Hey all;
A transistor thread in this group got me wondering about transistor
characteristics. I have seen low (and very low) on resistance MOSFETS (2
ohms and less) but haven't seen a similar spec in a standard BJT. In fact,
sifting through the Fairchild site, one can sort the MOSFETS based on Ron,
but the same isn't true for the BJTs. Are some BJTs lower Ron than others,
or is low Ron something that BJTs simply are not capable of? It's my
understanding that a MOSFET is more susceptible to damage from static
discharge than a BJT, and so it would seem to limit it's applications as a
switch if you weren't supposed to have any voltage at the drain before
things were powered up.
Some of the mythology about MOSFETs is based on not thinking things through.
They have very high input impedance, which means that unloaded, high
voltages can remain high voltages when applied to the gate. There's nothing
to dissipate the high voltage. A bipolar sees the same voltage on its
base from static discharge, but the relative low impedance of the input
means the "high voltage source" gets loaded down significantly, and isn't
high enough to do damage. The voltage doesn't change, the "voltage
sensitivity of the input" doesn't change, the impedance of the input of
the device changes.
Hence, MOSFETs are more prone to static damage when lying loose. It
was a real problem in the early days (or at least every time a MOSFET
was mentioned, they mentioned being very very careful about protecting
the device), though it was relatively soon after that the manufacturers
started putting protection diodes on the gates to help protect against
this (the protective diodes would not conduct normally, but would conduct
when the voltage on the gate went above a certain level, providing a path
to discharge the high static voltage on the gate).
But the worry about static damage was with the MOSFET lying loose. Once
it was installed in a circuit, you could remove the jumper around the
leads (some devices in the early days came with such a jumper, and
if it didn't all the articles warned about putting your own jumper
on the device before you started soldering). The circuit itself
would generally protect the device, because there was a very real
path to ground from the gate in the form of a resistor or something
else that would provide a path to ground (such as a coil in the case
of radio circuitry). The circuits could still be relatively high
impedance, but even with relatively high value resistors they loaded
things down enough to dissipate the static, if it ever got to that
point in the circuit.
The real exceptions would be circuitry where a MOSFET gate is somehow
open to the world, and there's nothing to protect that input. So
something like an electrometer that depends on a really high input
impedance to do it's work and needs the gate exposed to the world
might suffer. That MOSFET as an "active antenna" in that shortwave
receiver (where the device is more to transform a really high impedance
point of the whip antenna to a lower impedance that the rest of
the receiver can use) might be vulnerable, if the manufacturer didn't
put protective diodes in the circuit. Plug in boards that have a MOSFET
input connected to the connector might be vulnerable, if there is no
pullup or pulldown resistor on the gate that provides a discharge
path for static electricity on the pin.
Michael
