Geometry Effect on SiGe Heterojunction Bipolar Transistor Unit Cell for 1 W High-Efficiency RF Power Amplifier Applications

Abstract

The effect of geometry on the RF power performance of silicon–germanium heterojunction bipolar transistor (SiGe HBT) unit cells is investigated using various emitter finger spacing (S). Two unit cells, namely, HBT-1 and HBT-2 with the same emitter area of 8×0.6×10 µm3 but with different S values are thoroughly discussed. The S values of HBT-1 and an HBT-2 are 2 and 5 µm, respectively. The obtained measurements, including DC characteristics and small- and large-signal performance characteristics of high-breakdown SiGe HBT unit cells, are presented. The HBT-1 in class-AB operations at 2.4 GHz achieves an output 1 dB compression point (OP1dB) of 16.0 dBm, a maximum output power of 17.4 dBm, and a peak-power added efficiency (PAE) of 59.1%. Under the same testing conditions, HBT-2 achieves an OP1dB of 19.6 dBm, a maximum output power of 20.6 dBm, and a PAE of 64.5%. HBT-2 yields significant improvements in all power performance parameters compared with HBT-1, such as 3.6 dB in an OP1dB, a maximum output power of 3.2 dB, a PAE of 5.4%, and an improvement in the power performance figure of merit (FOM) of approximately 50%, which is attributed to the fact that HBT-2 has a lower thermal effect than HBT-1. The thermal effect affects both DC and output power characteristics. A 1 W power device fabricated by combining eight HBT-2 unit cells achieves a power gain of 14.5 dB and a maximum PAE (PAEmax) of 75% in a class-AB operation at 2.4 GHz. The power density is calculated to be up to 2.6 mW/µm2. These results demonstrate that SiGe HBT technology has great potential for high-power amplifier applications.