SiGe Heterojunction Bipolar Transistor Mesa Architecture

The base structure is segmented into two distinct parts: an NSEG base formed by non-selective epitaxial growth and a raised external base formed by selective epitaxial growth. This segmentation allows each part to be optimized independently - the NSEG base provides the foundational structure while the raised external base enables precise alignment features, thereby achieving high alignment precision without requiring the entire structure to be formed by complex selective processes

The raised external base acts as an intermediary structure between the NSEG base and the emitter. It provides a platform for forming the emitter with better critical dimensions and reduced peripheral junctions, while the NSEG base provides the underlying support. This intermediary structure simplifies the overall manufacturing process compared to traditional fully self-aligned approaches that require multiple complex alignment steps

The hydrofluoric acid etch procedure is completely extracted and removed from the manufacturing process. Instead of using HF etching to form the emitter, the patent employs a selective epitaxial growth process that naturally forms the emitter structure with precise dimensions. This eliminates the harmful chemical etching step while maintaining or improving emitter formation precision through controlled epitaxial growth

The manufacturing approach changes from chemical etching (HF acid) to physical/chemical deposition (epitaxial growth). By changing the fundamental process parameter from removal-based etching to addition-based growth, the patent achieves precise emitter formation without the safety and environmental hazards associated with hydrofluoric acid