(Invited) Role of GaN-Based Devices in Medium and High Power Conversion

All power converters rely on a switch that is predominantly based on Si technology today. Power conversion techniques largely served by Si transistors and diodes, are facing a roadblock where any further improvement in performance is now incremental. It is well recognized that high efficiency and high-speed compact power converters will offer more versatility, value and create platforms for newer applications. New functionality at the system level requires switches that can operate faster and at higher temperature without adding to the system losses. Studies show that the overall system performance over the system cost drives the technology to market rather than its component’s cost. Therefore it is crucial to evaluate the impact of any new device technology at the system level.

The evaluation of the effect of a more efficient GaN switch, as a replacement for Si-IGBT, on the overall system, has been possible due to the recently commercialized lateral 600V HEMT technology [1]. GaN-on-Si technology now offers a scalable and cost-effective solution for power switches. A 600V lateral HEMT, where the gate to drain distance absorbs the off-state high voltage with very low leakage current, can switch 1.5 times higher operating current and at a frequency which is at least 10 times or higher compared to the state-of-the art Si-based devices without compromising the efficiency of the converter. GaN switched converters can be designed to operate at higher frequencies (100s of kHz), with reduced size of the passive components leading to an overall reduction in the form-factor. In addition to performing at high frequencies, GaN-based devices can operate at higher temperature (>150^oC) thereby requiring smaller or no heat sinks; further reducing the form-factor of the converter. A 40% reduction in volume of a Photovoltaic inverter over the state-of-the art Si based inverter was realized by Yaskawa Electric Corporation using Transphorm’s GaN switches, due to reduction in size of the output filter and the heat sink. Thus Lateral GaN technology has effectively reduced the electrical and mechanical footprint in a converter by reducing the size of passive components and heat sinks, respectively [2].

Although well suited for the medium power applications, lateral GaN HEMTs become unattractive, both in switching performance and cost, for high power (10kW-1MW) application space since the chip area becomes large. A vertical GaN switch offers more attractive solution at these higher power ratings, where the required blocking voltages exceeds 1kV. Early results from CAVETs have established the fact that bulk GaN devices offer breakdown electric field almost 3 times higher compared to lateral HEMTs. In a CAVET or a CAVET-like design, the peak electric field is buried into the bulk of the material, enabling higher breakdown electric field, which renders lower R_on for the same rated voltage, compared to HEMTs. With a buried electric field far from the surface, one expects and CAVETs have demonstrated dispersion less output current without the need of complex field plate structures (an integral part of the lateral HEMT design). One challenge that is yet to be overcome in lateral HEMTs is single chip normally off operation. Our recent research has identified vertical device designs, using bulk GaN, in the form of a vertical MOSFET that warranties single chip normally-off devices. Single chip normally-off operation will further improve switching performance by eliminating bond wire and PCB trace related inductances.

Recent results on GaN substrates with defect densities lower that 10⁴cm^-2 and allowing electron mobility >1100cm²V^-1s^-1 [3] create a very encouraging picture for bulk GaN power devices. Development of implantation technology with high activation efficiency is an ongoing activity that has lately shown some promising diode operation achieved with [Mg] implanted GaN/n GaN structures [4].

In conclusion, Lateral GaN has made a remarkable progress over the past 5 years making it into a product that can be designed into various low and medium power converters. The same trend is expected to repeat for vertical GaN for higher power applications, which is less matured than its lateral counterpart. Vertical GaN, owing to its promising cost roadmap and high quality of material allowing high electron mobility is in an advantageous position over its competitors.

Reference:

[[1] S.Chowdhury and U.K Mishra, IEEE Transaction on Electron Devices, 60 3060(2013)

[2] Yaskawa Annual Report (2013) http://www.yaskawa.co.jp/ir/ir_document/annualreport/2013/en/ar2013e.pdf

[3] P Kruszewski et al. The International Workshop on Nitride Semiconductor (2014)

[4] T. J. Anderson et al, Electronics Letts. , 50, 197(2014)