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In datasheets they write '1 MHz' in the capacitance vs voltage figures. For example:

capacitance vs voltage figure of a Schottky diode

I suppose it is some sort of a standard to measure the capacitance at 1 MHz, but is there a dependence of the capacitance on the frequency or is it just the dependence on the reverse voltage?

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  • \$\begingroup\$ Faraz, S.M. et. al., Voltage- and Frequency-Dependent Electrical Characteristics and Interface State Density of Ni/ZnO Schottky Diodes contains a graph of capacitance over bias for frequencies from 1 kHz to 1 MHz showing significant rise more pronounced from 10 kHz to 1 kHz forward biased than elsewhere. \$\endgroup\$
    – greybeard
    Commented Mar 24, 2023 at 10:34

3 Answers 3

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The supplier has to measure capacitance using an oscillator and, that oscillator has to use a certain frequency. Given that a nominal value of 30 pF at 1 MHz implies a capacitive reactance of 5305 Ω, it's a fairly easy thing to measure.

For instance, if 1 kHz were used, the reactive impedance would be over 5 MΩ and, simple measurement techniques would be susceptible to diode leakage currents (these are quite high on Schottky diodes).

So, in a nutshell, stating the frequency as being 1 MHz is a means of giving confidence in the measurement method because you wouldn't want to use a frequency ten times lower (or worse) because of "other effects" such as leakage current.

I don't think there are any significant effects on capacitance related to frequency but, the higher you go (up to a certain point where transmission-lines effects take place), the more accurate the measurement is.

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    \$\begingroup\$ The purpose of writing "1 MHz" is now absolutely clear to me. Thank you! And now I think if it were any significant dependence, then the manufacturer would provide curves for several frequencies... \$\endgroup\$
    – Tyo Hoy
    Commented Mar 24, 2023 at 10:16
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    \$\begingroup\$ @TyoHoy when you are ready, please read this guidance document on closing down Q and A sessions. I'm not sure you were aware of it looking at your earlier questions. \$\endgroup\$
    – Andy aka
    Commented Mar 24, 2023 at 10:22
  • \$\begingroup\$ @TyoHoy good chances are that the part mentioned is typically meaningful in switching up to 1 MHz or so. That way the measurement is giving insight for real use. \$\endgroup\$
    – LuC
    Commented Mar 24, 2023 at 13:03
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Generally it is not dependent, and the info is provided merely for reference.

There may be devices which do exhibit frequency dependence. Which is to say, losses, or diffusion effects, or other physical processes are at play, not just your basic depleted junction capacitance.

I recently learned that SuperJunction structures have bias-rate-dependent resistance, which can make the capacitance look anomalously high or low, depending on how the measurement is taken (time between measurement points, measurement frequency, what conversion is used from impedance to "capacitance"). This is most important for MOSFETs (pretty much all types 200V and up use SJ technology), but various types of diodes (particularly higher voltage (>100V) Si schottky) may be using it as well.

I have confirmed this for one MOSFET. I may make some measurements of diodes at some point, but haven't planned on it yet.

I have seen some diodes not rated for capacitance, which is suspicious. Diodes SBR20A200CTFP is an example. I somewhat suspect they are using recovery time as an alternative to rating the capacitance. Note that recovery (as such) should be impossible as a "schottky". I suspect that it's actually highly nonlinear capacitance (with some associated hysteresis loss) at play here, which has a similar appearance to recovery. (And again, I have yet to measure and confirm whether this is the case, that capacitance and hysteresis is manifesting as recovery for this part.)

I haven't seen any cases yet, where such effects are much more than a curiosity, or that can generally get lumped into switching loss. So don't read this as cause for paranoia.

Literature jumping-off point: COSS Measurements for Superjunction MOSFETs: Limitations and Opportunities, Zulauf et al., IEEE Transactions On Electron Devices (2018)
Note the hysteresis loop described.

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Schottky diode junction capacitance depends on its reverse biasing, as you know. At higher frequencies, parasitic effects start to kick in. For example, the inductance tends to cancel out the capacitance if the frequency goes high. Therefore, 1MHz is a relatively proper way to measure the capacitance.

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