Materials optimization for high power silicon germanium heterostructure bipolar transistors at X-band frequencies

SiGe base layers with high (>0.15 Ge) can potentially extend the range of high power HBTs to X-band and higher frequencies, but in practice the actual performance of these devices is usually poorer than the anticipated results. Optimization and reproducibility of materials properties is essential for the successful development of these devices, and this paper reports microstructural characterization of high-Ge content films, simulates the performance of an HBT structure, and provides insights into how material defects impact device performance. High resolution X-ray diffraction indicates that the crystalline quality of SiGe layers is highly degraded with increasing Ge content, and interfacial relaxation becomes apparent for Ge>0.15. The SiGe layer thicknesses and Ge contents were measured using both optical ellipsometry and SIMS, and the results were comparable. Simulated data of an HBT transistor shows that minor variations in Ge content alter the cutoff frequencies and gain of the transistors, and base recombination times less that /spl ap/5/spl times/10/sup -7/ seconds severely impair device performance.