Modelling of the influence of velocity overshoot on the cut-off frequency in Si and GaAs heterojunction bipolar transistors

Abstract

We present self-consistent Monte Carlo simulations of the collectors of Si and GaAs heterojunction bipolar transistors (HBTs) with wide (700 nm) collectors such as are used in power transistors. Recent experimental data on peak cut-off frequencies f T of 76 GHz for GaAs/AlGaAs HBTs have provided strong direct evidence for extended velocity overshoot in the collector under base push-out conditions and there are theoretical results using fast semi-analytic techniques confirming this explanation. Our aim is to investigate how these regions of extended velocity overshoot might form and how they would affect f T at high current densities in such structures. Differences in velocity-field characteristics and velocity overshoot between Si and GaAs collectors have large effects which are quantified by calculating f T . Comparisons are also made with data from the semi-analytic results.

A theory for the nonlinear response of high power p-i-n photodetectors

“…It may be noted that the formation of an extended low field region is very analogous to the Kirk effect for bipolar transistors where extended velocity overshoot effects are know to be important. 14,15 In the detector case, however, for electron motion a carrier generated near the p ϩ contact experiences a large potential drop prior to entering the low field region, so that the overshoot is strongly localized and not expected to significantly affect the results. The drift-diffusion model should consequently work well.…”

Modelling the influence of high currents on the cutoff frequency in Si/SiGe/Si heterojunction transistors

“…This paper is an extension of our studies on f T in Si and GaAs bipolar transistors [10]. The extensions account firstly for how strain and alloy scattering alter carrier mobilities in SiGe, secondly for the influence of the heterojunction at the base-collector interface on f T and thirdly for how bandgap narrowing (BGN) affects f T .…”

“…The extensions account firstly for how strain and alloy scattering alter carrier mobilities in SiGe, secondly for the influence of the heterojunction at the base-collector interface on f T and thirdly for how bandgap narrowing (BGN) affects f T . The geometry and doping of the modelled transistor devices were based on the device grown by Wang et al [11] which was simulated in [10]; the details are given in table 1. We assume a strained Si 0.8 Ge 0.2 layer in the base region, with Si in the emitter and collector regions.…”

“…The calculation of f T is described in detail in [10]; f T = 1/2πτ EC where τ EC is the emitter-collector delay time; this in turn is the sum of delay times across different sections of the device. Of these, the most important are the base transit and collector signal delay times (τ B and τ C respectively), which are calculated from Monte Carlo modelling of the base-collector region, and the emitter charging time (τ E ), which is calculated by an analytical model.…”

“…The paper has the following outline. Section 2 contains a brief description of the method; full details are given in [10,15,16]. Section 3 contains the results of our calculations on carrier transport in bulk SiGe and on f T for Si/SiGe/Si HBTs described above, section 4 a brief discussion and section 5 a summary.…”