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Testing N-JFET gate - source breakdown voltage (FAKE jfets!) (Successful finale)
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spec:

--- Quote from: SilverSolder on February 01, 2019, 02:17:57 am ---I have a pile of PN4391 N-JFETs on my desk, I'm trying to verify the gate-source breakdown voltage.

The spec says 1uA at 30V.

I am measuring it by grounding Drain and Source, then applying a reverse voltage to the gate (from a 1uA current source), and measuring the current with an electrometer.

The problem is that barely any current flows at all... even at more than 100V compliance voltage, the current is in the low picoamps region - barely measurable...  I maxed out the current source at 120V, but the magic smoke resisted all attempts to get it to budge.

Am I doing something fundamentally wrong, or are the DUTs simply blowing away their specifications?  - made with newer processes that blows away older specs?  ... confused....

--- End quote ---
If that were a bipolar transistor the base/emitter junction would have broken down at between 5V and 20V (normally 7V). The base/emitter junction behaves like a low-power zener diode. In fact, there are circuits that use this characteristic.

When datasheet gives a beak-down voltage, it is a minimum break-down voltage and the fact that the gate source did not break down could indicate that you have very good JFETs.

A JFET looking into the gate is exactly like looking into the base of a  bipolar transistor. There is a diode between the gate and source and the gate and drain, just as there is a diode between base and emitter and between base and collector.

The only way to test a JFET, if you don't have a special JFET tester, is to build a simple circuit which you can run off a 9V battery.

I am not saying that your JFETs are not fakes, just that they may not be. :)
edavid:

--- Quote from: RandallMcRee on February 02, 2019, 07:42:09 pm ---Hmmm, my cheap component tester reliably identified JFETS versus transistors.
Have tested lots of jfets and never seen it misidentify anything.

--- End quote ---

It depends on what version of the firmware you have.  Markus worked on improving the JFET algorithm a couple of years ago... before that it was hit or miss with low current parts.

I don't think high IDSS and high VP JFETs can be tested reliably with the hardware available.

Kleinstein:
JFETs with very low and rather high threshold could be misidentified with the simple testers. The low voltage ones could behave remotely similar to an NPN, while the high threshold ones could be similar to a diode with D and S connected.

I would be surprised if the PN4391 (4-10 V threshold) is mistaken for an NPN. Due to the missing brake-down - it's also unlikely to be an NPN. The analysis from user spec  is quite conclusive in this point.  So it may sill be a JFET of a different type (e.g. PN4393).  With the low leakage possibly really good ones.
spec:
+ SilverSolder

Below is a little test circuit for your NJFETS- nothing fancy but it will tell you if the NJFETs are in fact NJFETs and if they are working,

The circle marked A is a multimeter set to the mA range.

Turn the pot to the fully negative line, connect the NJFET, and then connect the batteries, lower first.

The ammeter should indicate no current or a low current.

Turn the pot to make the gate less negative: the ammeter current reading should increase.

And that is it. :)

https://www.mouser.co.uk/datasheet/2/308/PN4391-1301540.pdf   (surface mount version, which I believe is electrically similar to the PN4391 leaded version that you have)
SilverSolder:
The component tester only came out to play after all the usual tests had been tried, including the circuit provided by @Spec above.  Subsequent tests with a DMM with a diode test function shows conclusively where the junctions are located on the device:



@RoGeorge:  The (fake) "Drain"-"Source" channel has infinite resistance until you exceed about 8.2 volts, where it acts like a zener diode.  If you reverse the current, it acts like a regular diode with a drop of about 0.65V

@spec:  The 120V was across the (fake) "Gate" - "Source" junction (I was trying to measure gate leakage) which is actually the Collector-Base junction, not the EB junction.  It looks like it was able to withstand 120V.


I buy stuff from China all the time - thousands of products a year.  99.9% of the time I am a happy customer where Chinese engineers and workers have done a good job. This crook isn't going to change my mind about that.

Looks like seller has other unhappy customers and should really be banned from selling on eBay with this track record:





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