(Failed?) Invention of the day:  The magnet-adjustable PIN diode  - Page 1
Forum section: electronics / general

author: RoGeorge
author_rank: Super Contributor
timestamp: December 26, 2024, 08:06:25 am
content: The tail for this one was like this:1. - w2aew posted a very nice 101 experiment about PIN diodes, then another video about a classic Tx/Rx antenna switch based on a PIN diode and a lambda/4 transmission line, and about a diode's reverse recovery time in general (https://youtube.com/watch?v=SBqLOrlA7QI) which made me put those experiments on my todo list2. - Noopy posted some beautiful pics of a decapped PIN diode (https://www.eevblog.com/forum/projects/different-die-pictures/msg5674113/#msg5674113), which reminded me once again about the w2aew's PIN diodes videos3. - me, not having any PIN diode at hand (and being too lazy yesterday night to scavenge for one, and too impatient to order one then wait for it), tried using an 1N4007 diode instead, because I've heard 1N4007 might substitute a PIN diode (with some limitations). So I've put at the output of the generator an 1N4007 in series with a 50\$\Omega\$ resistor to observe the recovery time.The 1N4007 instead of PIN diode experiment worked pretty well, the recovery time of a 1N4007 can be as long as a few microseconds, depending on the voltage level and/or the DC bias applied. But I wanted to make the recovery time even longer, and here's where I've diverged from the initial goal and got "creative" instead.I've recalled from the physics classes how a moving electron will steer in a magnetic field, and eventually will move in circles, and that's part of why a Hall sensor works. (The image in my had was that of a Hall-effect sensor overlapped with the image of a P(I)N junction.)Now, if I apply a magnetic field perpendicular on the traveling direction of the electron/holes inside a PN junction, I would expect to deflect the electrons sideways (and implicitly deflect the holes, too, to the opposite sideway) at least by a little (the electrons will move "diagonally" relative to the PN junction). That was supposed to make the PN junction (from an electron's perspective) to appear as being "longer" when a magnetic field is applied, so the electrons/holes will travel a longer path between anode/catode. Similar, I would expect the recombination time to differ when a magnetic field is applied.I would expect to see this "magnetic lengthening" of a PN junction reflected as a change of the recovery time in the diode (between other side effects).However, no matter how close I get with my small neodymium magnet to the 1N4007, and no matter the magnetic field orientation, I can not see any change in the recovery time of the diode.I didn't do yet any quantitative estimations of how much change would be expected, and this is mostly because I don't know enough about the inner workings of a PN junction and recombination times, and I don't know the magnetic field intensity either, probably a few mT to 10s of mT at most, but I would expect to see at least a little change on the oscilloscope waveform.But I couldn't observe any changes at all while rearanging the magnet position, or when changing the signal frequency, or the bias of the diode.So, why no change observed in the recovery time of a 1N4007 when a small Neodymium magnet is placed nearby? Is the magnetic field not strong enough? Is the idea wrong?(P.S.The goal is not to make an Rx/Tx switch, or to substitute a PIN diode with a 1N4007. Right now I would like to understand if the idea of "adjusting" a diode with a magnet can work at all, and if not, then why not? What am I missing, or doing wrong?)

author: RoGeorge
author_rank: Super Contributor
timestamp: December 26, 2024, 09:22:18 am
content: Meanwhile I've searched online, and found this morning there are quite a number of papers about the effects of a magnetic field upon a PN junction.That's a lot of reading, and some are behind paywalls, but at least it's a hint the idea of using a magnet to influence a diode might not be totally off.Adding a few links for the docs (no paywall):INFLUENCE OF A MAGNETIC FIELD ON THE CHARACTERISTICS OF A P-N JUNCTION DIODEGafur Gulyamov, Gulnoza Majidova, and Feruza Muhitdinova∗http://jase.tku.edu.tw/articles/jase-202401-27-1-0007.pdfChanging the Voltage of the p-n Junction in a Magnetic FieldGafur Gulyamov, Feruza Mukhitdinova, Gulnoza Majidovahttps://www.jstage.jst.go.jp/article/ejssnt/21/4/21_2023-047/_pdf/-char/enStrange that most are papers by Gulyamov et al.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 26, 2024, 02:13:29 pm
content: The reverse recovery time is not a flight time. So the idea of extending the path by forcing it to spirals does not work. As an approximation one can see the reverse recovery time as the minority carrier life time. So the time it takes to recombine for minority carriers.One may get rather long reverse recovery with a photodiode.

author: iMo
author_rank: Super Contributor
timestamp: December 26, 2024, 03:00:17 pm
content: Quote from: RoGeorge on December 26, 2024, 08:06:25 amBuy a large magnet on ebay and you may start with "magnet fishing" instead. A lot of physical activity and fresh air at water shore too, good thing instead of sitting in the lab the whole day..PS: and people make $$ with it as well..

author: RoGeorge
author_rank: Super Contributor
timestamp: December 26, 2024, 04:12:51 pm
content: Meanwhile I've tried to measure any VFvariation. As suggested by the papers linked in reply #1, VFshould increase. But I couldn't detect any change caused by magnets.I've tried either by pushing constant current then positioning the small magnet nearby, or by placing the diode near this much larger magnet (a magnetic clamp helper for welders):Measured this red magnetic clamp before, and found a B of about 65mT (with an oscilloscope and a piece of wire).Another try was to push constant current between the + and - terminals of a 4 diodes bridge, and to measure the voltage difference on the AC-in pins of the bridge (using a DC DMM with 1uV resolution), in the hope that one arm of the bridge will change more with the magnetic field, and thus will show a DC imballance, moving away from 0V.In practice it was a small difference, but constant (a couple of mV or so, depending on the bridge type, or depending on the DC bias current). There were other small DC variations, but they seemed related to the diodes temperature, and not related to the orientation of the magnetic field.Either with the big or with the small magnet, couldn't notice any correlation between the magnetic field and the diodes bridge imballance.

author: TimFox
author_rank: Super Contributor
timestamp: December 26, 2024, 05:01:26 pm
content: The reverse recovery time in a PIN diode (or similar) is important to estimate the stored charge in the device (DC current times recovery time).For RF switching, the stored charge gives a high conductivity for AC current, so long as the total integrated current during a cycle does not deplete that charge.That determines the low-frequency end of the useful frequency range, since the period of the AC sine wave is the reciprocal of the frequency.In the Hall effect, there is a net deviation in the current density vector direction that gives an emf across the device in the direction perpendicular to both the current direction and applied magnetic field.This shouldslightlylengthen the path for the current, inversely proportional to the cosine of that small deviation angle.

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2024, 05:20:36 pm
content: The one that at least conceptually goes in a circle because of magnets is faradar isolator or 'circulator'.I have trouble understanding exactly what is going on there, took me enough time to just to figure out what the arrow of time is in their definition, but it does 'curve' physically and its related to magnets lol

author: coppercone2
author_rank: Super Contributor
timestamp: December 26, 2024, 05:54:19 pm
content: I think you can focus that field down so its more intense around the diodes with some iron wedge shapes.Until I determined the field intensity is actually changing, I would not hold any results conclusive. Different magnets = different field lines, you don't know density. It needs a controlled setup some how, but its hard to make it.I think a electromagnet would be best. I bought a large iron lamination rod inductor that can probobly work as a magnet (in the mili henrry range). But for a static field, I think maybe a solid chunk of soft iron would be better, I don't know if laminations do anything useful at DC.

author: Analog Kid
author_rank: Super Contributor
timestamp: December 27, 2024, 12:17:31 am
content: Ro, maybe you should try using a big honking electromagnet; can probably generate a stronger field, and you can vary it to see the effects.

author: Sensorcat
author_rank: Regular Contributor
timestamp: December 27, 2024, 03:13:35 am
content: Insofar understanding of the operation principle of a Hall sensor is relevant here, I can add this:During stationary operation of a Hall device (constant current and constant magnetic flux density), there is no, or almost no* deflection of current path due to the mangetic field. Instead, the electric field that can be measured perpendicular to the current prevents such a deflection. An equilibrium is reached when the potential movements from both fields exactly compensate each other. Hence a deflection only happens in a non-stationary situation, when there is a change in current or flux density.* Since there are a lot of different geometries of Hall plates (semiconductor layout, electrode patterns), and both the current and the electric field are not uniform across the device, the statement is an approximation, the whole picture, so to speak. For every single point in the Hall device, current vector, current density, and electric field vector will be slightly different.

author: RoGeorge
author_rank: Super Contributor
timestamp: December 27, 2024, 08:19:14 am
content: I'm afraid I was wrong with this one. The reverse recovery time in a diode is dictated by the amount of charge stored in the junction, and by how fast that stored charge is removed. By definition, I=Q/t, so at a given current, the time is dictated only by the amount of charge, Q.Tilting the direction of the moving charges with a magnetic field, at best, might make the junction resistance to appear as being larger (like some sort of magneto-resistive effect, but I have no idea how magnetorestivity really works). I guess this tilted path is the main culprit for why the V forward is increasing in those research papers linked before, but again, wildly guessing, because I know nothing about the physics of semiconductors. Don't even know why the forward voltage on their diode is in that big, in the range of 10V..60V, what kind of diode is that? (for example the I/V curves in Fig.3 fromhttps://www.jstage.jst.go.jp/article/ejssnt/21/4/21_2023-047/_pdf/-char/en).In conclusion, I think now that the idea of increasing the recovery time with a magnetic field can not work at all, as Kleinstein and Tim Fox made me understand, thank you.As for the increase in V forward (which is something else than what I assumed, just that it is also with magnets), the increase in Vf, if there is any, I couldn't measure it. Tried yesterday all kinds of Si and Ge diodes and bridges. They don't seem to care about nearby magnets.