On Wed, 13 Sep 2017 20:43:52 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
>On 09/13/2017 07:52 PM, John Larkin wrote:
>> On Wed, 13 Sep 2017 16:29:34 -0400, Phil Hobbs
>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>
>>> On 09/12/2017 01:58 PM, John Larkin wrote:
>>>> On Tue, 12 Sep 2017 12:10:40 -0500, "Tim Williams"
>>>> <tmoranwms@gmail.com> wrote:
>>>>
>>>>> "Phil Hobbs" <pcdhSpamMeSenseless@electrooptical.net> wrote in message
>>>>> news:QfydnRoVYIpmXirEnZ2dnUU7-YfNnZ2d@supernews.com...
>>>>>> That's plausible, since the TC of avalanche current is small and negative,
>>>>>> so hot spots would avalanche slightly less. However, depending on the
>>>>>> construction of the diode, there may be some fairly significant lateral
>>>>>> resistance in the epi.
>>>>>
>>>>> That, and sensitivity to defects. Wasn't that the problem with the early
>>>>> diodes (before 1N4007 and the like arrived), they worked majestically well
>>>>> within ratings, but could be toasted at the drop of a hat from a little
>>>>> overvoltage?
>>>>>
>>>>> The implication being, I guess, faults in the junction, or even more likely,
>>>>> along its edge, causing uneven avalanche (or weirder things) and rapid
>>>>> failure under adverse conditions.
>>>>>
>>>>> Speaking of, when were guard rings developed? That, and careful control of
>>>>> surface states (purity, cleaning, and passivation), must've been critical
>>>>> steps towards reliable parts.
>>>>
>>>> Early silicon diodes, and some still I think, were just diced from a
>>>> big diffused wafer. Envision lots of edge damage.
>>>>
>>>>
>>>>
>>>>>
>>>>>
>>>>>> Something like a MELF package would reduce this, because the metal makes
>>>>>> contact over the whole surface of the die. Maybe the metal contact covers
>>>>>> the whole die in the SMT parts as well, I don't know.
>>>>>
>>>>> Hmm, interesting thought. Also good way to get heat out -- one terminal is
>>>>> even in contact with the junction side (for epitaxy or one side diffusion).
>>>>> The lead can share in the first tens of microseconds of heating.
>>>>>
>>>>> For pulses on the order of what JL's talking, it's all about die size, and a
>>>>> little about what's immediately touching it.
>>>>>
>>>>>
>>>>>> I haven't measured it myself, but I doubt that zener recovery is slow at
>>>>>> all. In reverse bias there's no stored charge in the junction, and so
>>>>>> nothing to shield out the applied field.
>>>>>
>>>>> Zener breakdown, per se, is a tunneling phenomenon, which I would think is
>>>>> pretty darned fast, both on and off. Avalanche should have a tail, but it
>>>>> might be so fast (i.e., charges sweep out "instantly" because of the high
>>>>> field and fully depleted junction) that you can't tell.
>>>>>
>>>>> BJTs under avalanche can take tens of microseconds to recover, but part of
>>>>> that is due to the three-layer design*. I'm not sure how much is comparable
>>>>> between BJTs and diodes.
>>>>>
>>>>> (*Hmm, it would entirely come down to the shielding effect of the base
>>>>> layer, no? And likewise, one should expect quicker recovery when smaller
>>>>> R_BE, or a B-E discharge circuit, is used. I should test that.)
>>>>>
>>>>> Regular power diodes are susceptible to dynamic breakdown (applying a high
>>>>> reverse voltage during / just after reverse recovery), and I would expect
>>>>> zeners to be as well. At least there are very few situations, where you'd
>>>>> need to worry about a zener diode's behavior as it goes suddenly from
>>>>> forward to reverse bias. And with the higher doping (of most "zeners",
>>>>> including true zeners, obviously), it would be that much harder to trigger.
>>>>
>>>> There is a second Grehkov effect: apply a high voltage across a diode,
>>>> so fast that it forgets to conduct. When it finally wakes up, it
>>>> breaks down in picoseconds.
>>>>
>>>>>
>>>>> Oh hm, speaking of doping, do they do zener diode families by doping
>>>>> density, diffusion time, or both to cover the range?
>>>>>
>>>>> Oh and and, power diodes are often PIN diodes, which gives a lot of room for
>>>>> charge drift shenannigans.
>>>>
>>>> Right, that's the first Grekhov effect, the drift step-recovery diode,
>>>> which typically uses PIN-structure high-voltage rectifier diodes.
>>>> Combine that with the second one, the sneaky over-voltage thing, and
>>>> you can get kilovolt edges at giant currents in picoseconds.
>>>
>>> And the FCC will have no trouble finding you. ;)
>>>
>>> Cheers
>>>
>>> Phil Hobbs
>>>>
>>>>
>>
>> https://www.dropbox.com/s/bwaulog6mzx8zkn/T222_Copper.jpg?raw=1
>>
>> https://www.dropbox.com/s/q82toc257fv43z8/DSRD_neg-2KV.JPG?raw=1
>>
>> Water cooled, -2 kV pulse, 1.4 ns wide, 100 KHz. That's the DSRD
>> version without the breakdown booster. That was for a tomographic atom
>> probe project.
>
>One of these days I'd like to see that running, if you still have it.
>
>Cheers
>
>Phil Hobbs
I don't think I do. Haven't seen it in years... might be in storage
somewhere.
I wound up using the C-B junction of a 1500V TV horizontal-output
transistor as the DSRD. The doping profile was accidentally right.
That transistor is long gone.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics