# Polarity issue in JFET transistor

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?

• It's referencing $U_{GS}$ to $U_P$, not zero volts -- so what's $U_P$? – TimWescott Mar 13 at 3:16
• @TimWescott Up is the pinch off voltage - the voltage that must be applied for a current to flow through the transistor – MagicMan Mar 13 at 3:20
• So in the second set of equations, there's nothing to say that $\left | U_{GS} \right| \ge 0\mathrm{V}$ always holds. – TimWescott Mar 13 at 3:28
• @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. – MagicMan Mar 13 at 3:39