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The idea struck my mind while I was looking at this typical application for the PCF8563.

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This RTC requires typically 250nA (interface inactive, like in circuit in power-down), but a general-purpose diode like the 1N4148 already leaks 10% of that. The gate reverse current of a JFET is only a fraction of that; max 1nA for the MMBF4391.
The whole circuit is very low voltage and dito power. Are there reasons not to use a JFET as a blocking diode here?

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They do make low-leakage diodes - on Digikey I found a diode with 1nA leakage at 125V, and if you want to spend $16, you can get one that has only 3pA leakage. If powered-down operation takes 250nA, then one of the low-leakage diodes will lose less than 0.4%. – W5VO♦ Dec 2 '10 at 15:52

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1N4148 is a general-purpose, legacy diode that you shouldn't use if you're looking for some level of performance.

From a very cursory glance at Digi-Key, the BAS116 (in stock, 0.25/10, 0.035/reel) has a typical ~400 pA, max 5 nA leakage at 75 V, 25 °C.

Not sure if it's an actual "series", but BASx16 seems to go to BAS416, a 75 V, 200 mA diode with 3 pA typical, 5 nA max reverse leakage @ 75 V, 25 °C. In-stock at Mouser.

The JFET you linked (MMBF4391) doesn't shut off until -10 V (though the MMBF4393 in that datasheet shuts off at -3 V), and the quoted 1 nA leakage is at -12 V (for either -1 or -3), which you can obtain? "Shutoff" is defined as where the JFET conducts 10 nA.

The diode's leakage is specified at 75 V, so if you're running it at 3-5 V the leakage will be much lower.

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3 pA vs. 5 nA, what the heck?! That's a factor 1000! But I admit, I should/could have had a look at low-leakage diodes first. That said, the JFET doesn't do bad at all, even better than the BAS416 (1nA vs. 5nA, max. values, which are the only ones that count) – stevenvh Dec 3 '10 at 8:09
@steven: Added more to answer about the JFET. It may be an interesting use, but it seems somewhat odd. Using maximum/minimum parameters when characterizing the power use of your device is extremely conservative and is more of an engineering-side thing than sales-side. – Nick T Dec 3 '10 at 15:48
Well, I'm not a sales person! If you design with typical values, the Law of Conservation of Misery says that the actual value will be closer to the maximum specified. And your circuit may not work, or at least not to its specifications. Worst thing is that you can't even blame the manufacturer. – stevenvh Jun 16 '11 at 13:41

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