Thermal performance of collector-up HBTs for small high-power amplifiers with a novel thermal via structure underneath the HBT fingers

We will describe the thermal performance of a special heterojunction bipolar transistor (HBT) structure for mobile communication systems, called a collector-up HBT. We calculated the thermal resistance between the HBT fingers and the bottom surface of a GaAs substrate using a finite element method (FEM). The results suggest that the thermal resistance of collector-up HBTs with thermal via structures can be reduced by 64% compared to the thermal resistance of ordinary emitter-up HBTs. They also show that the thickness of the InGaP emitter layer effects the thermal resistance of, and the temperature distribution in, the collector fingers of collector-up HBTs. Even though the thermal resistance of collector-up HBTs can be much smaller than that of emitter-up HBTs, a thermal interaction between the collector fingers still exists in multi-finger structures. We analyzed the temperature distribution in the collector fingers of a four-finger HBT structure and found that the thickness of the plated heat sink (PHS) was not sufficient to reduce the thermal interaction between the HBT fingers, and that optimization of the HBT location was needed to minimize the thermal interaction. We also found that the thickness of the InGaP emitter layer was the most important parameter for reducing thermal resistance, even in four-finger HBT structures. These calculation results can be used to reduce the temperature of collector-up HBTs and the temperature differences between the HBT fingers in the development of power amplifiers with collector-up HBTs.