how do BJT transistors get damaged from over current? - Page 1
Forum section: electronics / projects

author: coppercone2
author_rank: Super Contributor
timestamp: November 09, 2024, 01:56:51 am
content: I have a 2SC4636 current mirror I madethe data sheet says the absolute maximum current is 10mA or 30mA for 100uS.I put 60mA through it for like..a few cycles on and off, for maybe 2 second eachAfterwards it looked like the mirror lost a bit of accuracy (2% more error) and I thought maybe it was like drifting a little (it was more stable before) on the last digit of the dmmWhat has happened in this transistor on a physics level?

author: moffy
author_rank: Super Contributor
timestamp: November 09, 2024, 02:19:30 am
content: AFIK, excessive current can degrade the junctions by high energy collisions causing dislocations in the lattice and or excessive local heating causing a migration of atoms, not good either way.

author: Analog Kid
author_rank: Super Contributor
timestamp: November 09, 2024, 02:32:32 am
content: Good question by the OP. I'd also like to know more about this; how exactly does overcurrent/overvoltage damage a BJT? Obviously after a certain point there can be gross physical damage, like melting, but before that, what exactly happens, electrochemically speaking, that degrades performance or destroys the device?

author: coppercone2
author_rank: Super Contributor
timestamp: November 09, 2024, 02:53:01 am
content: Quote from: moffy on November 09, 2024, 02:19:30 amso did i shift the PN junction boarders ? Or did it like get randomly degraded in nanoscopic areas?

author: moffy
author_rank: Super Contributor
timestamp: November 09, 2024, 03:30:25 am
content: Dislocations create traps for carriers that will reduce the beta, diffusion tends to degrade the junction area where P type dopants have moved towards the N side and vice a versa, leading to a less effective junction, greater leakage etc.

author: coppercone2
author_rank: Super Contributor
timestamp: November 09, 2024, 03:42:58 am
content: Ok the beta decrease is the thing I could measure, because it put out 2% less current afterwards, from 52uA to 50uA.Are they related effects for the beta? Or do specifically dislocations decrease the beta, and the p-type atom shift causes increased leakage?I.e. I think you can use neutron radiation to cause dislocations right? Then you just have a low gain transistor, without the leakage.?

author: moffy
author_rank: Super Contributor
timestamp: November 09, 2024, 03:49:04 am
content: AFAIK dislocations will tend to reduce beta but it could depend on where they occur, the unwanted diffusion of dopants could have a number of effects, possibly a book that discusses semiconductor failures would be a better, more complete source of info.

author: coppercone2
author_rank: Super Contributor
timestamp: November 09, 2024, 03:55:04 am
content: Quote from: moffy on November 09, 2024, 03:49:04 ami opened one up before and it caught on fire and the pages flew after memaybe one that shows how they break might be more palatable thoughusually its like putting about 5 pool tables back to back, tricking it out with golf course hazards and dumping way too many balls on itI think I started hating them with they refer to the thing that you put current into as a "drain" and the outlet as a "source". Thats what they always teach you. Flowing water up a drain. Makes sense yeahOr is the water going down the drain back into the source? a metaphor for the eco system? the source is the ocean and the drain is a drain?Then they call it a gate, ok, thats actually pretty accurate. But the other one is called a base. So one of them has a base on the golf course (ok, like a border patrol headquarters where the cops have way more power then the individual) to manage flow, and the other one is just like a big ass robotic wall. So the base is actually some how encouraging tourism with the participation of friendly electrons and the other one works like a mall gate on black friday

author: CatalinaWOW
author_rank: Super Contributor
timestamp: November 09, 2024, 04:32:59 am
content: Quote from: coppercone2 on November 09, 2024, 03:55:04 amYou do realize they are two different kinds of transistors, with totally different operating principles and different parameters of operation.

author: coppercone2
author_rank: Super Contributor
timestamp: November 09, 2024, 05:04:48 am
content: really?

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2024, 11:03:30 am
content: We are looking for things other than general overheating from excessive power dissipation, so I have two ideas.One thing I have seen is where hot spots form on the semiconductor die. Then the eutectic layer where the aluminum metalization meets the silicon can melt, degrading the device and eventually causing a short.I think another possibility is where the breakdown voltage falls with current crowding, causing avalanche breakdown. Hot carriers will then damage the crystal structure similar to how reverse breakdown of the emitter does, but in a different area so I do not know how this will affect the transistor characteristics; maybe it will increase leakage? This is how avalanche switching works, but initiated at high voltage, and it often eventually results in transistor failure.

author: tooki
author_rank: Super Contributor
timestamp: November 10, 2024, 11:51:20 am
content: Quote from: coppercone2 on November 09, 2024, 05:04:48 amYes, obviously.A MOSFET and a BJT are extremely different devices. If they were the same thing, we wouldn’t use different names for them. If they were closely related types of devices, they’d have related names (like the MOSFET and JFET).For historical reasons, in English, when we say “transistor” without specifying what kind, we mean the BJT. (With one exception: when talking about the number of transistors in CPUs. We say “transistors” but they’re all MOSFETs.) Similarly, when we say a FET, we usually mean a JFET. And we normally just say MOSFET whenever we mean those. This, despite the fact that BJTs, JFETs, and MOSFETs are all types of transistors — it’s the “T” in all their names.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: November 10, 2024, 02:27:04 pm
content: Maybe Noopy could open up a xistor that is only partially damaged and do his macro photography and maybe the damage can be evident

author: David Hess
author_rank: Super Contributor
timestamp: November 10, 2024, 03:40:22 pm
content: Quote from: BILLPOD on November 10, 2024, 02:27:04 pmDamage from avalanche will not be visible, and is even reversible through high temperature annealing.Thermal damage could be visible though. I have even seen it in plastic packaged power MOSFETs where the encapsulation bubbles before failure.Electromigration is possible at high currents, however I am not sure about this for discrete transistors which have thicker metalization then integrated circuits.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 11, 2024, 06:06:40 pm
content: I'm also interested in this topic.Recently I bought an "Multifunctional withstand voltage tester" from Aliexpress, and it is a very nice gadget. It has an Li-Ion battery, and adjustable step-up voltage generator and two knobs for setting maximum voltage (20 to 3700V) and a knob for the current limit (0.1 to 3mA, but that one seems less accurate, and current generally gets limited to 0.5mA)And the thing really works. I can add diodes in series, and the avalanche voltage adds up quite nicely.It also shows 0.6V for a conducting diode. 1.8V for a LED (some 12V for a LED in reverse).And as long as you don't press the "test" button (which starts the HV generator), you can use it to measure externally applied voltages. It's not "really cheap" (Cost around EUR20), but it is quite nicely built and well worth it's money.Testing avalanche voltage is a quite useful test. Both to check whether you bought fake parts, and to check the condition of unknown semiconductors.I do know that a lot of power MOSfet's have specified avalanche ratings. but what about all those other FET's and BJT's, Thyristors, Triac's, etc?I know there are no guarantees, but my best guess is that it's quite safe to use this for short tests, but it may degrade the devices over longer times (hours, days, months, years?) if used continuously. (Which is not it's intended use anyway).

author: Randy222
author_rank: Frequent Contributor
timestamp: November 11, 2024, 06:59:19 pm
content: Quote from: David Hess on November 10, 2024, 11:03:30 amPower dissipation may not even be observable. A flash current at really small duration like 1usec or less can cause a tiny local area at a junction to melt, or in general terms it just gets damaged.Damage is usually from heat, even in a high voltage breakdown.

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2024, 09:33:35 pm
content: melt the silicon ?That makes it sound like its equivalent power to a strong etching laser pulse

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 11, 2024, 10:19:04 pm
content: I've worked in the semiconductor industry for over thirty years now, dealing with analog power, so I've seen some seriously weird failures.Common failure modes that would show a shift in matching of a device usually comes from severe heating of the die. You're talking about the current you put through the transistor, but not the voltage that it was seeing at that time. If you're running 60mA at 0.5V across the collector to emitter, that's only 30mW and you'll be hard pressed to heat things a lot. Now, if it's 500V across the device, that's 30W and you're likely to do some serious heat damage.Considering you said the transistor can handle 10mA to 30mA for 100us and you ran 60mA for 2s, a bigger likelihood is electromigration. In that case, metal atoms on the silicon die are literally being moved by the collisions and field of electrons flowing through it. As someone else noted, you can also get some dislocations of atoms in the silicon matrix (possibly moving dopants or knocking silicon atoms out of place in the crystal) that can degrade operation.Another common failure comes from high operating voltages where current is not running uniformly in the transistor. This was noted as 'hot spotting' occurs on the die and can result in thermal runaway. The issue is that one spot on the die is running more current at the same voltage as everything else, leading to more power dissipation locally. That heats the die in that spot, and beta (for BJTs) goes up or Vgs threshold (for MOS) goes down, and you end up running more current. More, current is more power is more heat. It's positive feedback and can destroy a piece of silicon quickly.Those are my likely guesses for what happened to your transistor.I've seen fun things like someone doing an overvoltage on an IC pin that triggers the ESD clamp in their application. Thing is, they had a big cap outside the IC on this pin and the part was in a hermetic package. When triggered, ESD clamps go low impedance to shunt away energy from an ESD strike, the issue here was that there was so much energy in the external capacitor that it caused the bond wire to fuse open. Well, a bond wire is inductive to a certain extent and so that caused things to arc over to a low impedance point on the silicon die, creating what looked like an explosive crater. You could trace the arc track from the bond pad over to the low impedance point, it was impressive.

author: coppercone2
author_rank: Super Contributor
timestamp: November 11, 2024, 11:27:15 pm
content: so that is a problem you can get by having excessively low ESR capacitors near IC's .. which is like a trend now for the low noise linear regulators. I thought it was REALLY easy to break that thing (LT part). They want like many uF of ceramic right up against a ultra low inductance leadless part, in my poorly protected labperhaps that is a good reason why you should not just blatantly trust the "ESD PROTECTED TO...... " feature on some parts .! That big DO diode can take that 'side effect'. People were saying it could have been inductance, but maybe there is more to it.And how does it interact with the 'common wisdom' that its safe after its soldered to the PCB?? It sounds like some parts might be safer getting zapped on a table then being touched mounted to a board!

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 12, 2024, 02:21:12 pm
content: Quote from: coppercone2 on November 11, 2024, 11:27:15 pmLT parts tend to be rather robust, as for it being one of the low noise ones from them I may have designed it (if not I likely know who did). The problem with using very low ESR capacitors in a lot of places isn't about ESD, but instead about voltage control.I designed some parts that could withstand 80V operating, they didn't actually die until closer to 120V, and we would screw up every so often and blow them up in the lab when putting a 70V supply on them. How? Low ESR capacitors coupled with just a couple feet of wire from the power supply. A good rule of thumb is that you can get 30nH of inductance per inch of wire (not twisted pair or coax, just a single wire). If you watch the voltage across the capacitor when you put that 70V immediately across the wires, it will resonate up to almost double, close to 140V! Yeah. BOOM! Simulations will show this as well. If instead you start at zero and slowly ramp the supply (well below the resonant frequency of that LC) then the voltage across the capacitor tracks up nicely. It's an LC tank circuit with high Q.That's where you need to understand the difference between ESD and rated voltages. ESD strikes are a limited amount of energy, HBM (human body model) is defined in JEDEC standards with 100pF and 1.5k at a given voltage being dumped into a device. For a 2kV HBM strike that's 200uJ of stored energy at a maximum current of about 1.33A during dissipation. If you have a 10uF low ESR capacitor (say 10milliohm) that gets to 30V because of resonating, that's 4.5mJ of energy and at a maximum current of hundreds of amps. If an ESD clamp turns on to somewhere around 1ohm, then a 2kV HBM ESD hit is less than 2W of instantaneous power. But if you're dumping that 10uF at 30V through the ESD clamp, then the power level gets massive (almost 900W!). ESD clamps are designed to turn on below the breakdown voltage of devices on the die, but above the normal rated voltage. They are supposed to turn on and protect the internals of the chip once a pin gets pulled above the rated voltage, but there is only so much energy they can take before being destroyed.'Common wisdom' that a part is safe after soldered to the PCB? That's very true. When on a table, the part itself has to absorb the ESD strike, there is nothing around it to take any of the energy. On the board? Now there can be capacitors, resistors, or other components to help take the energy of the hit and things are better protected.

author: coppercone2
author_rank: Super Contributor
timestamp: November 12, 2024, 10:03:44 pm
content: but you said that if there is a charged cap near it, and its shocked, it can bust the bond wire, so even if it has the highest class of ESD protection, it sounds like it would survive on the table but not if you zap it while its on a turned on PCBI must be mis under standing somethingBecause alot of people take off wrist strap after its soldered together. To me it sounds like the strap should never come off until its mounted in a closed chassisI think the IC was the LT305something. I feel like the big TO-220 ones almost never died in my experiments. I like using the LT1965 alot. I did notice though that it has some strange interaction with small DCDC converter modules though (needs alot more input capacitance to work right). If you build a board with those and its doing something funny, put a 100uF polymer cap infront of the LDO. The LT1965 actually does a 300mV drop out without any additions, and it uses a normal voltage divider

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 13, 2024, 03:00:33 pm
content: Quote from: coppercone2 on November 12, 2024, 10:03:44 pmYes, you are misunderstanding. You can simulate this or just make it a mental exercise: take a 10uF capacitor (even one with high ESR at 2ohm) charged to 15V and put it on the input of a part. Now, take a 100pF capacitor that is charged to 2kV and connect it to that capacitor through a 1.5kohm resistor. Even if ALL of the charge from the 100pF capacitor goes into the 10uF cap, you're talking about less than a 1.3V rise in voltage. This is where the 'common wisdom' about ESD safety after building a board comes from.What happened in the one case I saw was that someone took a pin to a DC voltage above the point that the ESD turned on. Typically, ESD devices are designed to turn on at 50% above rated voltage for a product while the internal breakdowns of most devices are another 50% beyond that. We did not design with tight margins, doing so would have turned into lots of production issues. Remember, busting (fusing) a bond wire isn't just about current, but current across time. A normal ESD strike at a current of up to several amps is not long enough in duration to get a bond wire to fuse.QuoteNot a big surprise there. Small DC/DC converter modules can run with a crossover frequency that may be close to the unity gain frequency of the LT1965. This can get the whole system oscillating. Add a big 100uF cap, and you push the crossover frequency so low the two are well away from each other.

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2024, 09:53:20 pm
content: It was not oscillating, just had the wrong voltage, and too much ripple, so I guess that is kind of oscillating, but it was not going totally crazy, it just looked like a really bad power supplyIn this case, you can adjust the voltage by varying the height of the capacitor (how much leads) up to a certain minimum height where it would start working normally. Like a really bad trimmer. I had it sticking out alot during testing and it seemed to work slightly better (closer to design voltage), then I reworked it and pushed it down half way, and the voltage increased, and finally later when I fit it flush with the board it was working fine.I almost wanted to see if I can socket the capacitor with through hole sockets (like a TO-3 socket that lets the leads go through to the other side of the pcb) so I can pull it up to reduce the voltage output, so you can use it on a plunger control, sometimes it gets tiring to spin things. A secret adjuster.I find it totally hilarious that its possible to 'set' a power supply this way"sweep the capacitor height from 1 to 3 centimeters to test the under voltage behavior of the circuit"

author: AnalogTodd
author_rank: Regular Contributor
timestamp: November 13, 2024, 10:31:16 pm
content: Quote from: coppercone2 on November 13, 2024, 09:53:20 pmYeah, that's oscillating. If it's just that the part doesn't have good supply rejection, you'll see ripple on the output of the linear regulator but it will operate at the right voltage. You're changing the LC on the output of the DC/DC and also changing the impedance the LDO sees. If you look, a 10uF ceramic cap has a self-resonant frequency of about 1.5MHz, putting it on wires that are just 1cm off the board can reduce that frequency to about 300kHz, 3cm can take it to under 200kHz. That's right in the range of the crossover frequency of the LDO.QuoteYeah, you're not using this to 'set' the power supply, just getting it closer to a stable state. When oscillating, all bets on the reference voltage for an LDO are off. Word of advice, don't make something like this for production, normal variations will mean some will work and others won't.

author: coppercone2
author_rank: Super Contributor
timestamp: November 13, 2024, 11:02:11 pm
content: Lol I just imagined treez adding this control for his lamp posts as a dimmerbecause you can pull the capacitor out a little bit with a pole"help with finding china vendor willing to spot weld a hook on top of electrolytic capacitor"