High Frequency Characterization and Modeling of SiGe Heterojunction Bipolar Transistors

High-speed, low voltage Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) have been designed, fabricated, electrically characterized and modeled. The SiGe HBTs are suitable for use in radio frequency (RF) integrated circuit (IC) applications and were fabricated using non-selective epitaxial growth. The design of the extrinsic base region has been investigated in detail. Transient enhanced diffusion of boron, caused by the extrinsic base implantation, was found to degrade DC and high-frequency electrical characteristics. It was also found that the low-frequency noise was affected by the base design. Furthermore, a hydrogen anneal was found to reduce the low-frequency noise in polysilicon emitter bipolar transistors. The high-frequency noise of SiGe HBTs was investigated experimentally as well as by device simulation. Noise parameter extraction methods based on direct admittance or Y-parameter measurement have been investigated in detail. Good agreement was found with conventional noise figure measurement. SiGe HBTs were fabricated to investigate the optimization of ion-implanted collector doping profiles. A novel concept was suggested where a low-energy (5–10 keV) antimony (Sb) implantation was combined with a standard selectively implanted collector (SIC) using phosphorous. Segregation of Sb was found to occur during the subsequent growth of the epitaxial SiGe base layer. The resulting broadening of the implanted Sb-profile degraded the DC-electrical characteristics of the device. The devices with an Sb-implantation exhibited a cut-off frequency of more than 60 GHz. A mixedmode circuit and device simulation methodology was developed to investigate the RF harmonic distortion of SiGe HBTs. The influence on harmonic distortion of the Ge-profile as well as the collector doping profile was quantified. High-injection heterojunction barrier effects due to the presence of a valence band offset at the base-collector junction were found to significantly affect the harmonic distortion. Devices with a Ge-profile retrograded towards the collector exhibited significantly reduced harmonic distortion. Increasing the Ge-concentration at the base-emitter junction led to reduced harmonic distortion for low current operation. A nonuniform collector doping profile was shown to suppress the harmonic distortion and increase the breakdown voltage.