A perspective on multi-channel technology for the next-generation of GaN power devices

The outstanding properties of Gallium Nitride (GaN) have enabled considerable improvements in the performance of power devices compared to traditional silicon technology, resulting in more efficient and highly compact power converters. GaN power technology has rapidly developed and is expected to gain a significant market share in an increasing number of applications in the coming years. However, despite the great progress, the performance of current GaN devices is still far from what the GaN material could potentially offer, and a significant reduction of the device on-resistance for a certain blocking voltage is needed. Conventional GaN high-electron-mobility-transistors are based on a single two-dimensional electron gas (2DEG) channel, whose trade-off between electron mobility and carrier density limits the minimum achievable sheet resistance. To overcome such limitations, GaN power devices including multiple, vertically stacked 2DEG channels have recently been proposed, showing much-reduced resistances and excellent voltage blocking capabilities for a wide range of voltage classes from 1 to 10 kV. Such devices resulted in unprecedented high-power figures of merit and exceeded the SiC material limit, unveiling the full potential of lateral GaN power devices. This Letter reviews the recent progress of GaN multi-channel power devices and explores the promising perspective of the multi-channel platform for future power devices. Published under an exclusive license by AIP Publishing.

Diamond on GaN integration for power semiconductor devices with efficient thermal management

Reza Soleiman Zadeh Ardebili

The international actions against global warming demands reductions in carbon emission and more efficient use of energy. Energy efficiency in the conversion and use of electricity, as an important form of energy in the modern life, has strong environmental and economical impacts. Power electronic devices determining roles in the overall system efficiency in many applications such as power converters and inverters. Gallium Nitride (GaN) devices have emerged as an superior alternative to the silicon devices and enabled unprecedented efficiencies. GaN-based high electron-mobility transistors (HEMTs) offer excellent characteristics such as high switching speed, low switching and conduction losses, small device size and high power density capabilities. However, such significant increases in the power density and the reduction of device size comes at the price of dramatic increases in the heat fluxes and appearance of hot spots in the device footprint that cannot be addressed using conventional methods. The integration of diamond and GaN has been highly pursued for thermal management purposes as well as combining their exceptional complementary properties for power electronics applications and novel semiconductor heterostructures. In this thesis, we discuss the key hindrances towards realisation of diamond on GaN substrates and devices, and we propose novel solutions for improving the feasibility of diamond-on-GaN integration for efficient thermal management of hot spots in GaN devices and development of diamond devices integrated with GaN devices in the same chip.The growth of diamond on GaN is challenging due to the high lattice and thermal expansion mismatches. The weak adhesion of diamond to GaN and high residual stresses after the deposition often result in the diamond film delamination or development of cracks, which prevents the subsequent device fabrication. In this thesis, a new seed dibbling method is presented for seeding and growing high-quality diamond films on cost-effective GaN-on-Si substrates, which result in significantly larger diamond grains and lower residual stresses (as low as 0.2 GPa) compared to conventional methods. Moreover, the excellent adhesion obtained by this method enabled a reliable polishing of the as-grown diamond films on GaN-on-Si without any delamination, resulting in smooth diamond-on-GaN substrates with sub-nanometer roughness. The high temperature hot spots in GaN devices are addressed by the near-junction heat spreaders using diamond films deposited on GaN. An analytical model for the heat spreading is presented to analyse the performance and optimize the heat spreaders, considering the geometry and thermal conductivity of the heat spreader and the substrate. Experimental demonstration of diamond heat spreaders on vertical GaN diodes together with the thermal characterization of their behaviour shows significant reduction of thermal spreading resistances, hot spot temperatures and temperature gradients from the device footprint, which highlights the impact of such heat spreaders for the thermal management of high power density GaN devices.The diamond films deposited on GaN were also used to demonstrate high-performance diamond transistors integrated on AlGaN/GaN-on-Si substrates with characteristics such as high breakdown voltage, high current density, and high on/off ratio, which opens many new possibilities for the development of power ICs with complementary logics, gate drivers and power

EPFL2021

LiNiO Junction Gate for High-performance Enhancement-mode GaN Power Transistor

Taifang Wang

GaN metal-oxide-semiconductor high electron mobility transistors (MOS)HEMTs) offer outstanding properties for next-generation power electronics devices. The high conductivity, high voltage blocking capability, high operation frequency, and device-level integration can be achieved on the same tech-nology platform. Moreover, the development of large-scale GaN-on-Si substrate reduces the cost of GaN lateral devices. That GaN power device has already been widely used in today's consumer electronics, and communications base stations with its superiority of power density, and energy efficiency.Despite the ideal side of GaN material for power transistor, the junction less structure of HEMT made it hard to achieve enhancement-mode (e-mode) operation. Existing solutions either rely on complicated processes or sacrifice device performance. A simple process, high performance, and reliable operation e-mode device are desired. This thesis proposes Li doped NiO as gate material combined with Tri-gate structure to achieve e-mode operation. This relied on a simple oxide deposition process, without using barrier recess or re-growth, and achieved a high-quality interface. These critical characteristics haven't been demonstrated on gate stack engineering for normally-off devices and reveal the outstanding stability of LiNiO as a junction gate.Moreover, to maximize the GaN power transistor performance boundary, a multi-channel junction gate device was developed, which uses multiple 2DEG channels to significantly reduce the device resistance while still operating at high breakdown voltage (VBR). The multi-channel junction gate de-vice presents state-of-the-art performance, stable operation, and simple process e-mode GaN transistor. Apart from achieving e-mode operation, the possibility of device-level integration was also explored. Reverse current conduction ability was achieved together with good on-state performance and voltage blocking capability. And the alternative method of regulating electric field to Tri-gate by grayscale lithography was developed. The results in this thesis reveal the great value of using oxide semiconductor LiNiO as junction gate, demonstrating high performance, e-mode, and reliable device, and unleashing the performance potential through multi-channel epitaxy.

EPFL2022

LiNiO Junction Gate for High-performance Enhancement-mode GaN Power Transistor

Taifang Wang

EPFL2022

Diamond on GaN integration for power semiconductor devices with efficient thermal management

Reza Soleiman Zadeh Ardebili

EPFL2021