diode or diode connected FET?

[sempjam]

I'm trying to get to grips with when you would want to use a diode connected FET instead of just a diode.

In my particular example I'm thinking about an rf dc rectifier (diode + low pass filter). In many cases I see people using a MOSFET with shorted gate/drain instead of a Schottky or other high frequency diode.

I'm wondering if anyone knows what are the advantages of one configuration over the other?

Thanks!

[T3sl4co1l]

Something like a PN4117 is sometimes used as a low leakage clamp.  Tie D+S together and use G to (D+S) as a diode, A-K.  This has rather high internal resistance, ca. 5kohms, so it makes a poor diode, but that doesn't matter much for something like an electrometer.

The recovery won't be any better than usual, because it's minority carrier conduction.  Normally you don't forward-bias the gate.  I'd expect the RF performance is worse than using the JFET properly (which works on fast majority carriers).

MOSFETs can be "diode strapped" (gate to drain), but this is only useful for generating a characteristic voltage (e.g., in CMOS current mirrors).  The body diode (source to drain) remains.  In monolithic circuits, the substrate is a separate connection, so this is okay, but four-terminal discrete MOSFETs are essentially hen's-teeth nowadays.

For RF rectification, schottky diodes are preferred.  They're fast (majority carriers, without having to worry about gates and amplifier bias!), and leakage isn't much of a problem.

Tim

[sempjam]

Thanks Tim! Much appreciated