SiGe HBT design tradeoffs between small signal and large signal RF performance

The paper examines the inherent SiGe HBT design tradeoffs between small signal and large signal RF performance for the first time. A higher Ge content and a higher Ge gradient in the base are typically desired to reduce RF noise figure, 1/f noise, phase noise, and to improve f_T/f_max. This, however, requires the use of less Ge retrograding into the collector, which worsens high injection performance, and could degrade large signal output power and power added efficiency. Similarly, the use of a high breakdown voltage device for a higher output voltage swing could inadvertently hurt large signal output power because of a decreased current handling capability. Measurements on 200 GHz peak f_T HBTs show that excellent large signal output power and power added efficiency can be achieved for 20 GHz operation, despite the inevitable breakdown voltage decrease with scaling.