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Interesting- SiC JFETs as the next-gen switching devices

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T3sl4co1l:

--- Quote from: mtwieg on July 09, 2023, 03:09:27 pm ---What specific device are you referring to? In my last posts I was referring to GaN FETs, but I'm guessing you're referring to some SiC JFET.
--- End quote ---

In regards to the topic I suspect (SiC JFETs), though those too.  I haven't seen GaN with less than -6V Vgs, I think; or maybe not even below 0, I forget.  Clearly they aren't symmetrical devices either!



--- Quote ---Just curious, what about that implies they're assymetric?

--- End quote ---

A traditional (diffused?) JFET is symmetrical in that S and D are formed by diffusion (or epitaxy or implantation, whatever works) around a G substrate.  Neither of the present technologies is symmetrical, and likely they specialize further, like having Vds dependencies (as most VDMOS do), rather than Vgd/Vgs (as JFETs or lateral MOSFETs might have).  Obviously, there isn't any way(?) to make a symmetrical 600V something or other, and it would have pitiful gain even if it did (gate operates on channel through huge depletion region), so just one side is made wider and lightly doped as the intended drain.

GaN are a bit something else, because they use a remarkable bit of physics (2DEG) which can probably only be fabbed as a lateral structure, but it's so much more effective than a regular FET that it hardly matters.

SiC is generally like an improved Si for higher voltages, and AFAIK they have all the traditional structures available that are used in Si (except maybe SJ, but it's not relevant in commercial rated MOSFETs i.e. up to 1 or 2kV), so you get JFETs, BJTs, MOSFETs and IGBTs for example, and I even saw an SCR the other day (which was very boutique priced, but definitely had a few advantages, like high forward and reverse voltage rating, and quite fast turn-on).  I think there aren't many ICs in SiC yet, but the planar process is there if anyone needs it (and I'm sure borehole, military and aerospace are deeply contemplating these possibilities).

At least in normal FETs, lateral vs. vertical is fairly easy to tell apart because of the predominant voltage dependency: lateral capacitances are dependent on Vgs (mainly Ciss varies strongly with Vgs, probably Coss does too, as well as varying with Vds due to ordinary substrate depletion), while vertical capacitances are dependent on Vds (Ciss is almost completely independent of Vds, with the only contribution being Crss which is the fraction of Coss where field lines terminate into G rather than S/SS; hence Crss and Coss vary in the same way).  An interesting consequence of this: a vertical MOSFET can be used as a surprisingly linear variable capacitor, if G-S is used as the capacitor, if it's biased to avoid turn-on (so, Vgs(min) < Vgs < Vgs(th), say -30 to 2V), and Vds is biased to adjust capacitance.  Of course since only Crss is changing, it's not a big effect, but it is linear (not dependent on Vgs).

I suppose for GaN, the capacitance voltage dependence doesn't matter much, because you have such a narrow Vgs working range.  So, it just is whatever the datasheet says it is.

Tim

nimish:
These have been around for a while, including the elusive normally off e-mode non-cascode SiC (V)JFET: https://ieeexplore.ieee.org/document/5433483 and http://dx.doi.org/10.1109/ISPSD.2009.5158070 for example. Same tricks as in junctionless transistors and e-mode HEMTs where you manipulate the structure to have the transistor off at 0V (by fully depleting the channel (?) via work functions I think) but I don't think you get the wacky ohmic contact GIT or Schottky diode gate behavior you see in commercial GaN HEMTs.

SemiSouth, unfortunately, went under since they were a decade too early.

The authors of electronics books ought to have updated them to not claim that there are no e-mode JFETs anymore....

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