Engineered substrates to enable high-volume manufacturing for GaN devices

By Steve Ohr, Consultant, Semiconductor Industry Analyst

While developers are making limited headway with GaN-on-silicon devices, wafer sizes, yields and performance scaling has been limited ―creating obstacles for mainstream GaN adoption. New substrate materials enabling high quality GaN epitaxial layers promise higher yields as well as performance scaling for GaN devices – even with a transition to 8- or 12-inch diameter wafers.


New substrate materials, matching thermal characteristics of Gallium Nitride (GaN) devices, may point the way to broader deployments of wide bandgap GaN-based devices for Power, LED and RF applications. Engineered substrates, whose thermal expansion coefficients more closely match to GaN epitaxial layers, will enable thick and high quality GaN semiconductor layers on 8- and even 12-inch wafers. The larger wafer sizes will support high-volume manufacturing, offer access to advanced CMOS manufacturing platforms and lower the costs of GaN devices in power supplies, RF transmitters and LED lighting banks. Thicker GaN EPI layers will enable higher voltage device roadmaps (up to 1,200V and beyond) as well as power devices with vertical conduction paths. This is the conclusion of Quora Technology (a two year-old startup), in partnership with memory chip maker, Micron Technology and foundry service vendor Vanguard International Semiconductor whose combined research and production results were cited earlier this month (March 7-8 in Brussels) in a paper presented at the CS International Conference https://www.cs-international.net/, a forum for compound semiconductor makers. Specially engineered substrates will open the doors for GaN to be more widely adopted ―eventually replacing silicon, projected Cem Basceri, president and CEO of Quora, the paper’s author and presenter. Everyone recognizes the potential of GaN power devices ― in terms of higher switching speeds, smaller form factors, and high-temperature performance. But they have been inhibited by either GaN’s stress issues on thermally mismatched substrates or by scaling issues on otherwise expensive alternative substrates, Basceri said. The use of carefully engineered 8- or 12-inch wafers ― coupled with 90% manufacturing yields ― will hasten the market penetration of GaN transistors and also open the path to commercial high power GaN diodes.

Manufacturing yields and scaling are chief among the factors holding back large-scale GaN deployments, Quora Technology believes. GaN epitaxial layers grown on silicon wafers can be fragile, especially when the stress levels increase exponentially with silicon substrate diameter due to different temperature coefficients, Basceri points out. The key to GaN-friendly manufacturing is achieving a very stable substrate and GaN structure altogether.


Improving yields for GaN devices

The result of significant R&D, Quora’s solution to high volume GaN manufacturing is a specially-designed substrate material, called “QST,” (Quora Substrate Technology) which enables very high quality GaN EPI layering. QST avoids the scaling issues of GaN-on-silicon, building devices with both high performance and low cost. It points the way to GaN-on-GaN like solutions, on 8-inch 12-inches. It is carefully designed to work with fab-friendly materials, Basceri explains, starting with a core material, closely matched to GaN thermal expansion characteristics, and wrapped with layers of thin films. These materials address thermal issues, electrical issues and several other key features for power, LED, RF and new applications. In each of these segments, the QST substrate, Basceri believes, opens doors to new commercial devices such as GaN diodes and LEDs with advanced wafer level chip scale packages.


Quora’s Substrate Technology (QST) includes a core material closely matched to GaN thermal expansion characteristics, and wrapped with thin films. These materials address thermal, electrical and mechanical issues and point the way to GaN-on-GaN like solutions. (Source: Quora Technology)



A joint presentation with the US Department of The Navy’s Research Laboratory (NRL) and Quora Technology will be presented at the CS Mantech 2017 Conference in May (https://csmantech.org ) focusing on in-depth material characterization studies. Of special interest is the thickness of GaN epitaxial layers on QST substrates (up to 16 microns), as this effects performance. A preliminary vertical Schottky barrier diode will be presented as well high electron mobility transistors (HEMTs). Earlier, Andrew Koehler of the NRL presented similar studies at the Workshop on Compound Semiconductor Materials & Devices (WOCSEMMAD) with the concluding remarks that engineered substrates will open the path for commercial high power GaN devices. (Experimental diodes are shown below.)



Engineered QST substrates enable high power vertical Schottky GaN diodes.(Source: U.S. Department of the Navy Research Laboratory)


Initially, the Navy researchers characterized 5-micron GaN device templates on a 6-inch QST substrate. The templates then served as base for realizing thick GaN epitaxy (15 micron), and subsequent evaluation of vertical Schottky-barrier diodes on these GaN layers.


While the U.S. Navy was clear that it could not officially endorse Quora Technology’s product offerings, its conclusions so far, verified QST technology as a solution for unleashing the full potentials of GaN.


Identifying market opportunities

The promise of Wide Bandgap materials, will be enabled by 10x lower losses for power conversion, 85% or more efficiencies in lighting, and 3x-to-10x faster mobile data for 5G at frequencies (up to 60 GHz).

Quora’s core technology has foundations extending from MIT to Micron, and R&D is currently funded by both Micron and Vanguard. Quora Technology acknowledges that it is having on-going collaborations with GaN device manufacturers and fabrication facilities ― including some of the best-known names in semiconductor markets.

While current GaN offerings support up to 600V transistors, Quora Technology’s own research and product development efforts with its partners include 1,200V and higher voltage power devices. The company plans to introduce GaN-on-QST wafers in to third quarter of this year, and offer 8-inch diameter “device wafer” foundry services at VIS fabs for power and LED applications in Q1 2018. In parallel, Quora will also support its customers with its reference designs for unique devices and applications.

A key care-about in Quora’s product rollouts will be the cost of the custom wafers. The current price of GaN-on-Silicon can be highly variable and expensive. To capture shares of the power discrete market, (which includes rectifier diodes as well as transistors), GaN discrete devices need to be available in the market at 10 cents per amp or less. GaN wafer cost and performance roadmaps should support this target, Basceri believes, but that kind of manufacturing efficiency does not yet exist. [ ]


Quora’s substrates enable epitaxy for enhancement-mode GaN HEMTs. The QST substrates enable switches support thicker epitaxial layers and build devices with high voltages and current densities. (Picture Source: IMEC)

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