have a breakdown voltage when connected in reverse bias if too much voltage is supplied and that voltage gets beyond the breakdown voltage the diode will actually begin conducting current in the wrong direction but this usually damages or completely destroys the diode I said that in a silicon diode the charge of the depletion zone is around point seven volts this is called the barrier potential or threshold voltage so to overcome the charge in the depletion zone we need to supply at least 0.7 volts across the diode we know that this doesn't work if the diode is connected backwards so let's see what happens in the diode if it's hooked up in the right way forward bias mode the negative charge of the cathode pushes more free electrons into the n-type region and further into the depletion zone since the depletion zone is gaining negative charge carriers it's positive charge decreases once more than 0.7 volts is applied enough free electrons flow towards the junction that the depletion zone shrinks and then disappears completely allowing electrons freely to flow across the junction now remember from before when we had those free electrons from the n-type region that came over to the p-type region to vacation in the holes well now they've found something more attractive they move out of the holes and onto the positively charged anode leaving holes behind don't worry those holes won't stay empty for long the free electrons in the n-type region now have easy access to cross the junction so they move right in filling the holes and the cycle continues free electrons from the cathode flow through the n-type region into the holes in the p-type region and on to the anode around and around in the circuit let's talk more about that point seven volts the point seven volts required to overcome the depletion zone is also the forward voltage drop in the circuit point seven volts is lost to heat energy if the diode was connected to nine volts then point seven volts would be lost leaving eight point three volts available to do whatever it is you're trying to do now not all diodes have a PN Junction and some diodes even let electricity flow through them backwards we'll talk more about these types of diodes and more in a future episode now there's never enough time for me to cover all of the information I'd like to share with you but hopefully you now have a basic understanding of why diodes work if you have a project that revolves around diodes I'd love to hear about it please post about it on the element14 community on element14.com forward slash the learning circuit happy learning [Music]

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Keywords: electronics, hardware, gaming, hacking, mods, weekly, element14, tbhs, benheck, madison, wisconsin, maker, engineering, circuit, battery, diodes, semiconductors, silicon, gallium, thelearningcircuit, tlc, stem_projects, susbstitution box, doping, n-type region, p-type region, boron, depletion zone, pn junction

Id: -SSkjWuUri4

Length: 9min 5sec (545 seconds)

Published: Wed Sep 26 2018