> Theoretically, unlike the JFET or IGFET, MOSFET's, whether N or P channel
> can be driven with either + or - Vg. That would imply bidirectional
> application.

Not quite, I'm afraid.
An enhancement mode MOSFET (by far the most normal type) (never actually
seen a depletion mode MOSFET) is on when -

- For an N channel, the Gate is more positive than the Source.
- For a P channel, the Gate is more negative than the Source

by a suitable voltage level (see spec sheet) in each case.

In both cases the FET will then conduct resistively and bidirecionally
between Drain and Source.

However,, as others have noted, there is in each case a reverse "body diode"
across Drain & Source which is an inherent part of the FET and cannot be
eliminated. This will conduct when the FET is reverse biases Drain to Source
even if the FET is off.

As noted by others, this can be overcome by operating two FETS in series
drain1 in,  source1 to source2, drain 2 out.
Join gate1 to gate2. Drain1 & Drain2 are now bidirectional inputs. Drive
gate1&2 positive relative to source1&2 to turn on (for an N channel). The
gate-source supply needs to effectively float relative to the input/output
signals. This is a useful tool when necessary (I have used it when
desperate) but can be difficult to drive due to the floating gate signal.



        Russell McMahon

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