The influence of collector dopant profile on breakdown voltage and cutoff frequency of Si‐based RF bipolar transistors

Abstract

The cutoff frequency ft and the collector-emitter breakdown voltage BVCEO are computed for various types of Si-based bipolar junction transistors with different doping profiles of the selectively implanted collectors (SIC). In particular the influence of the SIC profiles on the f t vs. BVCEO characteristics is investigated. It is shown that for slow BJTs the effects of collector profile variations are negligible. For fast HBTs, however, the actual SIC profile has a significant influence on the f t vs. BVCEO characteristics. Graded and steplike SIC profiles show an improvement compared to constant SIC doping, while retrograde profile deteriorate device performance.Motivation and introduction During the evolution of RF bipolar transistors, much efforts were spent to optimize the base design. Device engineers came up with concepts like graded dopant profiles, SiGe [1] and SiGe:C [2] base layers, elevated base structures [3], etc. Regarding the collector, selectively implanted collectors (SIC) were introduced to increase both the cutoff frequency f t and the maximum frequency of oscillation f max . In this work we focus on the collector-emitter breakdown voltage BV CEO and its relation to the cutoff frequency f t for differently designed SICs of Si-based RF bipolar transistors. This is done by two-dimensional numerial device simulation. To get a general picture, several distinctly different types of bipolar junction transistors were investigated: A SiGe HBT with a graded Ge profile in the base ("triangular" design) and a second SiGe HBT having a much higher Ge content in the entire base, thus allowing higher dopant concentration in the base than in the emitter ("true" HBT or rectangular design). Additionally two conventional NPN BJTs with diffusion respectively drift as the dominant mechanism for the electron transport through the base were simulated.

The influence of collector designs on fmax versus ft characteristics for different types of Si-based RF bipolar transistors

“…Most importantly, the hydrodynamic transport model and the Katayama-Toyabe impact ionization model [2] have been applied. The accuracy of the simulated results has been recently confirmed by comparison with measured data of an experimental SiGe HBT [3].…”

Performance trends of Si-based RF transistors

“…To check whether the selected set of models and model parameters is appropriate, Fig. 5 shows a comparison of measured and simulated results for a 40-GHz f T SiGe HBT [14]. Both the simulated breakdown and high-frequency behavior show excellent agreement with the experiment.…”

Performance Trends of Si-Based RF Transistors

“…To check whether the selected set of models and model parameters is appropriate, Fig. 5 shows a comparison of measured and simulated results for a 40-GHzfT SiGe HBT [ 15]. Both the simulated breakdown and high-frequency behavior show excellent agreement with the experiment.…”