Zerner diode can never get ESD damage?

....The voltage is clamped to Vf in one direction, and to the zener voltage in the other...so a zener diode can never suffer ESD damage?

Voltage breaks oxides. Current (or Joule heating) kills junctions.

A 2kV HBM pulse peaks at about 1.3A and this can fry integrated
circuit zeners with ease. I have designed and used "zener zap"
devices that can be spiked to short circuits by less than a
hundred mA.

Joule energy, Vz*1A*10uS (very rough) says microjoules in a
few cubic microns (integrated) or maybe a hundred cubic microns
(piece-part, close-in junction volume as thermal time constant
limits how much volume the heat can access). Temperature rise
that can be withstood goes to contact barrier system; pushing
the contact in a bare aluminum (cheapo) contact system past
the silicon-aluminum eutectic temperature will entrain aluminum
and spike the junction.

-Designed- "zener" (breakdown of any sort) devices are used in
ESD protection. GGNMOS clamps are one such. TVSes are another.
Difference being "-designed-", for the threat envelope. A 100mW
glass package zener, stands a pretty good chance of not liking
amperes of pulsed current.

If we talk about discrete zener diodes with usual power rating, you probably don't manage to damage it with standard ESD levels. At least I didn't when testing it as protection devices.

Thnaks, so if i have a MOSFET on a PCB....and i am worried about getting ESD damage to the FET gate...then all i need to do is to put a zener diode from gate to source (obviously with cathode to gate)...and then the MOSFET will never be ESD damaged.?

Hi,

Usually a zener should be sufficient to protect a mosfet gate.
But:
* some mosfets have built in protection
* a zener has relatively high capacitance. Check if this can cause problems with high speed switching.
* in none of my designs the gate of the mosfet is connected to a node leaving the PCB. Thus there never was the need to protect the gate against ESD. (ESD always comes from an external source)

Klaus

It depends. Expect the zener diode to absorb most of the ESD energy. Fast rising discharge current combined with diode inductance can cause nevertheless 50 or 100 V overvoltage.