JFET testing as 2 Diodes

Ken schrieb:
> I have used two different GM328 transistor testers that have both tested
> J111 and J105 JFETs as 2 Diodes. Why is that?

Could be a bug. What firmware version is running on those? I had a 
similar problem in the past when my transistor tester would identify a 
npn Transistor as 2 diodes. I updated the firmware to the last release 
from the Wiki (AVR Transistortester) and the problem vanished.

Technically this is not unplausible. The npn structure of a transistor 
is similar to a double diode. Same goes for a JFET, where the gate is 
insulated from both drain and source by a pn-junction.

Thank you for your reply, Axel.

The firmware is version 1.11K on the new tester.

I understand the construction of the JFETs.

The issue may be the J111 and J105 have a higher VGS (over 5V) and the 
tester does not correctly identify them. It could be the JFET spec. 
Because the tester correctly tests all JFET with a VGS under 5V such as 
the J112, J201, 2N5485.

I am willing to give updating the firmware a try.

Ken C. schrieb:
> The issue may be the J111 and J105 have a higher VGS (over 5V) and the
> tester does not correctly identify them. It could be the JFET spec.
> Because the tester correctly tests all JFET with a VGS under 5V such as
> the J112, J201, 2N5485.

Right. If the pinch-off voltage is higher than 5V, then the tester 
cannot drive the JFET to the point where Ids becomes zero. Still it 
should not identify the device as two diodes, because the Drain-Source 
connection is low-resistance (only 3Ω for the J105).

It would be a very odd double diode device where the two separate 
kathodes are connected by a low resistance like that.