Novel selective-plated heatsink, key to compact 2-watt MMIC amplifier

We describe an improved heatsink technology fully compatible with a standard MMIC processing that significantly decreases the thermal limitation of the MMIC format. By etching to reduce the substrate thickness by 75 micron only immediately under the active areas of the MESFETs and selectively plating solid gold heatsinks to replanarize the back of the wafer, a 45 percent reduction in the thermal resistance is obtained. Despite a very compact design, 2.4 mm MESFETs fabricated on 100 micron thick substrates demonstrated a thermal resistance of only 18 C/W. Using these devices, a 2 stage 2 watt power MMIC was designed to fit a compact 1.4 mm x 3.25 mm chip footprint. The nominal 2 watt, 7-11 GHz power MMIC amplifier was designed for phased array applications where small size, high power and high efficiency are primary concerns. With fixed off-chip tuning, the MMIC delivers 1.7-2.3 watts with 18-24 percent power added efficiency across the full 7-11 GHz band.