Npn Review and Trends for the Last Decade

INTRODUCTION The last decade has seen a massive device research effort in Si and SiGe NPN bipolar transistors. The fate of Si technology had already been prematurely declared a few years ago when it was estimated that conventional Si NPN devices would never see their cut-off frequencies go beyond the 5-10 GHz range. Surprisingly, the early to mid 90’s has seen an explosion in Si NPN bipolar device publications, in response to the rapidly growing RF mixed signal market. The demand for high speed devices and the premature introduction of SiGe in the early 90’s have pushed silicon chip makers to quickly develop and optimize new bipolar transistor processes. These new and improved polysilicon structures for Si NPN’s have yielded devices with speeds as high as 100 GHz, easily able to compete with early SiGe HBT’s, giving Si technology access to the communications market. The heterojunction bipolar transistor (HBT), conceptualized as early as 1957 , with a SiGe base seems to be the next candidate for RF communications and wireless application IC’s for the beginning of the new millennium despite Si technology’s rebirth through the use of advanced structures.