How does a semi-conductor fry itself, and sometimes become a conductor ? - Page 1
Forum section: electronics / beginners

author: MathWizard
author_rank: Super Contributor
timestamp: January 14, 2023, 12:19:10 am
content: IDK the impurity levels in a PN diode or transistor, but when they burn out short-circuit sometimes, what actually makes the good conducting path ? Is it the leads melt into it or some outer casing with lots of carbon or metals in it, burns or something ??

author: py-bb
author_rank: Regular Contributor
timestamp: January 14, 2023, 12:24:56 am
content: Electrons ending up where they ought not - electromigration.A really good case study is a flash memory cell (because it works on the principle, sort of)Also as things get warmer, stuff becomes "more migratory" anyway, this is why you can generally get away with more if you can pull the heat off "anything semiconductor related"It's a bit topic but that should get you started. If you don't know how your basic junctions work you need to do that first, wikipedia is a decent starting place.

author: MathWizard
author_rank: Super Contributor
timestamp: January 14, 2023, 01:46:33 am
content: Ok so it's happens anyways as they are used, and runs away when you fry the part. I've done a few equations on semi-conductors and junctions, like to find some mosfet gate capacitance. I have books on it, I'll get to it someday.

author: CatalinaWOW
author_rank: Super Contributor
timestamp: January 14, 2023, 02:28:16 am
content: There is more than one way that it happens. A severe over voltage can vaporize leads and metallization, which then condenses into a conducting path. Lower energy faults can heat the semiconductors until diffusion proceeds rapidly, eliminating the sharp gradients in doping that are critical to many semis operation. In other cases the heat can debond the chip from its substrate, allowing it to physically touch the frame or case or something else. Lots of interesting physics.

author: py-bb
author_rank: Regular Contributor
timestamp: January 14, 2023, 03:46:01 am
content: Quote from: MathWizard on January 14, 2023, 01:46:33 amIt doesn't happen "anyway" when they're used, this is what defines operating voltage.With computer-type-things, to switch faster requires more voltage, but that accelerates ageing (and can elevate the temperature which also increases ageing)Flash in 1998 (high end NOR) used to have ~100s of electrons in play to store/clear a value - yeah literally trapping a charge that tiny. Amazing stuff - at ambient temperatures it'd take about 10 years to "discharge" itself due to leaking.Your average semiconductor part which is quite big are practically immortal, entropy gets them (as it gets us all) unless you stray outside their operating voltages

author: T3sl4co1l
author_rank: Super Contributor
timestamp: January 14, 2023, 02:45:15 pm
content: Doping concentrations are on the order of 10^15/cm^3 (or ~10ppt), if you were curious. This is a bit higher than the intrinsic carrier concentration at room temperature. If we're talking about silicon that is.Failure can proceed in several ways:Straight up overheating: thermal carriers dominate over dopants and the bulk of the semiconductor goes intrinsic. And, since the carrier concentration is so much higher, conductivity is higher, regardless of applied voltage, i.e., any diode or transistor behavior is swamped by the resistive leakage current flowing through the thing. Which can quickly lead to further runaway failure, including hot-spotting, melting, etc.This occurs typically in the >200°C range for Si. Some metal and ceramic package parts are available with operating temperatures up here (2N3055 for instance!). There are some things that can be made to operate a bit hotter than this still, but the on/off ratio is basically awful so it's a matter of desperation, not having any choice (e.g. down-hole drilling sensors).Diffusion, I think, doesn't matter until quite high temperatures, by the way (>300°C), even for long term operation, and even for very fine pitch semiconductors.Package limits are generally lower, due to the glass transition temperature and subsequent expansion rate and loss of strength of the resins used. This is typically 150-175°C. Brief operation higher than this is possible, not to mention soldering, even for fairly extended periods of time (minutes at 260°C), but sooner or later the packaging material will delaminate or fatigue off the die and backing plate, shear bondwires off, etc., and the game is over.Once temperatures get to diffusion or melting levels, a few things are relevant. First, it's probably under fault conditions, so any runaway occurs very rapidly (milliseconds), and the energy level is excessive, enough not only to melt the device (including incorporating excessive aluminum from the metallization, and decomposing any nearby resins e.g. polyimide passivation or epoxy packaging), but to blow out the melt layer, and bondwire, forming an arc, which expands explosively, separating the package, fracturing the chip, and obviously leaving a great big melted crater in the middle of what's left.I don't know that carbon does a whole lot to silicon as such (if it dissolves, it looks like it doesn't introduce any trapping levels in the bandgap? -- which is the problem with many other contaminants, like copper), but any conductive deposits clearly aren't going to help.It's possible for a semiconductor to survive the less mechanical parts of this process. A TVS diode may reach ratings with T_J approaching or surpassing 175°C, but it might not fail persistently until it actually melts. I haven't tested this, but it might be interesting to play with just to see; it should be possible to ramp up the energy by just enough to push it over the intrinsic temperature, and this will give a noticeably different waveform, particularly in the tail with a thermal time constant. So, that would be measuring with a waveform applying a surge plus a constant current (the CC to measure the voltage drop due to internal resistance). Mind, you may not be able to test many pulses this way, due to mechanical failure modes (thermal cycling, package decomposition).Likewise, I've had diodes and transistors desolder themselves, or melt slots in the breadboard.I've also had transistors fail silently, when current stopped just as damage occurred, or "transistance" was still juuust enough to manage to eventually turn it off (avoiding a full runaway event), or supply current was limited to the same end. In this case, no cosmetic failure occurs, but the device remains a three-way short.Esoterica: apparently, in avalanche pulse mode of small BJTs, a discharge channel develops, starting with an avalanche process (a column of charge, accelerating via impact ionization -- an avalanche cascade) and maintained by actually heating the material so much in that tiny channel (we're talking 10s of nm wide here) that it becomes a low resistance (e.g. 2N3904 ~10 ohms in this mode). This is also a punch-through operating mode, since the hot (intrinsic) channel shorts collector to emitter, bypassing the base's "transistance". (So, it's not a C-B leakage multiplication effect, at least not once it gets going -- it is usually started in that way however). It takes some 10s of µs for this channel to "turn off" (apparently: cool down), which seems about right for a thermal time constant on such a small scale. Driven too hard, this channel turns into a melt zone (or electromigration or other ionic scrambling process) and a permanent pinhole results; this is the typical result of avalancing a power BJT, where even the junction capacitance alone (i.e., regardless of external load) provides too much energy to the site. The aftermath gives a fixed terminal resistance, typically some kohms, while the transistor overall works normally aside from that. (MOSFET failures can manifest in a similar way, where gate oxide punchthrough has occurred, but if you can deal with the 100s ohms G-S "short" circuit, it otherwise still works.)Have also seen diodes fail from excessive peak current, on very short time scales where thermal heating should not be relevant; admittedly this could still be weirdness due to being in the forward-recovery regime (uneven current flows?), but it could also be something like electromigration, simply due to the excessive peak current density, even if the average current density is very low. (To clarify, that was a snubber application. So, low average current, extremely fast peaks.)Tim

author: iMo
author_rank: Super Contributor
timestamp: January 14, 2023, 03:45:23 pm
content: The most semiconductor devices today are built in a couple of micrometers or even nanometers thin sandwiched layers almost on the surfaces of the Si (for example) crystals, the electromagnetic fields in the thin areas are large even with small voltages, so it usually comes to an ionization breakdown of a layer, with a lot of heat generated - such the layers evaporate, including the aluminum/copper/gold metalization. With high currents the heat loss evaporates the layers as well as their volumes are miniscule and the Si is a good thermal insulator (weak cooling through the bulk Si chip). The melted metal then may cause shorts as well..