A Polysilicon Contacted Subcollector BJT for a Three-Dimensional BiCMOS Process

A polysilicon contacted subcollector (PCS) bipolar junction transistor (BJT) was fabricated using selective epitaxial growth (SEG) of silicon to form the active region. The fabrication is the first step in the development of a novel 3-D BiCMOS process. To study the efficacy of the polysilicon collector contact, three types of BJTs were fabricated and their collector resistances were compared. These were the PCS BJT, a BJT fabricated in SEG silicon grown from a shallow trench incorporating a shallow collector contact with a buried layer, and a BJT fabricated in the silicon substrate with a shallow collector contact but no buried layer. The PCS BJT exhibited the smallest collector resistance as well as excellent device characteristics, demonstrating its viability for a 3-D BiCMOS process.