On the Substrate Thermal Optimization in SiC-Based Backside-Mounted High-Power GaN FETs

Abstract

This paper presents a discussion on the substrate thermal design of backside-mounted power GaN high-electron mobility transistors. After a review on the thermal properties of the relevant materials and their temperature dependences, design guidelines are proposed on the basis of 3-D thermal simulations; the results presented suggest that in SiC-based devices, substrate thinning does not typically improve the thermal resistance or the dynamic thermal behavior. Contrary to what happens in III-V GaAs-or InP-based discrete or integrated devices, therefore, microstrip design on a thinned substrate (as opposed to coplanar design on a comparatively thick substrate) is generally not thermally superior. This should make possible, from the thermal standpoint, the realization of coplanar multifunctional GaN-based monolithic microwave integrated circuits integrating, e.g., low-noise and power stages and avoiding the use of via holes. Index Terms-GaN, power modulation-doped field-effect transistors (MODFET), semiconductor device thermal factors, SiC.

Three-dimensional modeling of AlGaN/GaN HEMT including electro-thermal coupling effects

“…In addition, the thermal conductivities of the GaN and SiC vary considerably in the reported literatures [8]. The uncertainties of the thermal parameters could cause the simulated results deviating from the practical temperature distribution.…”

A Numerical Study on Comparing the Active and Passive Cooling of AlGaN/GaN HEMTs

“…An additional study in [23] investigated the impact of substrate thickness on the thermal resistance of AlGaN/GaN HEMTs that were backside mounted to either Cu or CuMo heat sinks. From [22] and [23], it was shown that the thermal resistance of a packaged AlGaN/GaN HEMT has a complex relationship to substrate thickness, gate length, gate width, and boundary conditions on the backside of the substrate. The results for our structure are shown in Figs.…”

“…It should be noted that these values may not be the minimum values for the active cooling case. In [23], it was clearly shown that if the thermal conductivity of the layers underneath the die are large, then die thinning can be used to reduce thermal resistance. However, if the conductivity is small, then die thinning may result in an increase in thermal resistance due to the lack of heat spreading.…”

Thermal Analysis of AlGaN/GaN HEMTs Using Angular Fourier-Series Expansion

“…Thinner chips have been used in GaAs power amplifiers for the purpose of reducing thermal resistance. Where highly thermally conductive substrates are available (e.g., SiC or Diamond), thinning the substrate is often disadvantageous as thermal resistance actually improves (i.e., reduces) with increasing substrate thickness owing to improved heat spreading [4,8,18,19]. However, substrate thickness is also determined by the microstrip transmission-line and thru-substrate via properties.…”

“…The SiC substrate thermal conductivity value 35OW/mK [19] was also deduced from datasheets of commercially available semi-insulating SiC substrates. The slight anisotropy of SiC thermal conductivity is neglected.…”