Are 4 terminal FETs (G, D, S, B) available? If so, what are they used for?

Check out the SD214

It was a very nice MOSFET switch. There were applications where it was a good idea to tue the substrate to the negative rail, though it would mean that you'd a more positive voltage on the gate to turn the thing on when the source was appreciably more positive than the substrate.

-- Bill Sloman, Nijmegen

I don't know that they are available, but I've seen 'em...

With the body to GND or -V, you don't have to worry about the body diode conducting and shorting out signals, so you don't have to cook up hacknied schemes like this

to keep the output from being pulled down by the diode (notice the 1N914 down to the 2N4401 to saturate the amplifier when it's unhooked). So it's like a JFET, but even better because the gate isn't a diode.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

You can buy a BSS83 from Farnell in the UK (order code 1081312)

(Stupidly expensive, but perhaps useful sometimes.)

If you design CMOS chips, then all of the MOSFETs have a backgate terminal. Depending on the processing, you may be able to connect the backgates of different transistors to different circuit nodes (so you could connect the backgate of each FET to its own source), or on some processes, all of the N-channel devices (or sometimes P-channel devices) have their backgates unavoidably tied together to the same node (that is also the substrate, i.e. the backside of the chip, and usually one of the supply or ground rails). Even if it is possible (in the process) and desirable (in the circuit design) to separately tie the backgate of each transistor to the source of that transistor, this is sometimes avoided because the layout can be much more compact (and therefore cheaper) if most or all of the backgates are tied together.

Chris