The impact of device architecture on the thermal response of AlN/AlGaN digital alloy field-effect transistors

A digital alloy is a superlattice-like nanostructure formed by stacking ultra-thin (≤4 monolayers) AlN and GaN layers periodically. Digital alloys allow for the tunability of the bandgap and electrical transport behavior. However, for them to be explored for electronic device applications, it is crucial that we determine their thermal properties, as this greatly impacts the thermal resistance and heat spreading within a device. Here we investigate the thermal properties of various AlN/AlGaN and Al_xGa_1−xN/Al_yGa_1−yN digital alloys (where x and y are the associated alloy composition) are investigated using the combined techniques of time-domain thermoreflectance and steady-state thermoreflectance. A highly anisotropic thermal conductivity of 9.6 W/m-K (cross-plane) and 39.8 W/m-K (in-plane) was measured for an AlN/AlGaN digital alloy (0.86/5.93 nm period thickness), while all measured Al_xGa_1−xN/Al_yGa_1−yN digital alloys measured a thermal conductivity of 2.9–3.3 W/m-K (cross-plane) and 8.6 W/m-K (in-plane). To investigate the influence of these thermal properties have on in-planedevice thermal transport, a number of die-level thermal management approaches are investigated on an AlGaN metal–semiconductor field-effect transistor using numerical simulations. The effects of the various cooling approaches on the device channel temperature were comprehensively investigated, along with guidance for material selection to enable the most effective thermal solutions. Specifically, we investigate the influence of substrate material, top-side heat spreader thickness/thermal conductivity, digital alloy thickness, and flip-chip design. Overall, this numerical study shows that it is possible to achieve high power digital alloy device operation with appropriate die-level thermal management solutions.