How long does it take for a diode to conduct?

On a sunny day (Fri, 13 Apr 2012 20:51:58 -0400) it happened Phil Hobbs wrote in :

OK, yes, I needed some reading up on this, it always fascinates me how Maxwell came to his equations:

It is funny how he uses a 'medium' (ether)...

But I will not stir that pot now.

Anyways, in the thoughts about all this I came to the following conclusions, that may be important to some, especially perhaps to those interested in optical effects.

The conclusion was, that if, late at night, (or any other time[1]) I was looking at a star, and blinked my eye, then that change (is it collapse of the wave function?) would have an effect on that star, and of course then on its environment like its planets too. Looking a bit deeper and adding the 'butterfly effect', where in a semi??? chaotic system a small change can do so much, I can, by simply blinking my eye, destroy and create civilizations on planets around other stars.

When we apply this deep insight into our smaller world, you can deduce, using simple math(?) that by merely LOOKING at obama we can destroy the entire US, let alone by voting for him.

Now this deep insight happened to me this morning, and I thought I had to share it with you, especially Jim, who always has such nice contributions to this newsgroup.

There was something else, but I forgot Oh, it is coming back now, I did not stop at that depth of insight, oh no, I realized that the fact that IF I blink my eye, and when, is set by all the subconscious influences, and the configuration of my neural net set by evolution and past experiences, and that in a way everything is interconnected and fixed, and what I think about what is happening and what we are even, is just a thermal effect- and lets go into what was it .. entropy... and we are like a thermal side effect, and the universe will cool down and become a cold crystal without us life forms as thermal effect moving about just to use a word. If this is true it is challenging, and if it was how did the universe come into being then in the first place if entropy was zero,, so there must be a bit more. This is good, as it would worry me to end up as an ice crystal, and what good is Maxwell on ice. So anyways that is about as far as I came today, hope this did not damage any weights in the old neural net, but even if it did, I had to write this, as this is how it (universe) unfolds. Just be careful blinking your eyes.

optical effects.

Yes, there was a pretty young female that blinked her eye and my world changed. So, I have corroborating evidence for your premiss. Mikek :-)

I've run diodes at +48 volts, on purpose. In the ones I used, current ramped up linearly to about 50 amps in about 100 ns.

No. I had a problem in an H-bridge servo amp where output inductance caused undershoot when the high side transistor turned off. The low-side transistor's body diode could develop more than 12 V forward voltage for several microseconds before it started conducting. I put an "ultra fast" diode across it to prevent the undershoot

Again, no way that can be inductance. Hundreds of V for a whole us?

Scope probing circuits with really high Di/Dt can be a huge challenge, so there may well be a lot of measurement artifact in that. A current probe on the diode might be helpful to determine the Tr of the diode. And, of course, a 700 A diode might be pretty slow, too.

Jon

No kidding! Why can't they make them the same speed as a hundred 10A diodes stuck together, or whatever?

Tim

Here's a question for the gurus here. If the diode under discussion is used to prevent reverse current into a transistor's b-e junction, does this phenomenon eventually cause problems with the transistor and how can it be prevented?

Thanks.

John S

If it allowed the transistor b-e junction to zener, yes, the transistor could be damaged. I can't quantify that; the duration of zenering would be small.

Better to use a schottky, or otherwise limit the base drive.

I fully accept that it is a true phenomenon, but still can't find any literature which gives the cause of the turn-on time, apart from device/lead inductance.

Can you cite any literature on this? I still haven't found any explanation apart from device inductance.

Did you read the app note I posted ages ago, at the top of the thread?

Cheers

Phil Hobbs

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OK since no one 'choked' on this the first time I said it. I would guess it=92s related to the minority diffusion time. Higher voltage parts are =91in general=92 going to have larger intrinsic regions, and the diffusion time should go something like the length squared. I=92m not sure exactly how this model would =91square=92 with John L=92s observation that at higher forward voltages the turn on time was smaller. (Unless, the higher voltage means more minority carriers, so there will be more of the =91lucky few=92 carriers that make it across in less than the diffusion time.)

George H.

Personally, I don't need to know how a diode or a transistor works, because I don't design semiconductors. I use semiconductors, so I need to know how they behave. The best way to determine that is not from academic papers, because they won't be applicable to real-world parts in any hard usable way; I don't know the diffusion profile of a diode that I buy from Mouser, and I wouldn't care to do the math if I did. So I work from data sheets, verified by experiment if there's the possibility anything exotic or unspecified going on.

Things like a hint that a PN diode might turn on slowly, or snap on recovery, is useful because it makes me experiment with real parts when it might matter. The tests I did on the 1N914 were interesting, and only took a few minutes. The effect was clearly not inductive.

Testing parts to destruction is fun and valuable, too.

recombination.

Jamie

Yes it is a true phenomena, but if you are looking for a simple straight-forward explanation you are in trouble. It's not inductance, but rather drift-velocity and recombination times of carriers in the PN junction. Especially at the transition. Any book on the physics of semiconductor devices should cover the effects. This is one reason schottky diodes are so fast. They don't have the delays due to P-N recombination times etc. They are semiconductor - metal construction.

The whole business is quite complex and highly dependent upon the structure of the various devices. Hence generally rules of thumb in this regard are hard to come by without some very exacting measurements. Usually switching times are just measured and listed in device specs rather than to dig down and examine the "why" of the actual device structure.

I appreciate your post and has save me from looking up the detail. I wasn't in denial of turn on time, but this effect didn't correspond with what I've been taught/learnt in the past. As an engineer, whilst I use practical numbers from datasheets and measurement I have found some understanding of what's going on invaluable. I was also assuming that any junction would be small and velocity sufficient to make the measurement impossible. Clearly I am wrong. Many thank for enlightening me.

RL

Thanks, I did look at this App note, but apart from acknowledging the existence of a turn on time, and how to measure it, it still doesn't give a clue as to the cause. Sorry if I can't see the wood for the trees! :-)

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I found it worthwhile to segregate the differences for higher voltage ratings and higher applied forward voltages. They tend to be in opposite directions.

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