Impact of Buffer Layer on the Thermal Performance of Multi Channel AlGaN/GaN Transistors

The formation of the two dimensional electron gas (2DEG) in AlGaN/GaN high electron mobility transistors (HEMTs) has enabled devices that can operate at both high frequencies (&gt; GHz) and high voltages (&gt; 3 kV). These advantageous characteristics have translated to the commercialization of superior DC converters and high-power density radio frequency (RF) devices. The current record performances of single channel AlGaN/GaN RF transistors, however, have been thermally limited to power densities that are significantly lower than the expected theoretical limits. One approach to improve the power density is the implementation of superlattice based multichannel transistors. The stacking of adjacent 2DEGs has been proven to increase the current density and transconductance of HEMTs.<br/><br/>To maintain a low ON-resistance, the heat generated in these nanostructured devices must be efficiently removed which requires in-depth knowledge of nanoscale thermal transport. Two challenges to overcome include the low cross plane thermal conductivity of superlattices and the high thermal resistance of a thick AlGaN buffer layer. This study investigates the impact of implementing a thinner GaN buffer layer with higher thermal conductivity. Furthermore, to assess the feasibility of simplifying the thin film structure, the thermal performance of non-castellated multichannel HEMTs is assessed via two techniques: scanning thermal microscopy (SThM) and Raman thermometry. For direct comparison, a multichannel device with a thin GaN buffer layer is demonstrated to achieve the similar thermal resistance to a device that is epitaxially grown on a AlGaN buffer layer that has fewer number of channels. A thermal finite element model (FEM) is developed to explain the discrepancy between the temperatures measured via SThM (surface temperature) and Raman thermometry (depth average). The peak temperature is found to be in the proximity of the topmost channel which highlights the importance of surface temperature techniques, such as SThM, for accurate extraction of thermal resistance. Finally, the challenges associated with SThM measurements on structures with small channel spacing and thick metal contacts is discussed and a numerical solution via FEM modelling is proposed.