Contributed by St.J. Dixon-Warren
One of the tedious tasks in transporting even the newest light-weight laptop is also needing to lug the “brick” charger along with it. In the last couple of years, we have seen attempts to market small-format, light-weight chargers that fit easily into a briefcase or pocket. The Avogy Zolt is one such attempt. One of the selling features of the Zolt is that it uses gallium nitride (GaN) transistors to make a more compact device – but, to our surprise, we found something different.
Gallium nitride has been the “holy grail” of power electronics for a number of years due to the physical properties that theoretically allow a GaN transistor to exceed the performance specifications of a silicon power MOSFET, both in terms of breakdown voltage and on-state resistance (RDS,ON). The challenge, however, has always been obtaining high quality GaN substrates at a reasonable price. Bulk GaN wafers are very expensive and obtaining good crystallographic quality is challenging. Higher quality GaN epitaxial layers can be grown on SiC and Si substrates, which allows for the formation of lateral transistor structures, but not for vertical ones. The availability of GaN-on-Si at reasonable cost was a particularly disruptive event, since it allowed conventional silicon processing tools to be used for processing the GaN-on-Si wafers.
Avogy Inc. claims to be a market leader in bulk GaN device technology. The Knowledge Center section of Avogy’s website includes links to papers on bulk GaN p-n diodes and vertical power transistors on bulk GaN substrates. The website also lists various GaN-based diodes and JFETs, as well as the Zolt laptop charger. Since we were interested in analyzing Avogy’s bulk GaN vertical power transistor technology, Chipworks recently procured a Zolt Laptop Charger Plus, Graphite.
Zolt Laptop Charger
Our lab immediately began a teardown of the device (seen in the following images) to see what we could find inside. The main PCB features several devices including a component, indicated with a red rectangle on the image, bearing an older version of the Avogy logo and the following markings:
The date code, 1527, suggests that the die was packaged in week 27 of 2015.
Inside the Zolt Laptop Charger
Zolt Laptop Charger PCB Top
Zolt Laptop Charger PCB Bottom
Avogy 150-00028 Package Photographs
We surmised that this device likely contained the bulk GaN power transistors we were seeking for our analysis. We quickly submitted the device to our deprocessing lab and obtained a very nice die photograph, which immediately confirmed that we likely had a vertical power transistor. The image revealed gate and source pads on the top surface of the die and a drain pad on the bottom surface, a common layout for vertical MOSFET power transistors.
Avogy 150-00028 Die Photograph
The next step was a cross section to confirm the transistor structure, which turns out to be very similar to a Cree SiC vertical power transistor that we analyzed in 2013. Then, to our surprise, SEM-based energy dispersive spectroscopy (SEM-EDS), showed the substrate to be SiC and not GaN after all. We now believe that the die inside the Avogy 150-00028 was likely fabricated by Cree, although it is also possible that it was sourced from another SiC foundry.
Avogy 150-00028 Die SEM Cross Section
Avogy 150-00028 Die Substrate SEM-EDS
We consider this to be a very curious finding. Avogy very publicly claims to be focused on bulk GaN power transistor and diode technology. They have announced the Zolt product with great fanfare and were given an Innovation Award at the 2015 Consumer Electronics Show. Their product literature does not explicitly claim that GaN technology is used in the Zolt, but most analysts, including the folks at PointThePower, have assumed that it does. Efforts to confirm, went unanswered by Avogy. Of course, whether the internal power switching transistor is GaN or SiC is immaterial to most users of the Zolt charger, but this information will be of interest to the power market in general as it provides a benchmark of what is really in production products.