MOSFET Gate Source Breakdown Voltage

[defaced]

Hola,

So I'm back to playing with MOSFETs again. Specifically the LND150 (http://www.supertex.com/pdf/datasheets/LND150.pdf). What a cool little piece of silicon this is, but alas, I have a question regarding gate to source breakdown voltage (G/S BDV).

Conceptually I understand G/S BDV. If the gate is seperated from the source by more than some voltage (20v in the case of the LND150), the MOSFET can be damaged/die. Cool, makes sense, however, doesn't the source follow the gate more-or-less exactly? So how could the gate separate from the source by more than even the slightest voltage except for screwy bias?

The reason I'm asking is that often in schematics I've seen back to back series zener diodes between the gate and source. The reason usually given is that if the input signal to the gate exceeds the G/S BDV, the MOSFET can be damaged. But since the source follows the gate, I don't see how these zeners really do anything, but since I see them often, I'm assuming I'm missing something.

[R.G.]

The gate is separated from the channel under it by a layer of highly pure glass that is 20 volts thick. The source can only follow the gate if it's hooked up as a source follower, and even then, it has some limits. What happens if the gate is driven negative and the source CAN'T follow it? Also, what happens if there is a static spark that drives the gate positive faster than the source can follow? And finally, what happens when the source simply hits the place where it's conduction can't keep up and the gate continues to climb.

A zener or a couple of them is very cheap insurance.

You're right about the case of the source follower, but it's the not-supposed-to-happen things that kill them.

[defaced]

Exactly what I was looking for RG. Thank you again.