Abstract
To optimize the thermal design of AlGaN-GaN high-electron-mobility transistors (HEMTs), which incorporate high power densities, an accurate prediction of the underlying thermal transport mechanisms is crucial. Here, a HEMT-structure (Al0.17Ga0.83N, GaN, Al0.32Ga0.68N and AlN on a Si substrate) was investigated using a time-domain thermoreflectance (TDTR) setup. The different scattering contributions were investigated in the framework of phonon transport models (Callaway, Holland and Born-von-Karman). The thermal conductivities of all layers were found to decrease with a temperature between 300 K and 773 K, due to Umklapp scattering. The measurement showed that the AlN and GaN thermal conductivities were a magnitude higher than the thermal conductivity of Al0.32Ga0.68N and Al0.17Ga0.83N due to defect scattering. The layer thicknesses of the HEMT structure are in the length scale of the phonon mean free path, causing a reduction of their intrinsic thermal conductivity. The size-effect of the cross-plane thermal conductivity was investigated, which showed that the phonon transport model is a critical factor. At 300 K, we obtained a thermal conductivity of (130 ± 38) Wm−1K−1 for the (167 ± 7) nm thick AlN, (220 ± 38) Wm−1K−1 for the (1065 ± 7) nm thick GaN, (11.2 ± 0.7) Wm−1K−1 for the (423 ± 5) nm thick Al0.32Ga0.68N, and (9.7 ± 0.6) Wm−1K−1 for the (65 ± 5) nm thick Al0.17Ga0.83N. Respectively, these conductivity values were found to be 24%, 90%, 28% and 16% of the bulk values, using the Born-von-Karman model together with the Hua–Minnich suppression function approach. The thermal interface conductance as extracted from the TDTR measurements was compared to results given by the diffuse mismatch model and the phonon radiation limit, suggesting contributions from inelastic phonon-scattering processes at the interface. The knowledge of the individual thermal transport mechanisms is essential for understanding the thermal characteristics of the HEMT, and it is useful for improving the thermal management of HEMTs and their reliability.
Highlights
The ability of gallium nitride (GaN) to form heterojunctions can be used to fabricate high-electron-mobility transistors (HEMTs)
The paper has three major topics: Firstly, we systematically investigated a material stack of a high-electron-mobility transistor (HEMT) on a silicon (Si) substrate
The thermal conductivity (κ) of the constituent materials and thermal interface conductance were measured by the time-domain thermoreflectance (TDTR)
Summary
The ability of gallium nitride (GaN) to form heterojunctions can be used to fabricate high-electron-mobility transistors (HEMTs). Aluminium gallium nitride (AlGaN)-GaN-based HEMTs offer high carrier concentration (~1013 cm−2 [1]) and high electron mobility (2000 cm2 /Vs [2]), resulting in a high current density and a low channel resistance. These properties offer great potential in the power amplifier technology due to their high-power and high-frequency performance. Energies 2020, 13, 2363 power density in the AlGaN-GaN-HEMT allows further miniaturization of the device with gate lengths down to 80 nm [4] This high-power density leads to significant self-heating, which decreases the device performance. Thermal bottlenecks that have to be considered in GaN-based HEMTs are: (i) the reduced thermal conductance of transition layers (here: AlGaN) [8], (ii) the heat transport across interfaces [9], and (iii) the reduced thermal conductivity due to phonon-scattering processes [10,11]
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