Does moderate reverse biasing make transistors age more rapidly? - Page 1
Forum section: electronics / projects

author: Fixpoint
author_rank: Regular Contributor
timestamp: December 15, 2020, 07:24:25 pm
content: In my project I have several 2N3904 transistors whose BE junctions are repetitively reverse-biased to up to -4.56 V (see screenshot). The spec says reverse breakdown could occur as early as -6 V. I never reach that point, but the reverse bias occurs with a frequency of up to 1 kHz. I wonder whether either the negative voltage or this constant "hammering" will make the transistors age faster or will make them suffer other detrimental effects.The BE currents are always below 60 µA, the CE currents are always below 50 µA, at least with my measuring setup using a multimeter in AC-DC mode. (BTW, at the higher currents, the reverse bias is only -1.6 V.)Is it ok to treat the transistors this way? Or do I need to modify the circuit so that the voltage is more contained? If so, what level should I aim for? Actually, I would like the transistors to survive without issues for at least 10 years at 0 to 40°C.

author: graybeard
author_rank: Frequent Contributor
timestamp: December 16, 2020, 02:25:06 am
content: The reverse bias base-emitter current creates hot electrons that occupy traps and the Si-SiO2interface of the base-emitter space charge region. When these traps are occupied they allow a current to flow along the surface which bypasses the base-emitter and makes the effective current gain of the transistor lower.It is called McDonald effect after Bruce A. McDonald who demonstrated in 1969 it was a surface effect by putting a "gate" over the surface that allowed him to modulate the surface current and resulting increased 1/f noise.You do not need to get all the way to breakdown for this effect effect to occur. Typically (but not always) the transistor base-emitter reverse bias specification is far enough from breakdown that you are safe if you stay within it.seeIEEE transaction on Electron DevicesED-17, No 10 Oct 1970 pp 871-878ED-17, No 2, Feb 1970 pp 134-135

author: graybeard
author_rank: Frequent Contributor
timestamp: December 16, 2020, 02:32:43 am
content: In looking at you plot I would zoom in on bottom of the negative going edge to make sure you do not have any ringing that exceeds the reverse base-emitter bias specification. Even a tiny amount of ringing greater than the spec will hurt the transistor over time.I just looked at thespec sheetand ii does specify 6V. The actual base-emitter breakdown on general purpose transistors like this one are usually close to 7V, so <6V seems safe.

author: Gyro
author_rank: Super Contributor
timestamp: December 16, 2020, 09:17:26 am
content: This very question came up just last month, hopefully this will help....https://www.eevblog.com/forum/beginners/bjt-reverse-be-voltage-below-vebo/

author: Fixpoint
author_rank: Regular Contributor
timestamp: December 16, 2020, 09:35:00 am
content: Thank you very much for your replies.In the thread mentioned by Gyro they seem to conclude that the transistor is safe as long as I don't cause it to breakdown.@graybeard, you say "<6V seems safe." On the other hand, you also sayQuoteSo, I am not quite sure what to make of it ...And there is still one question that hasn't been answered explicitly: Is it also safe to subject the transistor to this spikey alternating voltage (up to 1 kHz) like in my plot? I think the discussion so far only handled the non-alternating case. I imagine that this could be a difference for the lifespan of the transistor.

author: graybeard
author_rank: Frequent Contributor
timestamp: December 16, 2020, 11:06:30 am
content: The 6V specification is less than the breakdown voltage to ensure you do not degrade the transistor. General purpose transistors usually breakdown at near 7V. The other thread shows a breakdown of ~8V.Stick to the spec and you will be fine.The shape of your waveform does not matter so long as it never exceeds the transistor specification.

author: Peabody
author_rank: Super Contributor
timestamp: December 16, 2020, 03:23:07 pm
content: I'd like to ask a related question. If you switch high-side power with a PNP transistor, it appears that the transistor also acts in a reverse polarity protection function in that if the voltage on the collector is higher than on the emitter, current will not flow no matter what the base is doing. But this works only up to 8V. In my tests I found that current began to flow at a reverse CE voltage of almost exactly 8V with the base open. I don't remember for sure, but I think this was using a 2N3906.Is this another example of the processes discussed above? Is it something that could be relied upon, or is it a bad idea? I was unable to find any datasheet entry related to this phenomenon.

author: graybeard
author_rank: Frequent Contributor
timestamp: December 16, 2020, 08:02:05 pm
content: Quote from: Peabody on December 16, 2020, 03:23:07 pmIf the collector of a PNP is made more positive than the emitter, with the base open, you are forward biasing the base-collector junction and reverse biasing the base-emitter junction. If this voltage is high enough you can make the base-emitter junction breakdown.PNPs are less susceptible to McDonald effect then NPNs because the the carriers need to gain more energy to fill the Si-SiO2interface traps. However degradation still occurs at a much slower rate that it would in NPN.Is this another example of the processes discussed above?yesIs it something that could be relied upon, or is it a bad idea?bad ideaI was unable to find any datasheet entry related to this phenomenon.That is because it is not a normal mode of operation.

author: Peabody
author_rank: Super Contributor
timestamp: December 16, 2020, 09:44:35 pm
content: Ok. It just seemed it might be reasonable way to implement reverse polarity protection for something like an 18650, as opposed to having to add a reverse-oriented P-channel mosfet to take care of that. The PNP would handle both the switching and the protection. And you know, it would only be for when the battery is inserted backwards by mistake, which hopefully wouldn't happen all that often.