Inclusion of tunneling and ballistic transport effects in an analytical approach to modeling of NPN InP-based heterojunction bipolar transistors

Abstract

We describe an analytical approach to modeling NPN InP-based heterojunction bipolar transistors using a Gummel Poon model. Starting from an initial, physical description of the device's epitaxial structure and geometry, this physics-based model is utilized to simulate the device's operation using a thermionic-field-diffusion model for carrier injection at the emitter and diffusive transport across the base. This approach incorporates recent advances in understanding of device operation and improved models of parasitics. The cutoff frequency and maximum frequency of oscillation are calculated as a function of collector current density and compared with experimental measurements. For the cutoff frequency, the results of this model underestimate the high frequency performance of the device by as much as 50% at the higher collector current levels. To improve the model and to allow simulation of narrower base widths, a more rigorous analysis of tunneling through the emitter spike and ballistic transport across the base and base-collector depletion regions has been included. Quantum mechanical tunneling of electrons through the emitter-base spike has been taken into account for accurate description of the energy distribution of electron injection into the base. The resulting electron flux distribution is used as an initial distribution in a regional Monte Carlo simulator to model electron transport from the emitter edge of the base to the subcollector. The results are then used to calculate an average base transit time and base-collector space charge region delay time which can be incorporated in the analytical model.