Missing BJT parameter, \$I_b\$ max

Question

After 40+ years you would think I would know this, somehow it never came up, but for the life of me I don't see it.

Typical transistors specs list a plethora of values but one that always seems to be missing is \$I_{b_{MAX}}\$. That is, the maximum current you can pass through the base emitter junction.

Obviously you can't exceed the power rating across the base-emitter diode forward drop, so 626mW/0.7V ~ 900mA, but I have a feeling there is a fusing number for the bonding wire. Perhaps it is the same as the max collector current.. 200mA.

What number is the right one, if any?

I have seen it listed but I’m racking me brain to think on what. It was to do with a question on this site so maybe bc547, 2n3904 or maybe 2n2222..... — Andy aka, Feb 01 '18 at 19:32
It would be related to secondary breakdown due to current crowding, but that's normally spec in the SOA plot for IC-VC. It would be less than IC max — sstobbe, Feb 01 '18 at 19:56
Related: https://electronics.stackexchange.com/questions/158522/bjt-absolute-maximum-base-current — vofa, Feb 01 '18 at 19:57
Generally it is shown in power transistors' (BD139, BD243, 2N3055 etc) datasheets. But I cannot say the same thing for general purpose ones. — Rohat Kılıç, Feb 01 '18 at 20:13
You would think it would be a critical one that was always listed huh... odd. — Trevor_G, Feb 01 '18 at 20:15

score 1 · Answer 1 · answered Feb 01 '18 at 20:31

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Aside from the answers here the NXP BC847 lists a maximum Ibm as 100mA for a <= 1ms pulse.

It is less than the maximum collector current under the same conditions.

answered Feb 01 '18 at 20:31

Spehro Pefhany

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Yup seen those, but no indication of continuous. I was toying with an idea to use the BE as a diode in a current detection circuit for a [different question](https://electronics.stackexchange.com/questions/353458/distributing-an-electrical-current) , but the current gets a tad large. – Trevor_G Feb 01 '18 at 20:34
Maybe 50% if Ic is a good ball-park number in general though. – Trevor_G Feb 01 '18 at 20:36
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@Trevor_G Depends on how brave you are. 10mA is mentioned in the testing of Vbe. – Spehro Pefhany Feb 01 '18 at 20:44

score 1 · Answer 2 · answered Feb 02 '18 at 03:34

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The assembly house will use the same size bond wire for all 3 leads. Unless the base metallization from bond-pad into the base region has narrow metal, I'd use the same # as emitter or collector. Some old datasheets would, with pride, show die photos.

answered Feb 02 '18 at 03:34

analogsystemsrf

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Missing BJT parameter, \$I_b\$ max

2 Answers2