3-DIMENSIONAL MODELING OF THE HEAT-FLOW INTO A GAAS SUBSTRATE - INFLUENCE OF THERMAL PHENOMENA ON THE RF BEHAVIOR OF POWER HBTS AND TECHNOLOGICAL OPTIMIZATION

Using three-dimensional modeling of the heat flow into the substrate, and taking into account the variation of thermal conductivity with tem perature, we study the sensitivity of the thermal resistance R-TH to p arameters such as the substrate thickness, its nature (GaAs, Si, AlN o r diamond), the topology of the power source (length and width) or eve n the power density. Thus, we show that the transfer of active GaAs la yers onto a host substrate with a higher thermal conductivity such as Si, AIN or diamond is of greater interest than the thinning of the sub strate. We developed an accurate electrothermal model for the AlGaAs/G aAs heterojunction bipolar transistor (HBT) allowing prediction of the static and mainly dynamic (RF) behavior at a high signal operating mo de. We show that if the HBT layers are transferred onto a diamond subs trate, a 50% gain in RF power can be expected at 10 GHz. Finally, we p resent the technological solutions investigated for the transfer and t he heteroassembly of HBT active layers on substrates better suited for thermal dissipation. (C) 1998 Elsevier Science Ltd. All rights reserv ed.