Current Trends and Challenges in III-V HBT Compact Modeling

Abstract

This paper gives an overview on recent achievements in the modeling of GaAs or InP based HBTs. The emphasis lies on the description of weakly nonlinear behavior, and on advanced descriptions for 1/ f and shot noise for nonlinear simulation. Although compact HBT modeling already reached a high level of accuracy, certain limitations remain that will also be addressed. I. IGaAs-based HBTs have been for quite a while the devices of choice for power amplification in the lower GHz range, especially for use in mobile devices. The vast majority of GaAs-HBT MMICs fabricated today therefore are power amplifiers (PAs) designed to deliver around 1 W in the range 1 -2 GHz, powered from a battery. InP based HBTs are capable to achieve highest cutoff frequencies while maintaining reasonably high breakdown voltages. E.g., this technology is well suited to design modulator drivers for multi-Gbps optical links that require high voltage swings and highest switching speeds. Recently developed GaAs HBT technologies target base station amplifiers rather than mobile PAs, with tenfold bias supply voltage and output power: beyond 10 W at 2 GHz from a 28 V supply. This results in much higher output impedances, that allow for easier and, especially, broadband output matching.Since III-V HBTs have come to market more than two decades after their unipolar counterparts, compact models are not yet as well established. This can be seen from the fact that simply the number of models that are available in circuit simulators is much lower than the number of MESFET or HEMT models. There are mainly two modern HBT models commonly available. One is the FBH model that will be used as the reference model in this paper. The other one that follows the same philosophy although with different formulas, is the AgilentHBT model [1]. On the other hand, designers are often not sure if they can trust the model they get, if it is not well understood what effects are to be accounted for.It is the aim of this paper to review the state-of-the-art in III-V HBT modeling. The status of the models will be discussed, as well as some recent advancements, and finally some unsolved issues will be addressed. In order to keep this article readable while addressing all relevant issues, the different issues will be touched rather than providing an indepth discussion. For details please refer to the literature cited in each section. Collector Contact Base ContactEmitter Air Bridge Fig. 1. Micro-photograph of single-finger HBT from FBH Foundry. The 20 µm thick emitter airbridge serves as a heat spreader, that is capable of equalizing the temperature distribution in multi-finger devices at 28 V operation. The airbridge has been removed in the foreground in order to show the inner device. A. HBT Model RequirementsTo begin with, three of the expectations shall be recalled that led to the development of HBTs. These are • the capability to be operated at high power densities, • high linearity, and • low 1/ f noise. HBTs are vertical devices, see Fig. 1. The collector curren...