Why are BJTs more reliable than MOSFETs under severe weather conditions?

Question

I read in a textbook (Microelectronic Circuits by Sedra and Smith, pg. 494, (2010) sixth edition) that BJTs are preferred by the automotive industry due to their reliability under severe weather conditions. I understand that temperature affects the carrier concentration, but how does this cause BJTs to be more reliable?

The paragraph in question:

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When you write that you've read something in a textbook, post a reference to the textbook. — Nick Alexeev, Nov 29 '13 at 18:52
@David, I added the reference. Tried to find the answer in the book myself, but it looks like this is some kind of a general statement which was not intended to be proved. This is indeed a very good question. — Vasiliy, Nov 29 '13 at 21:43

score 12 · Accepted Answer · edited Jun 06 '17 at 12:51

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In electronics, when we talk about "environmental conditions" we are not talking about the weather.

Environmental conditions means all of the conditions under which the part must operate that are external to itself. For example, ambient temperature, humidity, mechanical vibration, mechanical shock, liquid immersion, caustic chemical spray, or other factors.

While the weather might affect some conditions like temperature and humidity, if a system isn't specifically made for deployment outdoors, we're more likely to be concerned about conditions generated by our own design efforts, like the choice of whether to include a fan in the enclosure to cool the circuit.

In the case of advantages of BJTs over MOSFETs, they're likely referring to BJTs' typical higher tolerance for ESD events compared to MOSFETs, as mentioned in a recent On Semi application note TND6093/D.

edited Jun 06 '17 at 12:51

Jonas Stein

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answered Nov 29 '13 at 22:44

The Photon

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I thought of ESD too, but I'm not sure that this advantage covers for, say, the potential thermal runaway of BJT due to current hogging when multiple BJTs are paralleled. Also, it feels like the writers refer to more than a single advantage in the book - just ESD does not require generalization like "environmental conditions". – Vasiliy Nov 29 '13 at 23:01
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I thought they might also be thinking of over-voltage events, but I wasn't able to find any references to back it up. Also I agree with Eric's point that the quoted statement might have been written into an early edition (the book was available when I was an undergrad 20 years ago) and not changed to keep up with the times. – The Photon Nov 30 '13 at 00:26
I'd say that you put in *other factors* one of the most important, which is environmental noise and interference. – clabacchio Jan 15 '14 at 10:25
@clabacchio, in the groups I've worked in, we talked about "environmental stresses" and electrical and electromagnetic stresses as two different things. But I don't know if there's a reason or standard backing that up or its just the luck of who I've worked with. Certainly I wouldn't be surprised if people at a different company or from a different country lumped all those things together as "environmental" stresses. – The Photon Jan 15 '14 at 16:52

score 6 · Answer 2 · answered Nov 29 '13 at 23:38

Note that this is the 6th edition of the book; that probably means that it's been around 20 years or more. In the 1990s, BJTs were a mature technology but MOSFETS were still relative newcomers to the field (ha ha). It's entirely possible that the part that you quote was there in an earlier edition and was never revisited.

Why are BJTs more reliable than MOSFETs under severe weather conditions?

2 Answers2