Fundamental limits for near-junction conduction cooling of high power GaN-on-diamond devices

Abstract

The integration of differing materials can enable breakthrough performance for semiconductor devices. One example is the integration of gallium nitride (GaN) and diamond to form GaN-on-diamond, which enables high-power GaN devices to achieve extreme power densities and, arguably, approaches fundamental limits for conduction cooling. Here, we examine the fundamental limits for near-junction phonon conduction cooling of GaN-on-diamond devices via finite element calculations of their lowest possible thermal resistance. A semi-classical transport theory for phonons interacting with interfaces and defects is used to calculate the in-plane thermal conductivity of a GaN epilayer and thereby accurately account for the thermal spreading resistance of the GaN layer. The device thermal resistance of a state-of-the-art GaN-on-diamond structure is predicted to be ∼13.0 K mm W−1 for a 12 finger device with 30 μm gate-to-gate spacing and a power dissipation of 5 W mm−1. For the same multifinger cell geometry and dissipated power, device thermal resistances as low as ∼10.0 K mm W−1 may be possible with assuming anisotropic but homogeneous diamond, as well as the absence of phonon scattering by external defects in the GaN layer and interface.