Gallium nitride (GaN) is a compound semiconductor that has tremendous potential to facilitate economic growth in a semiconductor industry that is silicon-based and currently faced with diminishing returns of performance versus cost of investment. At a material level, its high electric field strength and electron mobility have already shown tremendous potential for high frequency communications and photonic applications. Advances in growth on commercially viable large area substrates are now at the point where power conversion applications of GaN are at the cusp of commercialisation. The future for building on the work described here in ways driven by specific challenges emerging from entirely new markets and applications is very exciting. This collection of GaN technology developments is therefore not itself a road map but a valuable collection of global state-of-the-art GaN research that will inform the next phase of the technology as market driven requirements evolve. First generation production devices are igniting large new markets and applications that can only be achieved using the advantages of higher speed, low specific resistivity and low saturation switching transistors. Major investments are being made by industrial companies in a wide variety of markets exploring the use of the technology in new circuit topologies, packaging solutions and system architectures that are required to achieve and optimise the system advantages offered by GaN transistors. It is this momentum that will drive priorities for the next stages of device research gathered here.

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The 2018 GaN power electronics roadmap

AlGaN/GaN high-electron mobility transistors stressed under dc bias at various channel temperatures were studied using transmission electron microscopy for evidence of physical damage. Stressed devices consistently developed crack- and pit-shaped defects in the AlGaN/GaN crystal material under the drain-side edge of the gate, whereas side-by-side as-processed unstressed devices did not show these features. Furthermore, the amount of physical damage was found to correlate to the amount of electrical degradation as measured by the change in IDmax from before and after stress. The formation of these defects is consistent with the theory of damage from the inverse piezoelectric effect.

TEM Observation of Crack- and Pit-Shaped Defects in Electrically Degraded GaN HEMTs

The peak power density of GaN high-electron-mobility transistor technology is limited by a hierarchy of thermal resistances from the junction to the ambient Here, we explore the ultimate or fundamental cooling limits for junction-to fluid cooling, which are enabled by advanced thermal management technologies, including GaN–diamond composites and nanoengineered heat sinks Through continued attention to near-junction resistances and extreme flux convection heat sinks, heat fluxes beyond 300 kW/cm $^{2}$ from individual 2- $\mu \text{m}$ gates and 10 kW/cm $^{2}$ from the transistor footprint will be feasible The cooling technologies under discussion here are also applicable to thermal management of 25-D and 3-D logic circuits at lower heat fluxes

Fundamental Cooling Limits for High Power Density Gallium Nitride Electronics

The wide bandgap, high-breakdown electric field, and high carrier mobility makes GaN an ideal material for high-power and high-frequency electronics applications, such as wireless communication and radar systems. However, the performance and reliability of GaN-based high-electron-mobility transistors (HEMTs) are limited by the high channel temperature induced by Joule heating in the device channel. Integration of GaN with high thermal conductivity substrates can improve the heat extraction from GaN-based HEMTs and lower the operating temperature of the device. However, heterogeneous integration of GaN with diamond substrates presents technical challenges to maximize the heat dissipation potential brought by the ultrahigh thermal conductivity of diamond substrates. In this work, two modified room-temperature surface-activated bonding (SAB) techniques are used to bond GaN and single-crystal diamond. Time-domain thermoreflectance (TDTR) is used to measure the thermal properties from room temperature to 480 K. A relatively large thermal boundary conductance (TBC) of the GaN/diamond interfaces with a ∼4 nm interlayer (∼90 MW/(m2 K)) was observed and material characterization was performed to link the interfacial structure with the TBC. Device modeling shows that the measured TBC of the bonded GaN/diamond interfaces can enable high-power GaN devices by taking full advantage of the ultrahigh thermal conductivity of single-crystal diamond. For the modeled devices, the power density of GaN-on-diamond can reach values ∼2.5 times higher than that of GaN-on-SiC and ∼5.4 times higher than that of GaN-on-Si with a maximum device temperature of 250 °C. Our work sheds light on the potential for room-temperature heterogeneous integration of semiconductors with diamond for applications of electronics cooling, especially for GaN-on-diamond devices.

Interfacial Thermal Conductance across Room-Temperature-Bonded GaN/Diamond Interfaces for GaN-on-Diamond Devices.

We review the Raman thermography technique, which has been developed to determine the temperature in and around the active area of semiconductor devices with submicron spatial and nanosecond temporal resolution. This is critical for the qualification of device technology, including for accelerated lifetime reliability testing and device design optimization. Its practical use is illustrated for GaN and GaAs-based high electron mobility transistors and opto-electronic devices. We also discuss how Raman thermography is used to validate device thermal models, as well as determining the thermal conductivity of materials relevant for electronic and opto-electronic devices.

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A Review of Raman Thermography for Electronic and Opto-Electronic Device Measurement With Submicron Spatial and Nanosecond Temporal Resolution

Thermal conductivity of the substrate affects the performance of high power RF devices. It is a dominant limiting factor in current state-of- the-art GaN HEMTs on SiC substrate. Due to high thermal conductivity, diamond substrate is an attractive alternative for GaN HEMTs. We have developed device quality GaN-on-diamond wafers using CVD diamond and fabricated 0.25 μm gate length HEMTs. We present detailed electrical and thermal results of the fabricated devices, which show RF power comparable to standard GaN-on-SiC HEMTs. We demonstrate over 25 % lower channel temperature for these devices compared to GaN-on-SiC devices. Electrical results using DC and RF tests and thermal results using IR thermography and micro-Raman spectroscopy are included.

Electrical and Thermal Performance of AlGaN/GaN HEMTs on Diamond Substrate for RF Applications

GaN-based RF transistors offer impressive power densities, although to achieve the maximum potential offered by GaN, thermal management must be improved beyond the current GaN-on-SiC devices By using diamond, rather than SiC substrates, transistor thermal resistance can be significantly reduced It is important to experimentally verify thermal resistance, rather than relying solely on simulation expectations, using measurement results to aid further optimization The novel thermal characterization methodology presented here combines Raman thermography and simulation to determine the substrate thermal conductivity and GaN/substrate thermal resistance in GaN-on-diamond devices Measured GaN-on-diamond interfacial thermal resistance is similar to reported values for GaN-on-SiC, whereas the diamond substrate thermal conductivity is substantially higher, resulting in a significantly improved thermal resistance with respect to GaN-on-SiC, with great potential for further improvement

Achieving the Best Thermal Performance for GaN-on-Diamond

A light-emitting device includes a GaAs substrate, a light-emitting structure disposed above the substrate and capable of emitting light having a wavelength of about 1.3 microns to about 1.55 microns, and a buffer layer disposed between the substrate and the light-emitting structure. The composition of the buffer layer varies through the buffer layer such that a lattice constant of the buffer layer grades from a lattice constant approximately equal to a lattice constant of the substrate to a lattice constant approximately equal to a lattice constant of the light-emitting structure. The light-emitting device exhibits improved mechanical, electrical, thermal, and optical properties compared to similar light-emitting devices grown on InP substrates.

Long wavelength laser diodes on metamorphic buffer modified gallium arsenide wafers

The design and performance of a high efficiency Ka-band power amplifier MMIC utilizing a 0.15um high voltage GaAs PHEMT process (HV15) is presented. Experimental continuous wave (CW) in-fixture results for the power amplifier MMIC demonstrate up to 5W of saturated output power and 30% associated power added efficiency at 35GHz.

High Efficiency Ka-Band Power Amplifier MMIC Utilizing a High Voltage Dual Field Plate GaAs PHEMT Process

High voltage GaAs pHEMT technology with field plated gates has been in development at TriQuint since 2000, resulting in three processes that deliver state of the art performance up to Ku-band. We report on an S-band optimized version that will soon be transferred to production. At 3.5 GHz, it delivers power output density of 2.1 W/mm and 64 % PAE at 28 V. This technology is optimal for designing high power MMICs at S-band and provides a competitive alternative to GaN and SiC devices especially when cost, reliability and maturity are considered.

David M. Fanning

Papers

Electrical and Thermal Performance of AlGaN/GaN HEMTs on Diamond Substrate for RF Applications

Achieving the Best Thermal Performance for GaN-on-Diamond

Long wavelength laser diodes on metamorphic buffer modified gallium arsenide wafers

High Efficiency Ka-Band Power Amplifier MMIC Utilizing a High Voltage Dual Field Plate GaAs PHEMT Process

High voltage GaAs pHEMT technology for S-band high power amplifiers