Impact of Emitter Width Scaling on Performance and Ruggedness of SiGe HBTs for PA Applications

We present performance and ruggedness trade-offs for the different emitter widths of SiGe HBTs using GLOBALFOUNDRIES 1K5PAXE technology for power amplifier (PA) applications. The technology offers HBTs with low intrinsic base resistance (RBI) and low emitter-base capacitance $(\mathbf{C}_{\mathbf{BE}})$ which allows for wide emitter devices essential for high power density PA designs through improved emitter utilization. Load-pull measurements of HBTs with $\mathbf{W}_{\mathbf{E}}=1.2\mu m$ show ~1.5dB higher gain at 5.8GHz for a ~17% smaller footprint compared to $\text{W}_{\text{E}}=0.8\mu m$ HBT (for a fixed emitter area). However, smaller footprint increases thermal resistance which degrades ruggedness. Simulations show that the ruggedness can be recovered through power cell layout optimization or by using emitter ballasting techniques. This paper also shows the importance of good mutual heating model between devices for first pass design success and reduced design cycle time.