How susceptible to ESD damage are JFET gates?

Question

Are the gates of JFETs protected against ESD transients? Is it best to include back to back zeners like with MOSFETs?

Answer 1

Because the gate-source voltage of (say) an N channel JFET is controlled from around 0 volts to anything down to -15 volts, a zener diode isn't normally needed to restrict positive ESD. This is because the gate-source region will act like a forward biased diode with positive levels on the gate and this will "normally" protect: -

If the gate is forward biased, because of the PN junction, it will conduct (just like the base on an NPN BJT conducts current to the emitter).

A MOSFET gate-source voltage does not have this "feature" built-in hence, if you need to supplement the protection because of potential ESD, you might consider back-to-back zener diodes.

Answer 2

In the forward direction there's nothing to worry about- the junction conducts. In the reverse direction the diode will break down at some voltage, however unlike the "O" Oxide insulator in a MOSFET, the breakdown is reversible provided not too much current is passed through the junction. You have to be a bit careful because the power is much higher at the relatively high breakdown voltage than it is at the forward voltage, so probably the maximum current is much less. You can use a curve tracer to find the breakdown voltage provided the current is kept reasonably low (perhaps tens or hundreds of uA).

In general you should treat all semiconductors as ESD-sensitive, but JFET gates are not particularly ESD-sensitive.