NexGen Vertical GaN™ core technology Enabling a new benchmark in power electronics

Although difficult, it is not impossible to make GaN-on-GaN devices.

Many technology firms have significant programs under the GaN-on-Si umbrella, limiting the ability to invest in GaN-on-GaN technology and process. In addition, complex issues such as die size, wafer cost, wafer size (4″ GaN wafers vs. 6,8″ GaN-on-Si wafers); have led to the popular myth that GaN-on-GaN devices cannot be made.

NexGen has successfully built and sampled complete power systems with the world’s first commercially available GaN-on-GaN devices, NexGen Vertical GaN™, switching at up-to 3MHz.

GaN-on-GaN devices are 20 to 25 times smaller than Silicon devices, which implies more devices per wafer and higher yields, leading to a competitive cost structure even on smaller wafer sizes. With the avalanche and short-circuit capabilities of these devices, and their ability to switch 10-100x faster vs. the switching frequencies of current power semiconductors, NexGen Vertical GaN™ devices provide additional system savings as well.

The source of this myth is the way GaN epitaxy is grown in GaN-on-Si power semiconductors. Epitaxy is the technique of atomically growing a crystal, layer by layer, on the surface of another crystal. Gallium nitride (GaN) devices can be created on GaN layers that are heteroepitaxially grown on different substrates like silicon (Si) and silicon carbide (SiC) or homoepitaxially grown on GaN substrates.

Lattice mismatch between GaN and Si or SiC leads to stress-induced dislocations that alter and degrade the electrical properties of GaN and are the source of lower breakdown voltages and poor reliability. Epitaxially growing thick (>10µm) device layers on materials with mismatched lattice and coefficient of thermal expansion (CTE) results in wafer bow, warp, and cracking. In case of NexGen Vertical GaN™ which grows GaN-on-GaN substrates, both lattice and CTE are perfectly matched. As a result, very thick layers of GaN can be epitaxially grown on bulk GaN substrate which enables the fabrication of high breakdown voltage devices. Vertical GaN™ devices are also the only wide-band gap semiconductor material with repetitive avalanche and >10µs short circuit capabilities. In addition to this, the Vertical GaN™ devices also don’t suffer from current collapse phenomenon because they does not rely on 2DEG (sheet of electron charge due to polarization at the interface between AlGaN and GaN).

NexGen Vertical GaN™ devices can operate from 100 volts all the way up to 4,000 volts and they can switch at multiple megahertz switching frequencies. These devices are three-dimensional in structure, and that is why they are vertical. For higher voltage, we grow more gallium nitride. For more current, we increase the area of the device. Hence, NexGen uses all three dimensions of the device to make a junction field effect, power transistor.

Power level is a function of thickness of the EPI layer. The vertical GaN structure provides an advantage to our devices when it comes to higher power applications.

The industry is clinging to a persistent myth that higher switching frequencies is leading to difficult (if at all solvable) EMI challenges. This view has only merit if small, incremental frequency increases are considered. A small step from 130Khz to e.g., 260Khz only makes EMI implementation more costly because now the switching frequencies fundamental has to be filtered w/o a sufficient frequency increase to make EMI filter components smaller. But if one considers a switching frequency increase to e.g., >1MHz then the EMI filter components are becoming sufficiently small to enable a cost reduction, even when a fundamental has to be filtered.

There are two types of EMI – Conducted EMI and Radiated EMI.

In Conducted EMI, the operation range causes the input filters to become much smaller (100kHz must be filtered) and therefore NexGen’s power systems have a clear advantage on conducted testing. In Radiated EMI, we use a faraday cage that doubles as our heat dissipater as well, this dual-purpose implementation allows a compact design with robust conducted EMI performance

NexGen’s power systems show that it is possible to build >1MHz frequency designs, still meeting EMI requirements at reduced EMI cost.

NexGen Power Systems is providing the first power supplies operating at 1+MHz. These are not evaluation platforms but fully realized products to be sold to the market.