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Innovative Reliable Nitride based Power Devices and Applications

Periodic Reporting for period 1 - InRel-NPower (Innovative Reliable Nitride based Power Devices and Applications)

Reporting period: 2017-01-01 to 2018-06-30

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

The strategic importance of the devices developed within InRel-NPower is undoubtable, as it can be understood reading some recent announcement from the US department of Energy (DoE):
“For nearly 50 years, silicon chips have been the basis of power electronics. However, as clean energy technologies and the electronics industry has advanced, silicon chips are reaching their limits in power conversion -- resulting in wasted heat and higher energy consumption.

Power electronics that use WBG 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. This in turn means that power electronics can be smaller because they need fewer semiconductor chips, and the technologies that rely on power electronics -- like electric vehicle chargers, consumer appliances and LEDs -- will perform better, be more efficient and cost less.” 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 material properties and electrical behavior of the reference “step-graded” strain management buffer have been investigated, and at the same time developed an alternative buffer approach based on strained superlattices. In parallel, two different novel barrier configurations were optimized based on AlN and InAlN material, achieving low sheet resistance below 300 Ohm/sq. Fabrication of AlN templates or bulk substrates to be used as starting material for task 1.4 have also been explored. The AlN on sapphire templates were show to be of very high quality.

Optimization of state-of-the-art gold-free Ti-Al-based ohmic contacts resulted in an overall reduction of the parasitic resistivity of the device and a significant yield improvement, by reducing morphological degradation and improving the crystal quality of the metal. Extensive interaction with WP3 enabled rapid progress in eGaN process development. Activities on advanced architectures based on substrate removal and AlN substrate have sucessfully started as planned.

Improvement of yield results in higher production efficiency and reduction in manufacturing costs and also indicate a stable and reliable process flow. During the earlier months one of the major reasons for Yield loss was high Dynamic On-resistance followed by high off-state drain leakage. Several iterations of epitaxial and device architecture improvement was carried out, which led to the achievement of an average (and stable) Yield figure of >90%. Second focus of WP3 was the development of the eGaN technology including all the relevant process modules such as pGaN etching with a strong focus on device reliability.

To illustrate how efficient and reliable the developed GaN-devices can operate in a system two demonstrators for power conversion are being developed. At a first stage, the system specification for the target characteristics of the Motor drive inverter and the non-isolated DC2AC converter was determined. The compiled data are based on the preliminary electric key data of the GaN devices and the chosen reference drive systems, respectively. In order to optimize the performance of the demonstrators ON Semiconductor has implemented the mixed mode methodology, comparing different device architectures, and exploring the effectiveness of parallelization of several GaN devices to reach higher current levels. Further for the involved packaging technologies a transition from half-bridge circuitry to AMB-LTCC-module design was outlined and a design freeze for the test modules based on the defined topology and schematics was finalized.
Compared to the start of the project, good progress was made towards the optimization of low sheet resistance barrier materials. Both selected barrier materials, AlN and InAlN, reach the target (< 300 Ohm/sq.) and have in the meantime been further developed, e.g. in terms of leakage current. By the end of the project we target to achieve device with the best FOM in terms of the trade-off between on-state resistance Ron and off-state breakdown Vbr.

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. In the second part of the project, it is expected to apply this approach on large industrial devices from the partner OnSemi.

Dynamic On-resistance (or current collapse) is one of the major reliability concerns for GaN based power devices. Such phenomenon temporarily increases the On-resistance of the device due to trapping in the gate-drain access region of the device due to off-state stress, which might generate thermal issues leading to device failure. Dynamic RDS,on measurements were done on our “best” available eGaN split and compared with our depletion mode baseline wafers and a commercially available p-GaN based enhancement mode device.

In this project novel high voltage GaN bare dies will be integrated into packages with high-performance Active Metal Brazed (AMB) ceramics and thereon built planar interconnect technologies for better cooling and lower inductance. Only such flat 3D-stacks enable the new level of functional density, power density and efficiency in power electronics.
Reliability is crucial for power electronics systems. With reliability testing and a detailed failure analysis new knowledge on the novel GaN devices will be developed to be considered in designing new reliable systems.