FLEXIBLE, THIN-FILM, GaAs HETERO-JUNCTION BIPOLAR TRANSISTORS MOUNTED ON NATURAL DIAMOND SUBSTRATES

Thermal considerations hecome important for optimal operation of high power heterojunction bipolar transistors (HBTs) in applications such as the next generation of phased-array radar systems. In particular, devices grown on GaAs substrates suffer due to the mediocre thermal conductivity of the III-V compound semiconductors. The maximum achievable power density before device performance degrades is mainly limited by the junction temperature. As a consequence, a great deal of interest has arisen[ 1,2,3] in determining both the experimental and theoretical temperature profile in GaAs ICs. Nevertheless, these thermal constraints can be greatly alleviated using alternative packaging concepts for removing the excess heat[4]. With the recent advances in thin film handling and manipulation technologies, the active layers of electronic circuitry can be separated from their substrates on which they were synthesized and furthermore mounted onto different types of substrates with suitable thermal properties. Yablonovitch et a/.[51 have demonstrated the bonding of CiaAs epitaxial films onto arbitrary substrates using epitaxial lift-off (ELO) and palladium bonding techniques. This process relies on the extremely high etching selectivity between the substrate and a sacrificial or release layer[6]. Another similar process consists of back etching the substrate up to an etch stop layer and is hereafter referred to as the total substrate removal (TSR) technique. The resulting thin film can be transferred and bonded onto an efficient heat sinking substrate. Natural type IIA diamond has a thermal conductivity, K,h-dMllond of 20 W/cm/K, or 40 times greater than the thermal conductivity K,h_GaAr of GaAs. Recently, using TSR techniques with a combination of mechanical and CF, plasma etching, Sullivan et a/.[71 have transferred AIGaAs/GaAs heterojunction bipolar transistors (HBTs) onto CVD grown diamond and observed a significant drop in the total thermal resistance. In their work, a 500 nm thick layer of indium solder served as a bonding inter-layer between GaAs and the polycrystalline CVD diamond substrate. To investigate the ultimate thermal performance which may be achieveable, in the present work we transfer AlGaAs/GaAs HBTs directly onto natural, single crystal, type IIA diamond substrates. limited only by a IO nm Pd inter-layer.