Thermal Performance of Collector-Up HBTs for Small High-Power Amplifiers With a Novel Thermal via Structure Underneath the HBT Fingers.

The article describes thermal performance of a special heterojunction bipolar transistor (HBT) structure for mobile communication systems, called a collector-up HBT and calculates the thermal resistance between the HBT finders and the bottom surface of a GaAs substrate using a finite element method. An HBT is a bipolar transistor whose emitter/base junction consists of different materials. A GaAs HBT has a higher current gain, a higher breakdown voltage, and a superior RF performance when compared to Si bipolar transistors. Normal HBTs have an emitter-up configuration. Because of the extrinsic area underneath the base electrodes, emitter-up HBTs suffer from large collector capacitance that limits the RF performance.