Emitter-Base Profile Optimization of SiGe HBTs for Improved Thermal Stability and Frequency Response at Low-Bias Currents

We explore different vertical profile designs with optimized emitter-base (EB) junctions targeting both constant current gain $(\beta)$ across temperature and broadened $\mathbf{f}_{\mathbf{T}}/\mathbf{f}_{\max}$ curves for improved large-signal linearity. This work explicitly examines achieving a temperature-independent $\beta$ via profile design in SiGe HBTs, and explores the limitations using triangular and ledge-based Ge profiles at the EB junction. The effects of base width and the EB junction separation length are also investigated for reduced parasitic capacitance and improved frequency response at low-bias currents. This work presents the underlying theory, along with the measured results for the two optimization targets, both of which should aid in designing circuits with better linearity and stability across bias and temperature corners.