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I am trying to replicate the characteristics of n-channel and p-channel JFET, but I don't understand why voltage Vds has to be reversed in the p-Channel JFET for current to flow. I have these characteristicsenter image description here

which were measured in this measure arrangement. enter image description here

I would have expected voltage to be able to be applied in the opposing direction as well, but in this case I can not reproduce the characteristic given above. If anybody maybe knows why voltage can only be applied in one certain direction in JFET, I am very garteful for any help.

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You need to look at the silicon stack up for P-channel and N-channel JFETs. enter image description here https://laptrinhx.com/junction-field-effect-transistor-225458852/

You can see the PN junction formed between gate and channel. That's a diode and needs to remain reverse biased otherwise it will be a shorting diode and fry. Since that PN junction is flipped between P-channel and N-channel JFETs that means the voltage applied between gate and source also needs to be flipped to remain reverse biased.

Also notice that the JFETs is symmetrical so source and drain are interchangeable. So how do you consistently know which end is supposed to be the source?

By convention the source is labelled to be whichever the reference terminal for the gate, just like a MOSFET. Unlike a MOSFET it's not immediately evident which end it is because the device is symmetrical and would work either way. So it's not specific to the construction of the device. Instead, it's specific to the way the device is used in the circuit. You know which end is the reference terminal for the gate because it's the end that forms a reverse biased PN junction with the center slab of silicon (which is the gate, obviously).

This in turn causes the relative voltage between the drain-source assignments to be flipped between P-channel and N-channel JFET.

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