Prospects for Gallium Nitride-on-Diamond Transistors

Strategies aimed at improving the near junction heat removal of Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are presently limiting GaN device technology from realization of its true capability [1]. Approximately ten years ago, Cree demonstrated AlGaN/GaN HEMTs with power densities exceeding 40 W/mm [2]. Control of the GaN junction temperature requires integration of thermal transport solutions near the heat source to ensure optimal performance and reliable operation [3]. An approach under consideration is the use of Chemical Vapor Deposition (CVD) polycrystalline diamond inserted within microns of the device junction. Recent AFRL and Defense Advanced Research Projects Agency (DARPA) efforts have shown that replacing the epitaxial host substrate with high thermal conductivity polycrystalline diamond substrates can improve the GaN HEMT areal power density >3Xs [4-9]. This paper will examine the motivation behind the use of diamond, integration approaches, material/device results and key technological challenges going forward.