Extremely Low Thermal Resistance Architectures for AlxGaN1-x Semiconductor Devices

Abstract

Next-generation high-power radio-frequency (RF) devices increasingly demand transistors that operate efficiently with high gain at high frequencies. High-aluminum-content ultra-wide-bandgap (UWBG) AlGaN alloys have shown great potential for enabling such high-frequency RF technologies. However, the widespread adoption of AlGaN-based RF devices is limited by thermal-management challenges arising from the intrinsically low thermal conductivity of AlGaN, which leads to higher device thermal resistance for a given geometry compared to GaN RF devices. As a result, these next-generation devices are highly susceptible to self-heating. This study investigates the thermal behavior of UWBG AlGaN devices, focusing on the effects of AlGaN channel thickness, substrate technology, and high-k material integration on reducing device thermal resistance to enable high-power operation. Experimental results demonstrate a record-low thermal resistance of 3.96 mm$\cdot$K/W when an AlN substrate is employed and the AlGaN channel thickness is reduced to 5 nm. These findings provide valuable insights into mitigating thermal limitations in UWBG devices through device-level engineering and the strategic integration of high-k materials.

Figures (5)

• Figure 1: Cross-sectional schematic diagrams of the AlGaN HEMT devices: (a) thick-channel device with a 500 nm channel fabricated on AlN (AlGaN-500-on-AlN) and sapphire (AlGaN-500-on-sapphire) substrates with a simplified thermal resistance networks across the layers where $R_1$ is the resistance across the substrate, $R_2$ is the combined resistance across the buffer layer, $R_3$ is the resistance across the channel, and $R_4$ is the resistance across the barrier layers, and (b) thin-channel device with a 5 nm channel fabricated on AlN (AlGaN-5-on-AlN) and sapphire (AlGaN-5-on-sapphire) substrates. (c) Schematic illustration of the gate resistance thermometry (GRT) design used for thermal characterization. (d) Numerical setup, (I) the schematics the numerical setup with all the boundary conditions, (II) top view of the source, drain and gate in the numerical setup, (III) image of the experimental setup on the SanjSCOPE™ EZ500 with the 100X objective lens.
• Figure 2: Output characteristics of the fabricated HFET devices: (a) a 500 nm AlGaN channel on AlN ( AlGaN-500-on-AlN ) and (b) a 5 nm AlGaN channel on AlN (AlGaN-5-on-AlN). For each device, the drain current $I_D$ was measured by sweeping the drain–to-source voltage $V_{DS}$ from 0 to 10 V under gate-to–source voltages $V_{GS}$ of 0 V, -3 V and –5 V.
• Figure 3: (a) Comparison of the average temperature rise across the gate metal for different power densities for the four different devices with their respective numerical model results. (b) The respective thermal resistance comparison between the four devices.
• Figure 4: Comparison of thermal resistance value from this work and with reported work for GaN hirama_rf_2012tadjer_gan--diamond_2019kim_thermal_2025pavlidis_thermal_2024kagawa_high_2024, AlGaN mamun_al064ga036n_2023chatterjee_interdependence_2020 and Ga$_2$O$_3$qu_extremely_2024lundh_electrothermal_2024 devices.
• Figure 5: (a) Transient thermal response showing the temperature rise versus time for the four devices under a 25% duty cycle with a 400 $\mu$s period at a power density of 3.35$\pm$0.15 W/mm. The solid lines represent the corresponding numerical simulation results for each device. (b) Thermoreflectance imaging illustrating the temperature rise at 97.3 $\mu$s for (I) AlGaN-500-on-sapphire, (II) AlGaN-500-on-AlN, (III) AlGaN-5-on-sapphire, and (IV) AlGaN-5-on-AlN. Note: The color scale for III and IV is reduced to enable clearer comparison.