# JFET With Forward-Biased Gate Diode

#### Levi Applebaum

Joined Oct 21, 2016
6
I am wondering what equation governs the operation of an n-channel JFET when Vg is positive relative to the drain and/or source. I understand that if Vgs is positive but less than Vt, where Vt is the turn-on voltage of the PN junction, the depletion region will shrink, enhancing the channel as compared to when Vgs = 0. However, once Vgs = Vt, the depletion region is gone, and the channel can be enhanced no further. In this case, the gate will draw current. I understand that this is not what JFETs are normally used for, but I appreciate any help I can get. Thanks!

Since a JFET is symmetrical, Vgs and Vds can be interchangeable. What equations govern each current when Vd, Vs < Vg-Vt?

How about when Vs < Vg-Vt, but Vd > Vg-Vt?

#### Bordodynov

Joined May 20, 2015
2,801
At direct gate currents, the transistor starts to operate in a similar manner to a bipolar transistor. About 30 years ago I found formulas for this mode in one Russian book. I don't think you'll find it now, but I think it's on the Internet. Once it was considered as a promising logical element with very little switching work. If you don't need it for idle curiosity, you will find it. There is a special type of field-effect transistors with zero threshold voltage (with a normally closed channel) that work similarly to bipolar transistors. In Russian they are called BSIT.
https://dmkpress.com/files/PDF/978-5-94120-042-9.pdf

#### Levi Applebaum

Joined Oct 21, 2016
6
At direct gate currents, the transistor starts to operate in a similar manner to a bipolar transistor. About 30 years ago I found formulas for this mode in one Russian book. I don't think you'll find it now, but I think it's on the Internet. Once it was considered as a promising logical element with very little switching work. If you don't need it for idle curiosity, you will find it. There is a special type of field-effect transistors with zero threshold voltage (with a normally closed channel) that work similarly to bipolar transistors. In Russian they are called BSIT.
https://dmkpress.com/files/PDF/978-5-94120-042-9.pdf
Does this mean that the JFET will conduct approximately Ids = ~β*Igs, assuming Vgs, Vds > 0 and β is some positive constant?

#### Bordodynov

Joined May 20, 2015
2,801
Actually, there are two mechanisms working. Normal field and bipolar. The current gain depends on the current. At low gate currents, the gain is higher.