Design of C-up GaAs HBTs with backside emitter for small power amplifiers

Poor thermal conduction of a GaAs substrate in GaAs heterojunction bipolar transistor (HBT) power amplifiers (PAs) previously required thermal designs such as ballast resistors and a large finger pitch (∼ 30 μm). This paper describes the successful fabrication of ballast-free PAs with a small finger pitch (15 μm) using a collector-up (C-up) configuration with a backside emitter structure. Having via-holes directly under C-up HBTs is convenient in terms of thermal conduction; PAs composed of multi-finger HBTs in this configuration occupy one-third less area than those with HBTs in the conventional configuration. A small 32-finger C-up HBT we fabricated with a total area of 0.25 × 0.31 μm² delivered a power-added efficiency of 52% at 24.4 dBm in wide-band CDMA operation. The results show that C-up HBTs with a backside emitter structure have strong potential for microwave high-power application.