How does a PN junction fail ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: February 22, 2018, 06:49:33 pm
content: I've brunt out a dozen LEDs over the years, fried a few transistors/555 timer's. 1-2 times I connected a BJT and went over the 5V Vbe typical limit. Or something that made them explode, rather than slow cook.How do they fail from too much forward I and V and reversed I and V ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2018, 07:40:35 pm
content: Forward: a couple of ways.1. Heat. Too much heat and it melts. Or more likely, dopants diffuse and the diode ceases to be its intended configuration, which might lead to hot spots, increased leakage, reduced breakdown voltage, etc., in turn leading to actual melting later.2. Electromigration. This is why diodes have a peak forward current rating. You can't simply use a diode at higher and higher peak currents, for lower and lower duty cycles, just because it's not getting hot in the process -- eventually, the current density becomes so high in the contacts or device, that metal starts to get pushed around, and soon, contacts fail, and the device fails more catastrophically.At least, I haven't been told otherwise about that failure mode. I suspect that's the underlying mechanism.3. There may be something about saturation. I can't remember off the top of my head, if there is any ultimate limit on current density in a PN diode. At some point, if nothing else, avalanche would occur, giving an unlimited current density (unlimited in the sense that, whether it's still semiconducting afterwards or turning into a hot plasma arc, you're still getting an avalanche breakdown behavior).Reverse:Repeated overvoltage, causing avalanche. Modern diodes and transistors are usually quite tolerant of this operation, within reason. Early diodes (from the 60s, say) were rather vulnerable, something about purity and learning to structure the junction to avoid hot spots.That said, diodes and transistors still have to do their primary job best, so don't handle much avalanche energy -- or in the case of transistors, they often do handle a lot of energy, but it comes at the price of worse body diode recovery, and not handling unlimited currents (not like a proper TVS diode, which can handle huge currents).In high leakage types (schottky), there can also be runaway meltdown. mA+ of leakage at 10s of V adds up to whole watts, quickly! But by then it's too late... There are more or less two classes of schottky, trading a slight edge in Vf for a lot more reverse leakage. The application is typically anything that spends most of its time forward biased (e.g., PV reverse blocking), so won't get very hot and doesn't need to be anything special in reverse. Which type is best for, say, an SMPS, depends on duty cycle and reverse voltage.Tim

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: February 22, 2018, 08:07:10 pm
content: An important point oft not appreciated is that the actual silicon die in the package can change its temperatureveryquickly in response to loading. The thermal time constant may be in the tens of milliseconds region. If a really excessive current is applied it will reach breakdown temperature (usually >200C) before there is any noticeable warming of the case or heatsink. Sometimes, faster than a fuse can respond.

author: danadak
author_rank: Super Contributor
timestamp: February 22, 2018, 11:24:17 pm
content: There are also mechanical failures due to heat, hot spots, high I, etc, likedie cracking. Wirebond junction fails. Die attach issues. Some of these tendmore to be quality issues, but it does happen.Then there are ESD failures. And external environmental where packageintegrity is compromised. Space radiation issues, but then we divergefrom your original causal conditions.Regards, Dana.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 22, 2018, 11:51:07 pm
content: Quote from: IanMacdonald on February 22, 2018, 08:07:10 pmWhat's more, note that the thermal time constant is much longer than the transient events in question: transistors die in 10s of microseconds, while the heat takes tens of milliseconds to leave the die (and hundreds to leave the package).(Reminder that fuses take thermal time constants to fail, even under heavy fault conditions!)For transient events much shorter than the time constant, the temperature can be modeled as a pure integrator: apply an energy pulse, and temperature rises in a step.The volume being heated, is the junction itself: not even the whole die, and certainly not the base plate and packaging! This is very little material, hence why most avalanche ratings are only in the mid to upper mJ range.Typical example, a schottky diode that might be rated 40mJ avalanche. In this case, the avalanche structure is the guard rings around the schottky junction itself. Only a perimeter is dissipating that energy, so very little of total die area is heating up.Power MOSFETs tend to have higher ratings (100s mJ, large ones pushing over 1000mJ), partly because of the longer pulse duration and partly because they utilize more of the junction. (Vertical TrenchMOS has the tiny source, substrate and gate features tightly integrated on one face of the die, constructed edgewise (vertically), while the body of the die is the lightly doped drift region, where drain voltage is dropped, and avalanche current is dissipated. The backside of the die is strongly doped to form the drain connection.)On that subject, I'm not sure why IGBTs aren't rated for avalanche (or very much, if ever, at least). They would be subject to much more... interesting breakdown behaviors, potentially including the rapid avalanche switching behavior that BJTs exhibit, and maybe inducing subsequent 4-layer (SCR) latchup (despite valiant efforts to prevent latchup even under fault current conditions). As far as I know, they just die when ratings are exceeded -- the usual explanation here is a hole burned in the die where all the avalanche current flowed, resulting in excessive off-state leakage.On that note, the typical pinhole failure looks like a resistor in parallel with an otherwise okay transistor. I've caused MOSFETs and BJTs to fail in such a way that C-E / D-S looks like a resistor (usually ~ohms, sometimes 10k's ohms), but if you can apply base/gate voltage (without it being loaded down by excessive short-circuit current), the 'ON' region still works just fine.One way to reliably destroy power BJTs: set up an avalanche pulse generator with a proportionally-sized pulse capacitor. Avalanche spreads out quickly in small-signal transistors (hence why the humble PN2369 can sink a couple amperes in this operation), but not in power transistors, which have wide junctions. Trying to deliver, say, 10A or more through that small avalanche channel, burns a hole in the die. Pretty reliably, switchmode and HOT types fail after one or a few pulses, with a final R_CE(off) ~ 40kohms.Tim