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 Post subject: Transistor permanent hfe degradation in avalanche mode
PostPosted: Dec Tue 21, 2021 6:44 am 
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This experiment has been done before by others, but I wasn't convinced enough so I decided to run it myself.

I've picked 6 identical 2n3904 transistors and ran 3 of them in avalanche mode for a little more than an hour at about 2mA current (the Vbe breakdown voltage is around 7.5V, so the total dissipated power is around 15mW) while used the three others as controls. It turned out I didn't need that many.

I used a simple DMM to measure the hfe before and after; the transistors were left to sit for 2-3 minutes before the measurements, so that the temperatures could equalize. This turned out not necessary either.

My results are quite spectacular and I certainly didn't expect this much of a degradation in such a short time. Hfe started at around 180 and the three transistors that were running in avalanche mode showed hfe=60 after an hour.

The only thing I'm yet to figure out is what current is my DMM is using for hfe measurements, but regardless the results are quite conclusive.


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 Post subject: Re: Transistor permanent hfe degradation in avalanche mode
PostPosted: Dec Wed 22, 2021 5:38 pm 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
A bit surprising. 15 mW wouldn't be expected to do a lot of damage, but localized heating (hot spots) could be the problem.

https://www.researchgate.net/figure/Foc ... g4_2978085

Too bad you can't "de-cap" the transistors and look at the surface of the chip. I don't know how close the metal pattern spacing is on a 2N3904, but you might see signs of voltage break over, like little craters or traces. Would really need a good Failure Analysis lab to spot that. I used to participate in FA at Westinghouse Semiconductor and Siliconix. But we had microscopes, SEM, X-ray, etc. and could dissolve the epoxy.

Rich


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 Post subject: Re: Transistor permanent hfe degradation in avalanche mode
PostPosted: Dec Wed 22, 2021 6:35 pm 
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i am curious if different manufacturers have different breakdown for same part.


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 Post subject: Re: Transistor permanent hfe degradation in avalanche mode
PostPosted: Dec Thu 23, 2021 1:27 am 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
Vebo is probably not a production line test. They would measure leakage current at 6 Volts and if it's under 10 microamps, the transistor is good. They would not increase the voltage until the transistor actually goes in to breakdown.

https://toshiba.semicon-storage.com/us/ ... break.html

It can be hard to limit the peak current quickly enough on a production line tester, so testing at some low leakage point is usually good enough. Avalanche, as the word implies, is a rapid and sometimes destructive event.

Rich


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 Post subject: Re: Transistor permanent hfe degradation in avalanche mode
PostPosted: Dec Thu 23, 2021 7:11 am 
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Joined: Oct Fri 11, 2019 3:03 am
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Location: Seattle, WA
My intuitive guess is that no matter how much I limit the current (to 2mA in the above test), the base-emitter junction capacitance holds enough charge so that when the breakdown happens, it will produce a hot spot.
And these micro-breakdowns cause the degradation of the hfe of the transistor, as well as the noise that this circuit is famous for.


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