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RESEARCH REVIEW Slanted field plates spawn superior HEMTs

Higher breakdown voltage and improved RF characteristics result from the introduction of slanted field plates

CALCULATIONS BY SEVERAL GROUPS have shown that switching from a conventional field plate to slanted variant (see figure) trims the peak electric field in a GaN HEMT, leading to improved device performance. Putting this theory into practice has not been easy, but a US-Japan team has just succeeded in this endeavor, with results validating the superiority of the slanted field plate.

What’s more, this recent work shows that this form of field plate delivers better device performance than the common alternative, the multiple-field plate, which also cuts maximum electric field in GaN HEMTs.

Adding gates of nickel and gold via evaporation, first with the sample pointing normal to the source and subsequently tilted, created slanted field plates with metal deposited only at the drain side of the sidewall.

Finally, reactive-ion etching exposed contact holes for ohmic electrodes, before Ti/Pt/Au probing pads were added by electron- beam evaporation and lift-off.

HEMTs with a slanted field plates deliver a 66 percent increase in breakdown voltage compared with those employing a double field plate.

Team member Tetsuya Suemitsu from Tohoku University, Japan, argues that the weaknesses of the multi-field approach are not limited to inferior device performance – their fabrication requires two or more additional masks, depending on the number of the field plates employed. “This could be a penalty on cost, yield, and the turnaround time of the fabrication process.”

In comparison, the approach adopted by Suemitsu, plus colleagues in his department and at engineers at MIT, is simple. It is also versatile, because it can be applied to conventional field plates, thereby allowing a systematic study of breakdown voltage with different plate geometries.

The approach of the US-Japan team involves the adoption of field plates as part of the gate metallisation process. The key to producing the slanted field plate is to control the mass flow ratio of carrier gases during plasma-enhanced CVD of the dielectric film. “This can be done by programing a recipe of the PECVD system,” says Suemitsu.

Demonstration of the superiority of the slanted field plate begins with the growth of an AlGaN/GaN heterostructure on sapphire. This substrate is not as

common a foundation as SiC and silicon for the production of GaN HEMTs. However, the team selected it because they view the AlGaN/GaN heterostructure on a sapphire substrate as the most conservative material system.

“In future study, we will use [SiC and silicon] substrates and more advanced heterostructures, such as an InAlN barrier, to show the impact of the slanted field plate on the state-of-the-art devices,” says Suemitsu. “We believe that our slanted field plate process is applicable to any substrate and heterostructure.”

Aside from the addition of the slanted field plate, GaN HEMTs were fabricated with standard processes. Ti/Au-based ohmic electrodes were formed by electron-beam evaporation and lift-off, before chlorine-based inductively- coupled plasma etching provided device isolation and ohmic annealing was undertaken.

Engineers used SiCN as the passivation film, depositing a 200 nm-thick layer while varying the ratio of the two carrier gases, hydrogen to ammonia. After defining gate patterns, etching vertically with hexafluoroethane and horizontally with sulphur hexafluoride yielded a slanted profile for the dielectric.

72 October 2014 Copyright Compound Semiconductor

The team also formed a device without a field plate, plus those with two or three field plates, and compared the performance of all these transistors. The absence of a field plate led to a hard breakdown voltage of 83 V, while the addition of a slanted plate led to no hard breakdown up to voltages of 160 V. By defining an off-state breakdown voltage as the drain voltage at a drain current of 1 mA/mm, the slanted field plate design was shown to deliver a 66 percent increase in breakdown voltage compared with a double-field plate, and roughly a 30 percent increase compared with a triple-field plate.

RF performance also improved with the slanted field-plate: The maximum frequency for current gain was 28.8 GHz with this transistor architecture, compared with 12 GHz and 24.7 GHz for devices with two and three field plates, respectively.

Plans for the future include employing two-dimensional device simulation to optimise the design of the slanted field plate.

“We will also introduce advanced material systems such as InAlN/GaN heterostructures on SiC substrates, to push up the limit of the product of the cutoff frequency and breakdown voltage,” says Suemitsu.

T. Suemitsu et. al. Appl. Phys. Express 7 096501 (2014)

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