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Because MOSFET conduction priciple is based on field contriction.
Due gate is insulated, any charge can affect conduction.
In other hand, BJT conduction is based on electrons recombinations, and due Hfe gain is not so high, even high induced noises don´t affects it so dramactically.
I guess, you are referring to ESD strength in terms of tolerable voltage levels for standarized tests?
I doubt if it's reasonable to generalize the answer, but BJT junctions are basically able to adsorb a certain energy amount by avalanche breakdown. RF transistors with very small structures may be damaged by rather small ESD energy amounts however. It's not correct to say, BJT are unaffected by ESD.
Some MOSFET devices have zener protection circuits, so there ESD strength may be even higher than BJT. Unprotected MOSFET can be damaged however if moderate overvoltages are causing a gate oxyde breakdown.
The mosfet has a SiO2 layer between the gate and BODY, wich the BJT doesn't have, and this material is susceptible to the ESD.
Because the BJT doesn't have this kind of material it is not susceptible to that kind of damage.
If you care about accuracy you won't be so cavalier about
bipolars' ESD capability. It's easy to trash an op amp input's
Vio, Iib, Iio with a couple of hundred volts (hot carrier
charging drifts the E-B junction charavteristics).
A think oxide is more easily, catastrophically damaged than
a P-N junction, it's true. But it doesn't take catastrophe to
make a part not-shippable or not-usable.
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