Depletion mode MOSFETs
Depletion mode MOSFETs are cool. They are naturally switched on; they conduct between drain and source with no voltage on the gate. They act much like a JFET, which requires that you pull the gate 3 or 4 volts below the source in order to pinch off the flow of electrons between drain and source. EDN has a neat design idea about them this month. I always thought of Supertex (pdf) for these parts, they make high-voltage depletion mode FETs that you can use to make start-up circuits for line-operated power supplies like in the design idea. I am exploring them for a Harley generator regulator that I have been (re)-designing for 20 years. I got an article from Advanced Linear Devices (ALD) that talked about their really cool zero-threshold FETs, enhancement mode FETs that start to turn on just above zero volts. I had to reject the story because it was too much like an app note for their cool energy harvesting modules, but there was one cool line I cut and pasted for the blog:
In contrast, other depletion-mode MOSFET manufactures use the more common vertical DMOS structure. These devices are "medium power", with considerably higher threshold voltages (>1V) than Advanced Linear Devices’ EPAD ® products. Medium power depletion-mode MOSFETs are made by Clare Inc.; Infineon Technologies Inc.; Philips/NXP; and Supertex Inc. High power depletion-mode MOSFETs are made by IXYS Corporation.
So there you have it, from a company cool enough and confident enough to tell you who else makes parts similar to theirs. Here is a nice ALD app note about depletion FETs (pdf).