Well I just blew out the G-S junction of a fet. I'm going to redesign circuit a bit... but I was also thinking of adding a ~15V G-S zener. (V_GS max = +/- 20 V) Is there any downside I should think about? I guess I'd need some sort of current limit. (Resistor at a minimum)
But this CB junction thing. So I do have an N-Fet, I then drew the collector (of npn) attached to the gate, base to source... which leaves the emitter flapping around. I don't see how attaching the emitter to some reference V causes anything to turn on? Does the BFT25A have some predicable (lowish) Vcb?
8V? Have you tested that number? Then the reference voltage is in series with the cb junction? I don't need a lot of GS voltage so
8 V would be good enough. How hard (current-wise) can you zener a cb junction?
George, I believe Phil was referring to the use of the CB junction as a low leakage, low capacitance diode. the emitter is left floating. there was a discussion about this a few months back, the thread title is "JFET used as a diode"
(Assuming you wrote about a MOSFET rather than a JFET then) Strictly speaking you didn't blow the "junction" because a mosfet gate has no junction, what happened was you broke down the gate insulation dielectric material.
I think John Larkin reported that +/-20Vgs rated parts typically ruptured above 70V.
Yes, sorry--I thought I'd expanded on this in another post, but it hasn't appeared.
The CPH3910 JFET has a maximum V_GS of -15V. In my recent APD front end, I use a BAV99 series-connected dual diode with the anode connected to the source, the midpoint connected via a resistor to -5V, and the cathode connected via a BFT25A C-B junction to the gate. That bootstraps the already small capacitance and reduces the leakage to femtoamp levels.
The reason for doing it is if somebody opens up the instrument with the APD bias attached, the gate of the FET would get pulled away towards
-220V. The APD bias is current-limited to a few hundred microamps, so this is a complete solution.
Right, I blew out the gate and maybe made a junction. I'm not sure exactly how much overvoltage, but a quick calc would say 5 x 15V.. ~70V. (STN4NF20L) I'm not going to repeat the experiment... only 4 pieces left. :^)