Why short source and drain on a JFET transistor?

[giosif]

I am trying to understand a board where, occasionally, I see a JFET transistor (specifically, model MMBF4393LT1) with the source and drain pins on the same trace.
What would be the purpose of doing that?

Thanks!

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[Andy Watson]

The gate junction forms a good, low-leakage diode.

[David Hess]

The gate junction forms a good, low-leakage diode.

And at less cost and with greater availability than an actual low leakage diode.

[giosif]

Ok. Thank you both for the explanation!
Back to "reading" the board now...

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[T3sl4co1l]

Though to be fair, small BJTs (like BFR93) make better diodes -- less series resistance.

Tim

[danadak]

Depends on the JFET. Some cannot tolerate much I in the gate.


Regards, Dana.

[David Hess]

Though to be fair, small BJTs (like BFR93) make better diodes -- less series resistance.

Tim

They do but you can buy JFETs with guaranteed maximum gate leakage specifications while transistors just have to meet their maximum cutoff current specifications which are typically tested to 50 or 100 nanoamps.  If you want low leakage from a transistor used as a diode, you have to qualify and test them yourself.