Mathematical approach and optimisation of nanometric base thickness for a SiGeC HBT dedicated to radiofrequency applications

Abstract

In this paper, we present a mathematical approach to the nanometric thickness optimisation of a heterojunction bipolar transistor (HBT) SiGeC base, which is realised using the BiCMOS (bipolar compatible metal oxide semiconductor) industrial process. However, the use of these components in microwave applications and radiofrequency ranges imposes the use of complex shrink structures. Because the SiGeC base is the active portion of the transistor, the optimisation of its nanometric thickness is a crucial aspect in accurately predicting the characteristics of the component. A numerical modelling approach is investigated using our 2D simulator “SIBIDIF”, which is based on the drift–diffusion model (DDM). This method solves the continuity equations for electrons and holes and is coupled with the Poisson equation based on the concept of the finite difference mesh using a revised Scharfetter–Gummel approach and is solved numerically using the Gauss–Seidel method for matrix algebra.This optimisation improves the static gain of the transistor, the transition frequency (fT), and the maximum oscillation frequency (fmax) while reducing the thickness of the base from 100 to 30 nm. However, the model reaches limits for thicknesses less than 25 nm. The simulation results obtained in this study are compared to electrical characteristics obtained by measurements.