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Innovative Reliable Nitride based Power Devices and Applications

Periodic Reporting for period 2 - InRel-NPower (Innovative Reliable Nitride based Power Devices and Applications)

Reporting period: 2018-07-01 to 2020-11-30

Power Electronics is the technology associated with the efficient conversion, control and conditioning of electric energy from the source to the load; enabling generation, distribution and efficient use of electrical energy. The schematic representations of the challenging requirements for power electronics are shown in Fig. 1 Power electronics that use GaN-based semiconductors have the potential to change all this. WBG semiconductors operate at high temperatures, frequencies and voltages -- all helping to eliminate up to 90 percent of the power losses in electricity conversion compared to current technology. Systems employing GaN based devices have higher power efficiency, corresponding to lower losses, and higher switching frequencies, that allow to reduce the size and weight of the converters. This unique performance provides a qualitative change in their application to energy processing.

The project – objectives and contents
The overall objective of InRel-NPower is to develop robust and reliable Gallium Nitride (GaN)- and Aluminum Nitride (AlN)-based power devices for high and medium power electronics systems targeting energy conversion efficiency applications and bringing the wide band gap (WBG) semiconductors power devices another step towards the wide usability in the energy saving environment exploiting the full potential of this semiconductor material. Packaging is also carefully addressed in this project thanks to two innovative packaging solutions that will allow the exploitation of the full capability of the GaN material. The target is to bring the European semiconductor value chain partners a step further towards the frontiers of the production and application of robust high and medium power devices.
The major issue of GaN-on-Si is the lack of understanding of the impact of the high defect density on the long term performance. In this project we obtained insights on new mechanisms that determine this behavior, published in several high-impact publications. Buffer structures with improved performance were demonstrated. Novel HEMT configurations were optimized, achieving low sheet resistance far below 300 Ohm/sq.
AlN on sapphire templates with very high quality were fabricated. These served as the substrate for the epitaxy of extreme wide bandgap HEMT structures. Transistors fabricated out of this material show a critical electrical breakdown strength well above the capabilities of GaN material.

Optimization of state-of-the-art gold-free Ti-Al-based ohmic contacts resulted in a significant yield improvement on AlGaN HEMT technology from the foundry ON-Semiconductor. The normally-off transistors developed by ON-Semiconductor showed high yield > 83% on all critical parameters assessed on large power bars (70 mm). Activities on advanced architectures based on substrate removal and AlN backside deposition (LSR) have been successfully achieved. Finally, a novel AlN-based HEMT technology with functional transistors having low leakage current, remarkable breakdown voltage and high temperature stability have been achieved.

Yield and reliability are the two key factors to initiate and accelerate market adoption of new/emerging technologies. During the second half of the project ON Semiconductor’s primary focus was to develop reliable eGaN devices using a p-type doped gate architecture. Several epitaxial, process and device architecture development were carried out resulting in manufacturing of eGaN power devices, which met all reliability (dynamic RDS,on, ohmic contacts, pGaN gate and buffer) and Safe Operating Area (Power-to-Failure area higher than hard-switching operating regime area) requirements. Furthermore, a Baseline activity was performed in order to evaluate the stability of the eGaN process resulting in >80% yield figure considering over several Lots/Wafers.

Within three varieties of planar half-bridge designs (Siemens demonstrator, Bosch verification and technology demonstrator) and a non-isolated DC2AC converter the increased performance of such low-inductive module designs with GaN HEMTs inside could be approved. The assigned advantages of these next generation III-V compound devices - e.g. steeper switching gradients and faster switching capability, lower power consumption in the control circuitry, and derivable from those higher power densities and higher efficiencies - were confirmed by wide band gap adjusted hardware design and realization, and attending functional measurements.
Basing on those attained results from module design and functionality, a subsequent generation of devices close to an engineering grade that represents one-step before industrialization could be possible.
We have developed AlN/sapphire template material with very high crystal quality and a perfect surface morphology.
On these substrates, we grew HEMT structures consisting of extreme wide bandgap materials. State-of-the-art sheet resistance was obtained. Transistors in this material have lateral breakdown voltages above 4kV.
For GaN-on-Si, new understanding of the correlation between defects and electrical performance was achieved. A mechanism of how the nucleation layer can impact the vertical conductivity of the buffer stack was revealed.
A different type of buffer stack was developed, resulting in thinner buffer that shows a higher breakdown voltage and a reduced trapping.

First demonstration of GaN-on-silicon HEMT technology with ultra-low leakage (< 1 µA/mm) up to 3000 volts has been achieved by using a local substrate removal and AlN ultra-wide bandgap backside deposition. This result represents a substantial progress beyond the state of the art. Furthermore, this approach has been successfully applied for the first time on large normally-off transistors delivering more than 10 A as shown by the enhancement of the breakdown voltage. Fully functional AlN-based transistors have been fabricated with low leakage current, superior breakdown field and temperature stability as compared to GaN HEMTs. A record three-terminal breakdown voltage for AlGaN channel HEMTs above 4 kV has been achieved.

Dynamic ON resistance plays a crucial role in determining switching efficiency and reliability of the power devices. Higher electron trapping/high dyn Ron locally reduces the channel density resulting in higher Ron/lowering of efficiency/increased junction temperature, ultimately resulting in device failure. The eGaN devices developed within this project, showed an Ron increase of only 30% for the entire rated voltage range of up to 650V. Competitor devices were benched marked under the same measurement conditions and the result was that ON Semiconductor’s eGaN devices were found to be >2x better.

The gained experiences within WP4 will support and direct the design and development processes for future wide band gap basing power electronics applications.
These results impress under consideration to the explored research grade of the new high voltage blocking intrinsic normally-off devices. Demands for improvements towards later industrialization, with respect of device’s stability under dynamic operational-related loads – e.g. prevention and mastering of the observed VDS-raise in on-state – are identified and addressed back to the device’s developers and manufacturer.