Measuring Leakage current of a FET

snipped-for-privacy@gmail.com a écrit :

No need of fancy meters. Just get some 10M/100M resistors from gate to ground and a low bias current opamp to buffer/amplify it. Depending on the current value you want to measure, some JFETs opamps will do (at room temperature) or use the Nationals LMC60xx series which will get you into fA. Serious layout is necessary there.

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Thanks,
Fred.

** Groper alert !

** The gate leakage of a JFET is a very small current as the effective resistance is many gigohms. If it were my problem to investigate and if limited to only basic test equipment I might try using a 10nF * polystyrene

• capacitor charged to a few volts DC to apply a gate bias and operate the JFET as a source follower.

Then, monitor the voltage drop across the resistor ( say 4.7 kohms) connected from FET source to 0 volts and see how it slowly falls. Leakage is then given by:

I = 1 exp-8 x dv/dt

Eg , if the source resistor voltage drops by 1 volt in 100 seconds,

I = 1 exp-8 x 1 /100

= 1 exp -10

= 100 pico amps.

BTW:

Great care with cleanliness and insulation will be needed not to spoil the low leakage of the JFET - maybe just build the test jig circuit as a " bird's nest " ie with no PCB and in mid air.

....... Phil

Fred and Phil have given you good advice. The leakage current of a JFET's gate can be very low, less than 1pA under the right circumstances. But be careful in your measurements, it's a strong function of drain current and drain-to-gate voltage. We have a detailed family of curves in AoE, page 137, showing a 2n4868 with under 1pA for Vds under 10 volts, but rising to 2.5nA for Vdg = 30V and Id = 0.1mA, jumping by nearly 10x to 20nA for Id = 1mA. In fact there's a region of operation where the JFET's gate current is nearly 5uA. So you need to know how you want to use your JFET in your design and adjust your measurement scheme accordingly.

You may also want to modify your circuit configuration. For example, if you use a second JFET wired in cascode, with its gate to the first JFET's source, so the critical JFET always has low Vds, you can enjoy a sub-1pA leakage at any operating voltage and current. In fact, such an approach can mean you don't even need to take the painful low-current measurements! But one point, you'll need to carefully choose the cascode JFET, because it must have a high enough Vgs to give the lower one a reasonable Vds at your desired current. While doing this be sure to keep an eye on the wide part-to-part Vgs variation JFETs suffer. One working example I have found, IIRC, the low-noise BF862 can be nicely cascoded with an mmbfJ310. That's just another factoid you can stuff away for later use.

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 Thanks,
    - Win

I should point out these currents are at 25C room temperature, the current goes up dramatically with increased temperature.

That's a pmbfJ310 I've been using (i.e. made by Philips). The batch I have in stock has Vgs of -2.25V at 3mA, and -1.55V at 10mA (it has an Idss of about 30mA). This means a Vds of 1.5 to 2.5 volts for the lower JFET, depending on amplifier current. These are fine operating voltages for my bf862 JFETs and insure very low gate-leakage currents. My amplifier design has a 1.2nV noise level, a 1pA input current, gives -3dB at 45MHz, with a 280V/us slew rate. I don't know its input capacitance, but it's very low.

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 Thanks,
    - Win

Thanks for a good piece of information! I just want to add that it also depends on temperature, as any diode reverse current. About 8% increase per K at 25C.

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Sven Wilhelmsson
http://home.swipnet.se/swi

Thank you everyone for the great notes of knowledge. I am going to try both the methods (by Fred and Phil). Phil, I have seen AN47 by Jim and i assume thats what you meant by building circuits in air. Also, thanks Will i will try these JFETs - i have heard about BF862 as a great choice for low noise applications.

I for one would be interested to have a look at that amplifier circuit. Is it a standard JFET diffamp + common emitter OTA? (ps: when is the next AOE coming out? Alot of the transistors in the present edition are obsolete now, same with opamps etc etc. Will it still be just the two of you?)

A few cents worth of advice for measuring JFET leakage. The AD549 will come in handy, its readily available, and has the lowest bias for buffering or amplifying leakage. This is probabally obvious, but with the other techniques mentioned above, a really low-pass filter will be necessary. Some of the ultra-low leakage JFET transistors have a leakage comparable to their rediculously small noise current. If you put a capacitor on the gate of a cascoded JFET the voltage will random walk more than it discharges.

Be interesting to hear from a semico test engineer to hear how they get those perfectly smooth JFET leakage (and current noise) plots.

/Andrew

In article , Sven Wilhelmsson wrote: [....]

It also varies with the drain current of the FET. This means that in the active area, the curve doesn't look like a diode or a resistor.

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kensmith@rahul.net   forging knowledge

I am still awaiting my opamps to be shipped to me. But i have the OPA637, jfet input opamp with input curret of