Zener Diode Capacitance - Page 1
Forum section: electronics / projects

author: mawyatt
author_rank: Super Contributor
timestamp: May 09, 2022, 03:22:42 pm
content: This is a topic you normally don't see discussed much, and most data sheets lack. On Semi has a document "Zener Theory and Design Considerations" which is a nice reference.https://www.onsemi.com/pub/Collateral/HBD854-D.PDFWe developed an add-on DC Bias Fixture for our LCR meters (TH2830 & IM3536) and measuring Zener/Avalanche & diode capacitance was the first task ahead.https://www.eevblog.com/forum/projects/bias-network-for-lcr-meter/Our interest lies in the detailed capacitance vs voltage behavior of various PN junctions including conventional, SB and breakdown (zener/avalanche) types for some advanced analog signal processing efforts.For more details on the measurement information see the above thread, and the ON Semi PDF starting on page 24. Note the relative simple setup in the above thread vs the more complex setup in the reference (Figure 20, pg 25).Here's an example of a 1N4742A Breakdown Diode Capacitance vs Voltage behavior. Note that at the breakdown voltage the significant increase (decrease in impedance) in apparent junction capacitance, this is somewhat misleading because the effective junction capacitance isn't actually increasing but the PN junction conductance is and the meter interprets this as an increase in capacitance. Included some other popular diode measurements, SB340, 1N5817, 1N4007, 1N4148.Also, when making these kinds of measurements on PN junctions, and other highly non-linear devices (certain electrolytic capacitors), one must be careful and use a very low voltage excitation, otherwise the non-linearity will "rectify" the AC excitation waveform and introduce errors. You can actually "see" this with the DC Bias adapter mentioned by using the Monitoring terminal with a Hi Z DVM, leave the Bias terminal open for no DC bias. Now apply the AC LCR stimulus and note the DC DVM DC Bias readingAnyway hope some folks find this useful.Edit: Added 1N4742 corrected (with text) graphs using Screen Shot instead of Preview.Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2022, 06:19:42 pm
content: Nice!What's the "V2" curve, it's flat?Tim

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 09, 2022, 07:14:08 pm
content: Sorry to nitpick. OK, maybe not.You need to show both capacitance and loss, especially in an exploratory case like this. You need to state whether you're using a parallel or series model. It might make a huge difference. Or none! Finally, even though this probably isn't terribly frequency sensitive, you need to state the frequency. You mentioned level is important, so it should be on the graphs too. The danger in omitting this stuff is people are likely to look at those graphs and extrapolate to conclusions that might not be valid.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2022, 10:18:52 pm
content: I'm assuming it's, either whatever an LCR meter says is capacitance, or just magnitude impedance (well, conductance) assumed as capacitance. But if it's a proper vector impedance analyzer, agree, that would be nice to know.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: May 09, 2022, 10:54:03 pm
content: Quote from: T3sl4co1l on May 09, 2022, 06:19:42 pmThanks, although not sure what V2 you are referring too?Best

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2022, 11:07:50 pm
content: The,The first is 1N4742 so this seems redundant, but it's rather different with the apparently flat(!?) capacitance before breakdown!.....Oh it's just zoomed to the edge there, isn't it? I see.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: May 09, 2022, 11:21:31 pm
content: Quote from: Conrad Hoffman on May 09, 2022, 07:14:08 pmNitpick all you want!!Evidently the text on the graphs didn't get the frequency text get included, these are all at 1MHz just like the usual data sheets use (was on graphs but didn't get copied when using Preview from Clipboard)!!Also, this is the reported Cseries from the IM3536, didn't include the series R because we don't require that data at the moment and it wasn't recorded.Anyway, thanks for spotting the missing text....now need to use another way to port the graphs, maybe use a screen shot.Best,

author: mawyatt
author_rank: Super Contributor
timestamp: May 09, 2022, 11:55:34 pm
content: Quote from: T3sl4co1l on May 09, 2022, 11:07:50 pmYes this is just zoomed in to show the breakdown region of a 1N4742, and as mentioned the text didn't get thru the clipboard to preview transition. Also noted the Y axis didn't get the pf designation!! Really botched these graphs up, need to find another way to capture these and include the text in the capture.Indeed the capacitance is almost flat vs. voltage around and up to the breakdown point, and according the the On Semi reference the capacitance doesn't change much during the breakdown. This makes sense as the PN junction capacitance is generally viewed as a parallel plate analogy, where the depletion region forms the dielectric between the charged layers which resemble plates in a parallel plate analogy. Reverse voltage causes the depletion region to expand and thus move the charge layers further apart and reduce capacitance. During avalanche any charges in the depletion region get swept away and contribute to PN junction current but not displacement capacitive current.Anyway, an interesting characteristic of breakdown in PN junctions where it doesn't increase capacitance during breakdown as one might expect, but thinking about the solid state physics does make sense regarding On Semi's statement:ZENER CAPACITANCESince zener diodes are basically PN junctions operated in the reverse direction, they display a capacitance that decreases with increasing reverse voltage. This is so because the effective width of the PN junction is increased by the removal of charges (holes and electrons) as reverse voltage is increased. This decrease in capacitance continues until the zener breakdown region is entered; very little further capacitance change takes place, owing to the now fixed voltage across the junction. The value of this capacitance is a function of the material resistivity, ρ, (amount of doping − which determines VZ nominal), the diameter, D, of junction or dice size (determines amount of power dissipation), the voltage across the junction VC, and some constant, K. This relationship can be expressed as:Cc= nth root ((KD^4)/pVc)After the junction enters the zener region, capacitance remains relatively fixed and the AC resistance then decreases with increasing zener current.Best,Edit: Added screen shot graphs for 1N4742 which includes missing text when Preview was used, see original post.

author: mawyatt
author_rank: Super Contributor
timestamp: May 10, 2022, 04:40:17 pm
content: A few more popular Zener Diodes capacitance plots (used the screen capture instead of Preview so the text wasn't dropped as in previous graphs)Also, included both Cseries and Cpar. Note that Cp better represents the diode capacitance during breakdown, also the impedance phase angle begins to deviate from an ideal capacitance angle of -90 degrees as breakdown is closely approached and was -35 & -62 degrees for the 6.8V and 4.7V zeners at the last data point shown.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: May 10, 2022, 07:06:52 pm
content: Wow, the reversed 1N4148 has less than 1pF, nice measurements!Quote from: mawyatt on May 09, 2022, 03:22:42 pmThat was interesting, too, thank you.

author: mawyatt
author_rank: Super Contributor
timestamp: May 10, 2022, 07:40:07 pm
content: Quote from: RoGeorge on May 10, 2022, 07:06:52 pmYes the 1N4148 has a low capacitance. SMD types are usually around 0.8pF. Here's one in a SOD-323 package with a zero bias cap of 0.657pF, measured at 1MHz. You need a good stable LCR meter and DC Bias Fixture to do this, and the DC Bias Fixture we posted seems to fit the bill nicely and cost less than $50 to build (not including design or build time of course)The mentioned thread on mislabeled 1N4148 diodes was also measured with this fixture, and the fixture used 1N4148 diodes!! Thank goodness we used the proper onesBest,

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: May 10, 2022, 08:56:09 pm
content: Great stuff!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2022, 01:23:21 am
content: Interesting; Cj(max) has always been 4pF or so, apparently it's usually much better than that now. JEDEC specs, go figure?Which is very much in line with clamp type ESD diodes, the kind with bandwidth into the low GHz for some of them, with similar capacitive loading, and clamping into a paired TVS or board supplies. I've never been shy to use 1N4148 or relatives for ESD.Tim

author: mawyatt
author_rank: Super Contributor
timestamp: May 11, 2022, 03:46:43 am
content: It seems some "others" have a different view of the 1N4148 and what JEDEC represents!!https://www.eevblog.com/forum/projects/incorrect-labeled-1n4148-diode/What I found equally surprising with the very low 1N4148 capacitance, was the rather high value capacitance of the low power 500mw Zener diodes.Yes, over the years we've used the 1N4148 in many places including ESD and over-voltage protection.Makes one wonder if some of the more expensive "special purpose diodes" are nothing more than a 1N4148Best,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2022, 06:41:37 am
content: Repurposing and selection/testing are certainly nothing new in the industry...PADs are just JFETs, innit? If you don't mind the Rds(on) ==> RS, of course!CRDs too, JFETs strapped the other way. Though who cares as they're essentially obsolete. (Depletion MOS is moderately available nowadays, greatly improving a lot of those applications.)Apparently some BJTs can have quite low leakage (C-B) too.Glass packaging is obviously a nonstarter for low leakage diodes. Proper 1N4148s at least, are supposed to be gold doped, which will increase leakage. They tend not to be very good. If they're just doping them higher though, they might be okay? But then they'd be like BJTs, 10s or 100s of ns t_rr, nevermind. Some like this are available though (options for higher voltage SMD diodes are thin, 200-400V being available with average ("fast") recovery).Wouldn't be surprised if they use the same dies to supply multiple zener numbers. TVSs too. A zener doesn't necessarily need to handle surge, while a TVS doesn't have to be accurate; they might well be selected from each other (do TVSs have bimodal distributions of Vr?). They should still be scaling by capacitance though (if not necessarily by power level, which is always package limited anyways, who gives a crap if it's "1/4W" or 300 if it's still a SOD-123?).1N400x, haven't been, for a long time. Been themselves I mean. Even a 4001 breaks down typically over 400V. 1N4007s break down like 1600V. When they tell you in the datasheet they're binned and you have your choice of 50-400V or 600-1000V parts, they aren't kidding. They aren't telling you what they really are, of course, why would they want to be informative; but it's been this way for probably a really long time.Also TL062, 72, 82... whatever the hell that was about.I wonder which all 2Nxxxx's were bins of identically manufactured dies. Things like 2N2222 vs. 2N4401; 2N5088 vs., well not sure what else in JEDEC, but BC847 has a similar spec I think; etc. I mean, literally thousands of long forgotten numbers in there, nevermind even just the more obviously binned series, 2N3903/4, etc.Tim

author: magic
author_rank: Super Contributor
timestamp: May 11, 2022, 08:37:49 am
content: I still haven't seen a sensible explanation what they meant by TL072 and TL082, but TL062 is a very different partNothing beats TL4581

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 11, 2022, 09:32:57 am
content: Is TL4581 a typo for "1458" and they just went with it?Tim

author: magic
author_rank: Super Contributor
timestamp: May 11, 2022, 10:45:31 am
content: No, it's an alias for NE5532.Don't ask

author: mawyatt
author_rank: Super Contributor
timestamp: May 11, 2022, 01:03:06 pm
content: The masks and process setup to make a semiconductor wafer of die is very expensive, so using one die type wafer for many different part numbers makes sense. There's certainly not that many fabs making discrete diodes or transistors as what are considered as manufactures. I mean you could probably find 100 "manufactures" of 1N4148 diodes, but likely only 5 actual wafer fabs and 15 package sources. Same situation way back in the tube days. RCA, Sylvania, GE and so on made specific tube types and swapped, so a Sylvania 6L6 was from RCA and a RCA 12AU7 was from Sylvania and such.All this makes sense as the economy of mass production where the non-recurring costs like masks and semiconductor process setup get smothered by the massive quality of product, how else can you get a 1N4148 for $0.01Best,

author: oz2cpu
author_rank: Frequent Contributor
timestamp: May 13, 2022, 05:02:21 pm
content: i like your curves mike, what software do you use together with the TH2830 ?to make the curves ?I also got an TH2830

author: mawyatt
author_rank: Super Contributor
timestamp: May 13, 2022, 05:44:23 pm
content: This was done on a Mac using Numbers which is a standard Mac application.Forgot to mention, these measurements were with the Hioki IM3536 and not the TH2830. Soon after we acquired the TH2830 we needed measurements up to 1MHz, thus the IM3536There's some sort of axiom or something that says, "Never buy what you need, always buy greater, since your needs will soon grow beyond what you have!"Best,

author: razvan784
author_rank: Regular Contributor
timestamp: May 14, 2022, 09:22:21 am
content: Quote from: T3sl4co1l on May 11, 2022, 06:41:37 amFairchild and National Semiconductor used to have "Process" number designations for discrete transistor designs. Datasheets for the usual part numbers included (and some still do) statements such as "Sourced from Process 20". Old databooks from the 70s contained Process-specific datasheets in addition to part-specific ones. These can be found online. Attached some representative examples from 1977 and 78.Interfet do the same, they call them "Geometries"https://www.interfet.com/geometry-types/. So these companies are / were pretty transparent about this.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 14, 2022, 10:19:08 am
content: Yeah, have seen "process" before, though no docs on what all they mean by that.Huh, somehow never realized 2N2222(A?) is gold doped...Tim

author: mawyatt
author_rank: Super Contributor
timestamp: May 14, 2022, 12:40:24 pm
content: Had completely forgot about those "Process Types" in the old National Data Books, thanks for the reminderBest,