Development of III-Nitride HEMTs on CVD Diamond Substrates

F. Faili1, D.I. Babid1, D. Francis1, F. Ejeckam1, J.D. Blevins2
1. Group4 Labs, LLC, 39500 Stevenson Place, STE 207, Fremont, CA
2. Air Force Research Laboratory, Sensors Directorate, Wright-Patterson AFB, OH
Keywords: GaN, high-electron-mobility transistor (HEMT), CVD diamond, thermal management

Abstract

AlGaN/GaN high electron mobility transistors (HEMT) semiconductor technology holds promise for revolutionary improvements in the cost, size, weight, and performance of a broad range of military and commercial microelectronics. AlGaN/GaN HEMTs have demonstrated incredible power densities exceeding 40 W/mm. However, this new generation of high power microwave devices faces significant thermal challenges due to ever increasing power densities. Due to its relatively low thermal conductivity, GaN is unable to effectively remove heat generated during device operation. Heat is typically conducted through a hierarchy of material structures, each of which adds thermal resistances. Exploiting the true capabilities of GaN is a compromise between the desired RF performance and the realities of current thermal solutions. Efficient thermal management is essential for realizing GaN’s intrinsic capabilities. Current GaN RF transistor performance is limited by localized heating in a very thin AlGaN epitaxial layer near the gate region. Ideally, the optimal solution would be to integrate thermal management directly into the GaN device thus minimizing design challenges for the system integrator. SiC is presently the substrate of choice for the epitaxial growth of high performance nitride structures. An alternative to SiC is to integrate a high thermal conductive heat spreading material as close as possible to the gate region. Diamond provides the highest thermal conductivity of any known material and would be an ideal heat spreading material. The superior thermal conductivity and insulating properties of polycrystalline diamond (1000-1500 W/m-K) is 3-5X’s more thermally conductive than SiC and 5X’s greater than copper. Incorporation of diamond films near the active device junction would spread heat away from the gate and entire device thus offering the potential of pushing GaN to its thermal performance limits. Fabrication of GaN-on-Diamond HEMTs involves transferring an AlGaN/GaN epitaxial layer from its host silicon and/or sapphire substrate and bonding to a 100um thick CVD diamond substrate. Current state of GaN-on-Diamond HEMT development includes ft = 85 GHz, 5 W/mm, PAE=60% power at 10 GHz. Significant technological challenges remain such as improving wafer bow on large (>50 mm) wafers and removing the lower thermal conductivity AlGaN nucleation layers . Manufacturing process development details and performance of AlGaN/GaN high-electron-mobility transistors (HEMTs) fabricated on freestanding diamond substrates will be reported.

Paper 6b.2.pdf