Why are discrete four-terminal MOSFETs so hard to find?

Although the FETs in on a monolithic chip are symmetric, many discrete FETs have a very different structure which tries to maximize the usable surface area as well as source/drain connectivity. The bulk substrate connection on a transistor or chip has excellent current-handling capability, and if one were designing an NMOS LSI chip in which every single transistor needed to have its source or drain tied to a common point, performance would probably be optimized by having the substrate serve as the source or drain for all the transistors. Most chips, however, use the bulk connection as a common base, wasting its current-handling abilities, but allowing the source and drain connections of each transistor to be independent.

A typical "discrete" MOSFET will in fact be not one transistor, but dozens or hundreds of transistors in parallel. Because all the transistors are supposed to have their drains tied together, using the substrate as the drain won't cause the same design problems as it would in an LSI chip. Since the substrate can be very well solidly connected to an outside terminal, such a design will both improve drain conductivity, and also eliminate the need to use top-side metal for the drain connection, thus allowing the use of more metal to connect the sources. Unfortunately, if the transistors are arranged so that all their sources form a "mesh" (good for connectivity), that will leave their bases as isolated islands. While it would be possible to run metal tracks to connect all the bases together, doing so would require either subdividing the source-connected metal into many strips (degrading performance) or adding an extra metal layer and an extra insulating layer (significantly increasing cost). Since each base section has the metal layer for the source connection sitting directly above it, it's much easier to simply have have the bases as well as the sources connect to that.

It is so because if you operate a MOSFET as it is normally done (body diode reverse biased) there is no difference if the Bulk is connected to the Source or to a voltage that is even more negative (N-channel) respectitively more positive (P-channel) than the Source.

If you want to build your own logic gates, transmission gates, etc. with single N- and P-channel MOSFETS the CMOS-IC 4007 is probably what you are looking for, althoug not all of the 6 MOSFETs included can be connected completely random (one P-/N-channel pair is configured as inverter, one pair is partially connected to V+ and GND; only one pair is completly free).

Here are examples.

"Are discrete 4-terminal MOSFETs just not that useful?"

Some potential uses include logic level translation and IC protection. The fourth pin changes the effect of the intrinsic body diode from one which shorts output to input (or vice versa) making the circuit asymmetric, to a diode which is biased off for positive voltage signals. If you look at the datasheet for a Phillips GTL2000, you find the fourth terminal inside the IC is symbolically tied to ground as it is in the physical construction. If you want to duplicate that with discrete devices, you need the fourth terminal to be separate. This allows you to do the same type of translation and protection without the highly restrictive absolute maximum voltage as well as change other parameters such as maximum current, RDS on, etc. of that device. The GTL2000 has 23 FETs (22 for data, one for a clever biasing trick) connected with the sources and drains each brought out to separate pins, the body connections all brought out on the same pin (ground), and all of the gate connections tied together and brought out to a single pin that will be tied to the voltage that produces the desired clamping voltage. Other ICs that are used similarly have similarly limited specs except one from maxim that allows higher voltages but has two fets in series (with higher RDSon for positive and negative voltage) and requires a negative bias voltage or the lower clamping limit will preclude a logic level 0. As a result, if you want a bidirectional logic level clamp and input protector that will protect a device from accidental connections to 13.8V, you need to roll your own. Someone has already mentioned the mosfet analog switch application, which could be expanded to cover a variety of discrete applications. And in some cases separate source pins and body tabs might allow heat sinking high side and floating transistors to the PCB ground plane without an insulator and surface mount devices could be soldered to the ground plane. But this might not provide the desired benefits due to higher internal resistances.

Given that most engineers have probably never held a 4 terminal device in their hands, there are many clever applications that might not have been constrained by the supply.