Efficient Approach to Optimization of f/sub γ for Graded-Base SiGe HBTs

Abstract

Most studies of f T for SiGe HBTs have been focused on one or 2 components of the total delay time, τ EC [1][2][3][4]. The present work takes into account all crutial components of the total delay time τ EC to optimize f T , including the transit times across the quasi-neutral regions (τ E and τ B ) and the depletion layer (τ BC ) and the charging times (τ JEB and τ JBC ) due to the junction capacitances (C EB and C BC ). Because of no minority carrier charge stored in the collector, τ C is not included. Also, τ EB across the thin forward-biased EB depletion layer is ignored. f T is thus expressed as