BJT: reverse BE voltage below VEBO - Page 1
Forum section: electronics / beginners

author: fmzambon
author_rank: Contributor
timestamp: November 04, 2020, 08:37:36 pm
content: Hello,I have a doubt about applying a negative voltage to the BE junction of a BJT, i.e. having the emitter at a higher potential than the base in case of an NPN BJT.I know that most BJTs have a fairly low maximum VEBO, often 5 or 6V. And, from what I read, going beyond this limit can permanently damage the BJT by reducing its gain.But what happens if I apply a reverse BE voltage less than VEBO? Does damage still happen, just at a slower rate? Or can I assume that voltages below VEBO will not cause any harm at all, no matter for how long they are applied?Please forgive me if this question has already been answered. I tried searching but I couldn't find an answer to this particular question.

author: Gyro
author_rank: Super Contributor
timestamp: November 04, 2020, 08:52:59 pm
content: Exceeding the VEBOcauses an Avalanche breakdown - that's what causes the damage to the transistor characteristics. As long as you don't avalanche the BE junction, there should be no long term shift in characteristics.

author: Circlotron
author_rank: Super Contributor
timestamp: November 04, 2020, 08:57:07 pm
content: Olde timey SMPSs that used a power bipolar transistor often drove the base slightly negative to speed up turnoff. Those things ran almost forever if not abused. Same with the horizontal output stage of CRT monitors and TV sets IIRC.

author: David Hess
author_rank: Super Contributor
timestamp: November 04, 2020, 10:05:17 pm
content: Quote from: fmzambon on November 04, 2020, 08:37:36 pmReverse Vbe up to breakdown has no damaging effects and higher reverse Vbe extends the Vce rating beyond that of shorting the base to the emitter. Reverse Vbe also sweeps charge out of the junction for faster turn-off.Note that when hfe is low as in power applications and where gold doping is used to reduce carrier lifetime for faster turn-off, limited Vbe breakdown has no practical damaging effect because hfe is already low.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2020, 10:50:50 pm
content: Operation between forward bias, Ib < Ibmax, and reverse bias < Vebo, is 100% acceptable.I suppose the leakage current that flows, as you approach Vebo, could contribute to the wear mechanism as well, but it's very small of course, uA rather than mA. So it basically doesn't matter.The voltage does matter, because the wear mechanism has an activation energy. Below some voltage, no wear occurs at all. Probably like 70% of Vebo or something.So staying with Vebo(max), expect decades, maybe centuries of operation, assuming no other failure mechanism.Tim

author: TimFox
author_rank: Super Contributor
timestamp: November 04, 2020, 11:19:45 pm
content: A classic book on solid-state amplifier noise, C. D. Motchenbacher and F. C. Fitchen, “Low Noise Electronic Design,” John Wiley & Sons, New York, 1973 discussed repairing the damage caused by excessive reverse bias on the BE diode by "annealing" the junction by passing a large forward current through it. They quoted a research paper which demonstrated this effect, but did not recommend the procedure. Instead, they discuss circuit modifications to discrete audio amplifiers to avoid reversing the bias by more than a diode drop, or so. Besides reducing the current gainhfe, the avalanche (from excessive reverse bias) worsened the noise of the transistor.

author: fmzambon
author_rank: Contributor
timestamp: November 04, 2020, 11:31:10 pm
content: Thanks everyoneFrom the replies it seems like it's as I had imagined: it's the avalanche breakdown that damages the BJT, not the reverse voltage per se. So at lower reverse voltages where breakdown does not occur there is no damage.For the record, the BJT that would be subject to this situation is aZTX458(or rather 45 of them) and the reverse voltage would be that of a coin cell, around 3V.Only one thing baffles me:QuoteI thought that the leakage through a reverse biased BE junction was MUCH smaller than that, like in the pA range. At least according to this post:https://www.eevblog.com/forum/testgear/fluke-8800a-design-detail/msg339015/#msg339015Am I missing something?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 04, 2020, 11:59:52 pm
content: Last time I tried B-E as diode, I found the leakage unacceptable (many uA) by 4V or so. And found inverted operation more effective, despite the low hFE:https://www.seventransistorlabs.com/Images/Low_Offset_Clamp.png(The Q12 line is a reference voltage, and the through line gets clamped to <= that voltage.)Though that should still be fine with low uA leakages. I did that years ago, and don't remember anymore what all I tried. Maybe it was partly because I was trying to do something with the collector too?Testing, I find 2N3904 gives:8.13V at 0.18uA8.15V at 24.9uA2N3906: 9.74V at 0.1uAIn any case, leakage increases exponentially around the threshold. It's always leakage, it just might be ~mA in avalanche or ~pA at low voltages. Looks like the threshold is pretty sharp after all, so indeed, you wouldn't expect to see ~uA until near threshold.Tim

author: Circlotron
author_rank: Super Contributor
timestamp: November 05, 2020, 12:18:22 am
content: Quote from: David Hess on November 04, 2020, 10:05:17 pmAhhh... gate drive of fast switching bipolars... a lost art. Same with resonant SCR forced commutation. And soon (maybe already?) with continuous mode reverse recovery management of PFC boost diodes.

author: mag_therm
author_rank: Frequent Contributor
timestamp: November 05, 2020, 12:37:11 am
content: It was common in the old days of switching transistors:Firstly, not to overdrive into saturation.Then, a capacitor across the base resistor was used to snap the base negative.Motorola: "High Speed Switching Transistor Handbook" year 1963 Article 5.7 shows how to optimise the capacitor to remove the base charge during fall time.The method is to observe the "bump" in the collector voltage fall time andadjust the capacitor until the "bump" just goes away, and no more.

author: Circlotron
author_rank: Super Contributor
timestamp: November 05, 2020, 01:33:58 am
content: Also having an inductor in series with the base to control the -di/dt of the reverse base current. Having it too fast would cause the collector current to initially fall very quickly then the remainder would stretch out and gradually tail off. Some details in this book somewhere ->https://archive.org/details/ThePowerTransistorInItsEnvironment/mode/2up

author: fmzambon
author_rank: Contributor
timestamp: November 05, 2020, 09:57:23 pm
content: Thank you everyoneI don't have much equipment (yet), so measuring these parameters (hfeand very small leakage currents) with any accuracy is beyond my capabilities.That's why I have to rely more on theory and less on measurements.

author: fmzambon
author_rank: Contributor
timestamp: November 21, 2020, 10:38:00 am
content: Well, I had to try myselfI connected 10 ZTX-458 NPN transistors in "parallel" and applied 3V reverse BE voltage (see picture. Row 17 is the collectors, row 18 is the bases and row 19 is the emitters. The red alligator clip is positive and the black is negative).First of all I used my good DMM (a Fluke 289) totry tomeasure the total leakage current throught the 10 reverse biased BE junctions. In the lowest range it has a resolution of 10nA, and despite this the value kept oscillating between 10nA and 20nA regardless of whether it was connected or not.That means that the leakage current of each single BJT (at 3V) has to be less than 1nA, otherwise I would have seen something with 10 of them in parallel.Then I used an old old old old OLD (did I mention that it's old?) multimeter to measure the hFEbefore and after leaving the 10 BJTs connected like that for a week. I had to use that crappy multimeter because that one has a transistor tester and I don't have a dedicated component tester. Still, comparing it to itself should give some vaguely meaningful numbers.samplehFEbeforehFEafter#1165165#2170171#3186187#4168168#5174171#6185191#7170174#8205211#9180184#10172175I have no idea what the specs of that old multimeter are, but I fail to see a decrease. In fact the trend seems to be a slight increase, though that's most likely other factors (temperature, the multimeter becoming more optimistic after a week and so on).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 21, 2020, 03:21:42 pm
content: Yeah, like I said, the charge is small, it will take centuries to see an effect. 3V might also be below the activation threshold anyway?Tim

author: magic
author_rank: Super Contributor
timestamp: November 21, 2020, 04:19:31 pm
content: Beta (and pretty much everything else about transistors) depends on temperature. That's probably the actual effect you have observed.The transistor test circuit in DMMs is pretty simple and easy to replicate: 10Ω from emitter to ground, collector to 3V, 220kΩ from base to 3V, measure the voltage across 10Ω. Not very accurate. Maybe some meters are better, but that's what I found in a UNI-T.

author: fmzambon
author_rank: Contributor
timestamp: November 21, 2020, 05:19:54 pm
content: Quote from: T3sl4co1l on November 21, 2020, 03:21:42 pmCenturies should be enoughQuote from: magic on November 21, 2020, 04:19:31 pmI had no idea that the meter circuit could be that simple.I assumed that the meter measured the collector and base currents to calculate the ratio. Or, more likely, that it fed a fixed base current and measured the collector current.But yeah, the circuit you describe makes sense

author: magic
author_rank: Super Contributor
timestamp: November 21, 2020, 05:23:53 pm
content: The simplest 3.5 digit DMMs use very simple ADCs (like ICL7106) which only measure voltages in the range ±200.0mV. Everything has to be scaled to that sort of voltage. The only exception is resistance measurements, where the ADC actually is reconfigured to report the ratio of the resistor to a reference 1kΩ,10kΩ,etc resistor inside.

author: fmzambon
author_rank: Contributor
timestamp: November 21, 2020, 05:34:17 pm
content: Quote from: magic on November 21, 2020, 05:23:53 pmHmmm, ICL7106, that definitely rings a bell.The (old old old old old...) DMM that I used to measure the beta values uses just a single big DIP chip with some discretes around it. I'm not 100% sure, but I've already read the "ICL7106" name somewhere, and it may very well have been while opening that meter to change the battery.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2020, 11:04:33 pm
content: Quote from: Circlotron on November 05, 2020, 01:33:58 amAh, so that is what that inductor is about. My first choice is usually to use a capacitor and voltage step to remove a controlled amount of charge.