I'm wondering what determines the polarity conditions in a JFET - is it that this transistor can only operate when the p-n junction is in reverse biased polarization? If so, what about the states of operation that the transistor can be in? There are three of them, and the polarization mode is only one. Don't they result from additional polarity conditions?

While browsing through various books as well as websites, I came across a statement that says: "JFET Transistors can only operate with a reverse bias of the p-n junction, so there is only one way of polarization":

-n-type junction

\$ U_{DS} > 0, U_{GS} < 0 \$

-p-type junction

\$ U_{DS} < 0, U_{GS} > 0 \$

But I have also encountered the definition that: "Depending on how the unipolar transistor is polarized, it can operate in three different areas:

  • in the cut-off area - when \$ |U_{GS}| > |U_{P}|, U_{DS} \$ - any

  • in the active region - when \$ |U_{GS}| < |U_{P}|\$ and \$ |U_{DS}|\$ <= \$ |U_{DS} SAT| \$

  • in the saturation area - when \$ |U_{GS}| \$ < \$ |U_{P}| \$ and \$ |U_{DS}| \$ > \$ |U_{DS} SAT| \$ "

And it says that these areas of operation are methods of polarity at the same time (or more precisely, areas of operation are associated with different methods of polarity, but there is, after all, one) - I completely don't get that!

These definitions are confusing to me. So which one of them is correct? Is there one way to polarize as in the first definition? Does the second definition fit more with the ways of polarity? Could someone please clarify this for me?

  • \$\begingroup\$ It's referencing \$U_{GS}\$ to \$U_P\$, not zero volts -- so what's \$U_P\$? \$\endgroup\$ – TimWescott Mar 13 at 3:16
  • \$\begingroup\$ @TimWescott Up is the pinch off voltage - the voltage that must be applied for a current to flow through the transistor \$\endgroup\$ – MagicMan Mar 13 at 3:20
  • \$\begingroup\$ So in the second set of equations, there's nothing to say that \$\left | U_{GS} \right| \ge 0\mathrm{V}\$ always holds. \$\endgroup\$ – TimWescott Mar 13 at 3:28
  • \$\begingroup\$ @TimWescott Yeah. This is why these definitions are confusing to me. The first says that the method of polarity is one. The second says that depending on a given method of polarization, a JFET can operate in three different areas. I can't make the connection between the first and the second. They are kind of contradictory to me. \$\endgroup\$ – MagicMan Mar 13 at 3:39

Seems to me that you have seen a translation error. It's not that JFETs are 'prohibited' to be polarized, it's just reverse biased. Well, if you bias it directly it doesn't work…

The JFET is a depletion device and in fact it 'pinch off' (block the flow of the current) when the gate is at some voltage below the source (for the N-channel, how much is part dependand). Your second definition has the absolute value signs because for P-channel most signs are reversed.

By the way, be careful: the saturation zone in a FET (junction or MOS) is completely different than the saturation of a BJT (in fact, it's its linear zone…). It's a bad naming that confuses a lot of people.

Up (or Vp) is the pinch off voltage, the threshold of total depletion. If you reverse bias the gate over that, no current passes (there's always some leakage and parasitic stuff, but that's the idea). When the channel is open, the Uds (Vds) determines the kind of response it has on the signal (mostly if amplifies or works like a variable resistor, read the rest of your page)

  • \$\begingroup\$ Yes, it's about reverse bias, thanks for the correction. What I understand from this is that the polarity method is one. In that case, are these transistor operating states (in the cutoff region, in the active region, and in the saturation region) not ways of polarity but... states of operation? I wonder why in so many resources I have used these operating regions are described as " ways of polarity". \$\endgroup\$ – MagicMan Mar 13 at 1:10
  • \$\begingroup\$ @MagicMan JFETs are "supposed" to be operated so that the diode is reverse-biased throughout the entire AC input cycle, both up and down. This usually means some kind of DC bias sufficient to keep the diode out of forward-bias. You can operate it forward-biased -- especially if it is just slightly so. For example, almost all of the two-wire, cheap (no IC with an opamp inside) electret microphones use a JFET that will be biased at 0 V and therefore about half of the audio signal is applied reverse-biased and about half is forward-biased. Not so good. But workable. \$\endgroup\$ – jonk Mar 13 at 5:17
  • \$\begingroup\$ States is not quite convincing… usually we talk about zones of operation and they are somewhat fuzzy. Just like the diode which has a knee in direct bias where it's starting to conduct, all the semiconductor have these soft boundaries. Except thyristors maybe, since there's positive feedback in the trigger structure \$\endgroup\$ – Lorenzo Marcantonio Mar 13 at 16:37

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