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

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SilverSolder:
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....

David Hess:
That seems weird to me also.  Try running the same test on a 2N3904 base-collector junction to make sure that your test setup is working.

duak:
Any idea when the parts were made?  I remember in the late 80s one of my colleagues mentioning that his friend at National Semiconductor said that because the Japanese, and later, most everyone else cleaned up their fabs and processes, the spreads on things like Hfe, breakdown voltages and leakage currents tightened up and really got better.  By then, virtually none of the parts in a lot would have specs in the bottom quartile.  The JEDEC spec for that FET was probably written in the 60's to reflect what could be reasonably delivered and couldn't be changed.  Things have got a lot better.  I remember getting a failure rate  of between 0.1 and 1 %  for US made RAM chips.  When we switched to Japanese, the failure rate essentially went to zero, they were cheaper and used less power.

When you made the leakage current test at 30 V, did you try increasing the temperature to say, 125 C?

Cheers,

SilverSolder:

To avoid going insane, I stopped messing around with sensitive instruments and instead grabbed a handheld meter with a diode test function...  and found 2 diodes, with both cathodes on the pin that's supposed to be the Source!  ...which is the wrong way round for an N-Fet even if the cathodes were connected to the Gate!

Looks like I've got a batch of fake components - they are clearly marked as PN4391 JFETs (and were sold as such), but they behave more like bipolar transistors...  or something!  eBay...  gotta love some of the enterprising folks you find there.

Kleinstein:
One diode from gate to source is normal. With an open gate the JFET can behave odd and might fool you in thinking there is a diode.

A good test to see if it is a JFET base current sink circuit:  Gate with a protective resistor to GND, source to GND with a resistor (e.g. 1-10 K) and than measure the drain current from a voltage source of some 12 V and the voltage over the source resistor.

The gate currents are usually maximum values and sometimes older typical currents. Newer parts may be better in the typical values.

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