Geometry effect on SiGe heterounction bipolar transistor unit cell for 1W high-efficiency RF power amplifier applications

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 (5). Two unit cells, namely, HBT-I 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-I 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 2.4GHz achieves an output 1 dB compression point (OP _1dB) of 16.0dBm, a maximum output power of 17.4dBm, and a peak-power added efficiency (PAE) of 59.1%. Under the same testing conditions, HBT-2 achieves an OP_1dB 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 3.6dB in an OP _1dB, a maximum output power of 3.2dB, 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 1W power device fabricated by combining eight HBT-2 unit cells achieves a power gain of 14.5 dB and a maximum PAE (PAE_max) of 75% in a class-AB operation at 2.4GHz. The power density is calculated to be up to 2.6mW/μm². These results demonstrate that SiGe HBT technology has great potential for high-power amplifier applications.