Going Vertical With GaN Devices

Gallium nitride has long been on the horizon for a variety of uses in semiconductors, but implementing this on a commercial scale has been relatively slow due to a variety of technical hurdles. That may be about to change.

The wide bandgap of GaN makes it particularly attractive material for power conversion applications. Still, actually realizing its benefits in commercial devices has been a challenge. As a general rule of thumb, power converters should be able to operate at up to triple the expected RMS AC voltage in order to allow a safety margin for power surges. Most computing devices use 120V or 240V wall current, while electric vehicle chargers typically supply 400 volts, and many industrial applications depend on a 480V two-phase supply. Power converters for these applications therefore need an operating range of more than 1000V. So far, defects have limited the high voltage reliability of GaN devices.

The first obstacle is the GaN material itself. While free-standing GaN wafers exist, so far they are limited to only 100mm in diameter. GaN on silicon is easier to manufacture and economically attractive — larger wafers can support more devices at approximately the same per-wafer cost — but the large thermal expansion and lattice mismatch between GaN and silicon leads to cracking and high defect levels.

Rick Brown, co-founder and CTO of Odyssey Semiconductors, estimated that freestanding GaN wafers deliver defect levels between 10³ and 10⁵ per square centimeter, compared to 10⁸ to 10¹⁰ per square centimeter for GaN on silicon. To realize blue GaN lasers, optoelectronics researchers needed to grow GaN layers on GaN wafers. Brown expects power conversion applications will ultimately need to use GaN substrates as well.

Device-grade SiC wafers are also only available in relatively small sizes. Relative to SiC, GaN devices can be smaller because of the material’s ability to operate at higher frequencies. On the other hand, SiC is a better match with silicon, making reliable SiC devices on silicon substrates more achievable.

An additional constraint for GaN is the lack of robust area-specific doping. In silicon and silicon carbide, ion implantation allows manufacturers to create arbitrary p-type and n-type regions as needed for a desired device structure. In these materials, a post-implant anneal activates dopants and repairs implant-induced damage to the crystal structure. The GaN material, in contrast, can’t tolerate such high temperatures.

Alternatively, it’s possible to create doped regions by etching and regrowing a GaN region with the desired doping. Defects tend to accumulate at the interface between existing and regrown material, though. Furthermore, this same interface is subjected to the maximum current density during device operation.

Without an area-specific doping technology, GaN devices are limited to horizontal HEMT structures. In these transistors, current travels along the interface between two dissimilar materials, such as GaN and AlGaN, mediated by an applied gate bias.

HEMTs are unsuitable for very high voltage applications for a number of reasons. They are normally “on” devices, requiring a gate bias to prevent conduction through the channel. At high voltages, normally on devices are both unsafe and inefficient, so they are generally paired with silicon-based control logic. Planar structures require more area for a given device than vertical structures. That’s especially true in power devices, which require space between terminals to prevent arcing.

High voltage Si and SiC devices use a vertical design instead, with the source and drain on the front and back sides of the substrate, respectively. The gate bias modulates dopant wells on either side of the channel. The separation between terminals facilitates electrical isolation and minimizes the overall system footprint.

According to Brown, Odyssey Semiconductor has developed a rapid annealing process that protects GaN during ion implantation and dopant activation. The process enables area-specific doping and vertical conduction devices. The company expects to make engineering samples available to potential customers this year. If early results hold up, GaN may be on the verge of actually realizing its potential in high voltage applications.

Fig. 1: SEM darkfield images of GaN film (L) and GaN nanowire (R), showing aligned columns of N atoms (yellow) and Ga atoms (red). Source: NIST

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