

Gallium Nitride Power Transistors Priced Cheaper Than Silicon - rbanffy
http://spectrum.ieee.org/tech-talk/semiconductors/design/gallium-nitride-transistors-priced-cheaper-than-silicon

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kazinator
This article has more meaty info:

[http://spectrum.ieee.org/semiconductors/materials/the-
toughe...](http://spectrum.ieee.org/semiconductors/materials/the-toughest-
transistor-yet)

Highlights:

> _In the GaN FET, on the other hand, the two-dimensional electron gas already
> exists naturally. So a positive voltage applied to the drain immediately
> pushes current from source to drain. Thus the amount of current is varied by
> applying a negative voltage to the gate, which restricts the number of
> electrons available to flow from source to drain. A large enough negative
> voltage turns off the flow altogether. Thus in contrast to a silicon FET,
> which is normally off, a GaN FET is normally on._

(Somewhat misleading; silicon FETs that are "normally off" are enhancement-
mode transistors. Of course, there are depletion-mode MOSFETs, and of course
JFETs. There are even power MOSFETS that are depletion mode, see here:
[http://www.ixys.com/documents/appnotes/ixan0063.pdf](http://www.ixys.com/documents/appnotes/ixan0063.pdf)
In any case, this confirms the new transistor to be a depletion mode device.
This has implications for biasing which could be inconvenient in some
applications. In any case, it means it's not simply drop-in replacement for
enhancement-mode MOSFETs in existing designs.)

> _One of us (Mishra) has succeeded in making bipolar GaN transistors. But
> they are not yet as reliable as the FETs because at the moment it is very
> difficult to make p-type material good enough to use in a bipolar
> transistor. Applying electrical contacts to the material, as is necessary to
> connect the device into a circuit, often wiped out the semiconductor 's
> p-type character._

If these challenges are solved, the idea of a new kind of BJT is exciting.

~~~
csirac2
Biasing is indeed non-trivial, and most designs require temperature
compensation to keep drain current constant. In fact, it's pretty easy to fry
a GaN part without proper bias sequencing at power-on and off. At least it is
with the parts I'm using.

Edit: parts like MAX881R [1] make it pretty easy though :)

[1]
[http://datasheets.maximintegrated.com/en/ds/MAX881R.pdf](http://datasheets.maximintegrated.com/en/ds/MAX881R.pdf)

~~~
madengr
I've used the MAX881 for some lower power GaAs, but it's limited to a few mA
gate current. TI has an opamp I used that can drive capacitive loads with +/\-
30 mA, so it makes a good gate driver, whilst allowing a decent amount of gate
bypass capacitance. For temp comp I have found once you get the temp vs IDS
curve, it's identical for devices within a lot, and only need to offset for
pinchoff, which varies device to device within the same lot.

------
csirac2
I've been working with GaN parts for RF after being out of the hardware game
for a few years. GaN was around but nowhere near as prolific as it is now. I
thought SiC would have made more inroads than it has. GaN parts just seem
incredible in every way (to someone like me at least) - amazing performance,
efficiency, power handling in such a small package, more impressive impedance
matching on wideband parts - it's just a shame the basic problem of harmonics
still hasn't been whisked away by magic :)

~~~
madengr
Still issues getting the heat out, but yes, GaN is pretty amazing. 10 years
ago I was load pulling and matching sub 1 Ohm (24 mm) HFET die for 12 watts.
Now you can get 120W in the same periphery.

~~~
dskhatri
Packaging is indeed critical to get heat out. There have been some cool (no
pun intended) developments on this front such as the DirectFET [1], LFPAK [2]

[1] [http://www.irf.com/technical-
info/whitepaper/directfet.pdf](http://www.irf.com/technical-
info/whitepaper/directfet.pdf) [2]
[http://www.nxp.com/documents/leaflet/939775016838_LR.pdf](http://www.nxp.com/documents/leaflet/939775016838_LR.pdf)
[3]

~~~
madengr
Interesting. I have never seen RF power parts in flip-chip, but I suppose
there is no reason why they couldn't. Now Cree did have a 25W, 3 GHz FET in
SO-8 package, which is kind of strange given package inductance, but they are
have moved to QFN now.

------
ChuckMcM
These are pretty cool, this is a datasheet for the 100V part : [http://epc-
co.com/epc/Portals/0/epc/documents/datasheets/EPC...](http://epc-
co.com/epc/Portals/0/epc/documents/datasheets/EPC2036_datasheet.pdf) which in
this case is an enhancement mode transistor (so 'N' channel, or positive Vgs
to turn it on)

The thing that should smack you in the eye when you read the datasheet that
that it has a Theta(j-ambient) of 1100dC/W [1] which is why this tiny part is
limited to 1A continuous current (1 amp x 1 amp x .065 ohms == .065 watts and
a temperature rise of 71 degrees C. But it can be switching probably close to
80 watts while doing that.

I'm a bit surprised though that it doesn't list the switching time.

[1] You can get that down to 100dC/W if you solder it to a 1" square on 2oz
copper but then what is the point of having such a tiny switch :-)

~~~
madengr
There is that guy growing sheets of diamond, if he isn't off'd by the diamond
cartels. I suppose diamond semiconductors will supplant GaN some day. I'm read
a few years ago about cold cathod parts, but I guess it never panned out.

------
raverbashing
Good article until the end

"Lidow says EPC decided to first go after applications at 200 V or less in
order to pursue new applications silicon can’t easily reach, a category that
includes virtual reality and small medical imaging systems."

Sorry for the expression, but "dafuq did I just read"

What does VR has to do with a 100V power transistor? _nothing_

Small medical imaging systems may benefit from this (X-Rays? Mini-MRIs?) but
they're not _essential_ (also medical devices are kind of non-price sensitive,
so)

------
qzcx
If I am understanding correctly GaN mainly used in power components since it
can handle larger voltages. Is it possible that with GaN getting cheaper that
it might start moving towards other component areas that Si dominates? Or is
GaN's usefulness primarily in handling power?

~~~
xellisx
That's what I am wondering. There was a mention of RF, but don't go into
detail. If they can use this advancement in the CPU / GPU area, I think a new
"golden era" might happen. New CPUs and GPUs each year would be nice. =)

~~~
qzcx
Thats part of my other questions. How small is the current GaN technology? How
much can it build on silicon technology?

