Power Rectifiers: High-voltage GaN trumps SiC
Multiple channels and innovative edge termination enable GaN Schottky
power rectifiers to combine low epitaxial costs with operation up to 5
kV
BY YUHAO ZHANG AND MING XIAO FROM VIRGINIA TECH AND HAN WANG FROM THE UNIVERSITY OF SOUTHERN CALIFORNIA
RECTIFIERS providing high switching speeds and withstanding up to several kilovolts are in much demand. They are needed in power electronics systems used in the electricity grid, renewable energy processing, and industrial motors (see Figure 1).
Figure 1. Application space of power rectifiers with different voltage and current ratings.
Today, the most widely used device for rectification is the bipolar silicon p-n junction diode, despite its major drawback – a very slow switching speed, stemming from poor reverse recovery. A superior alternative that allows fast switching is the SiC Schottky barrier diode (SBD). However, its performance has only recently caught up with that of the silicon p-n diodes, and its epitaxial and fabrication costs are far higher.
Another material offering even more promise for high-voltage rectifiers is GaN: compared to silicon and SiC, it has the upper hand in several key areas, having a wider bandgap, higher mobility, and a higher critical electric field. Drawing on these strengths, several companies have launched GaN power devices with a lateral geometry, operating at up to 650 V. This geometry is challenging, limiting current and power capabilities, because current conduction takes place in a layer just a few nanometres thick – that’s a consequence of the two-dimensional-electron gas (2DEG) channel. Note that this impairment does not arise in high-voltage silicon and SiC devices, because they usually have a vertical architecture, with current spreading into bulk materials.
One major drawback, resulting from the limited current capability of GaN power devices with a lateral geometry, is the need for larger die sizes when accommodating high voltages and high currents. As well as increasing chip costs, the larger die induce large capacitances and charges, compromising device switching speed.
To overcome these challenges, our team at Virginia Polytechnic Institute and State University (Virginia Tech), working in collaboration with engineers at Enkris Semiconductor, Qorvo and the University of Southern California, has developed a novel high-voltage lateral GaN technology that features multiple channel materials and innovative anode structures.
We fabricate our devices from 4-inch AlGaN/GaN-on-sapphire wafers that host five stacked 2DEG channels and are produced by Enkris. A five-fold increase in the number of 2DEG channels compared with a conventional structure increases current capability by at least that factor, and delivers a corresponding reduction in sheet resistance. Around the multi-channel fins is wrapped a new anode architecture, comprising p-n junctions. This structure shields the Schottky contact from a high electric field and suppresses the leakage current for the five-channel device to below that for a single-channel counterpart. The performance of the novel multi-channel GaN Schottky rectifier is impressive, boasting a power figure-of-merit that exceeds the unipolar SiC limit, and is among the highest in all high-voltage rectifiers.
Stacking 2DEG channels
When engineers design power devices, efforts centre on the concurrent realization of a high breakdown voltage and a low on-resistance – for a lateral GaN rectifier, the latter is the product of sheet resistance and anode-to-cathode distance. While this distance is usually determined by the breakdown voltage, the on-resistance hinges on the sheet resistance, which can be trimmed by increasing the mobility and density of the 2DEG. As the number of channels increases through stacking, the density of the 2DEG increases proportionally. In turn, die size for a specific current rating can be greatly reduced, leading to smaller capacitance and charges, and ultimately a higher switching speed and lower losses.
Figure 2 (left) 2DEG density versus 2DEG mobility for the single-channel
and multi-channel AlGaN/GaN materials reported in the literature and
measured on the wafer detailed in this article. (right) cross-sectional
scanning electron microscopy image and top-view photo of the 4-inch,
five-channel, GaN-on-sapphire wafer produced by Enkris Semiconductor
Inc.
The idea of turning to multiple channels is not new. Around the start of the previous decade, researchers in the US and Japan pioneered AlGaN/GaN multi-channel epitaxy, using MBE. However, this growth technology is rarely suited to high-volume production of large-diameter wafers.
Very recently, the most common approach for the production of compound semiconductor devices, MOCVD, has been used to manufacture multi-channel structures on a variety of large-diameter substrates, including silicon, SiC, sapphire, and GaN. It is this growth technology that Enkris has employed to produce 4-inch, five-channel, GaN-on-sapphire wafers that feature a 2DEG with a density of 3.7×1013 cm-2 and a mobility of 1475 cm2 V-1 s-1. The corresponding sheet resistance is just 110 Ω/sq, a figure at least two times lower than the best value for a single-channel wafer. We estimate that the cost of this multi-channel GaN-on-sapphire wafer is no more than a third of that of a 4-inch SiC wafer.
Cranking up the voltage
Scaling up the voltage of multi-channel devices is much more challenging than it is for their single-channel counterparts because the increased charges from multiple channels threaten to induce electric-field crowding. However, overcoming this challenge is crucial for Schottky rectifiers, because their blocking voltage tends to be limited by the peak electric field at the Schottky contact region.
Holding the key to suppressing electric-field crowding is proper edge termination, which may also shift the peak electric field away from the Schottky contact region. For lateral Schottky rectifiers, the common approach to edge termination is to add a field plate (see Figure 3(a)). However, if this is to be effective, there must be precise control over the field plate geometry, such as the thickness of the dielectric and the length of the field plate. In addition, the design and production of the device must account for complex interfaces between dielectrics and semiconductors. Unfortunately, it is not uncommon for the device to exhibit instability when operating under high electric fields, or at high temperatures.
To tackle all these challenges, we have developed a new termination structure that uses a p-GaN layer grown on AlGaN/GaN (see Figure 3(b)). Thanks to vertical depletion enabled by our p-n junction, the electric field lines in the Schottky region spread out, and their distribution is more uniform. What’s more, the peak electric field is re-directed from the Schottky contact to the edge of p-GaN termination, a shift that shields the Schottky contact from the high electric field. Compared with the field plate, our p-GaN termination possesses a wide design window, in terms of doping concentration and p-GaN thickness, and it produces minimal dielectric interfaces. Another key attribute is that the fabrication is fully compatible with today’s foundry process for manufacturing the p-gate normally-off HEMT, opening up possibilities for monolithic integration of high-voltage rectifiers with GaN power ICs.
Figure 3. A conventional field plate termination (left) and Virginia
Tech’s novel p-GaN termination (right). Two channels are drawn to
illustrate multi-channel structures. Minimising device leakage
When developing our high-voltage multi-channel devices, the challenges we faced included minimising leakage current. Our solution is the junction-fin-anode. This is a three-dimensional anode structure, formed by wrapping p-n junctions around the multi-2DEG-fins (see Figure 4(a)). With this architecture, the p-type material provides strong depletion of the 2DEG channels. When the device is reverse biased, the junction-fin assists the Schottky contact for charge depletion, while shielding the Schottky contact from a high bias.
Our design can be evaluated with an equivalent circuit model of the entire rectifier (see Figure 4). This model includes an equivalent series connection for a sidewall SBD, a junction-fin-gated HEMT, and a p-gate HEMT. As the reverse bias increases, the sidewall SBD is pinched off, and then the two HEMTs. The voltage drop on the sidewall SBD is clamped at the threshold voltage of the junction-fin-gated HEMT, which is merely a few volts. This clamping occurs regardless of the reverse bias at the cathode, which can reach thousands of volts. Operating in this manner, the leakage current of the entire rectifier is equal to that of one of the sidewall SBDs biased at a few volts.
In our prototyped device, we realise the junction-fin structure by regrowth of p-GaN on top of the fin, and the addition of a p-type nickel oxide at the fin sidewalls. The resulting rectifier delivers a blocking voltage up to 5.2 kV, and when operating at 90 percent of this limit, the leakage current is just 1.4 µA/mm. The specific on-resistance is 13.5 mΩ cm2. Based on all these values, we find that the power figure-of-merit for our device exceeds the SiC unipolar limit and is among the highest in all multi-kilovolt power SBDs.
We have also fabricated large-area devices. They are capable of handling a 1.5 A current, have a leakage current measured in microamps, and a total charge of 13 nC (see Figure 5). Compared with commercial SiC SBDs with similar voltage and current ratings, our multi-channel GaN SBDs exhibit a significantly lower forward voltage and charges. 
Figure 4 (top) Virginia Tech’s multi-channel Schottky rectifier with a
junction-fin anode. (bottom) Equivalent circuit model of the rectifier,
comprising a series combination of a sidewall Schottky barrier diode, a
junction-fin-gated HEMT, and a p-gate HEMT. The internal voltage
distribution under a high reverse bias (VR) is also illustrated, with
the threshold voltages of the junction-fin-gated HEMT and p-gate HEMT
marked as VTH1 and VTH2, respectively. The voltage drop on the sidewall
Schottky barrier diode is clamped at | VTH1| regardless of VR.
These impressive characteristics show that our design, incorporating innovations in multi-channel materials and junction-fin anodes, promises to pave the way to a new generation of high-voltage GaN power devices that combine a low epitaxial cost with fast switching characteristics and high-power capabilities. Thanks to these attributes, our multi-channel lateral devices are will-equipped to extend the reach of GaN devices into high-voltage power electronics.
Further reading
M. Xiao et al. “5 kV Multi-Channel AlGaN/GaN Power Schottky Barrier Diodes with Junction-Fin-Anode”, 2020 IEEE IEDM, 5.4.
M. Xiao et al. IEEE Electron Dev. Lett. 41 1177 (2020)
Y. Ma et al. Appl. Phys. Lett. 117 143506 (2020)
