Full Band Monte Carlo Comparison of Wurtzite and Zincblende Phase GaN MESFETs
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INTRODUCTION The particular features that make the III-nitrides and SiC attractive for high frequency, high power device applications are their relatively high breakdown voltage, high saturation electron drift velocity, low dielectric constants, and for SiC, high thermal conductivity [1-3]. To the authors’ knowledge, only a few other theoretical studies of the behavior of GaN based FETs have been made [4-7]. The theoretical study of GaN based devices is frustrated to some extent by the paucity of reliable transport parameters. As such, numerical studies using advanced drift diffusion and hydrodynamic simulation tools [8] that require extensive parameterization are, at present, challenging. Alternatively, less parameterized models, such as the full band ensemble Monte Carlo technique that can proceed relatively independently of experiment, are presently more suitable for studying transport in GaN and in modeling GaN based devices [9]. It has been shown that Monte Carlo simulations using an analytical band structure approximation are generally insufficiently detailed to properly describe high field transport dynamics [10]. Inclusion of the full details of the band structure is necessary to provide an accurate accounting of the breakdown properties of a device. For these reasons, we employ a full band ensemble Monte Carlo simulation to examine the relative performance characteristics of wurtzite and zincblende phase GaN MESFETs operated near breakdown. The model is
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summarized in the next section. In section III, the calculated results are presented and conclusions are drawn in Section IV.
MODEL DESCRIPTION The full band Monte Carlo technique relies on the knowledge of fairly well known parameters i.e., lattice constants, energy gaps, phonon energies, dielectric constants, etc. From these quantities a reasonably accurate description of the band structure, impact ionization transition rate and phonon scattering rates can be obtained, enabling study of the basic transport properties of the material and its related devices. The full band Monte Carlo simulator can thus be developed without extensive experimental knowledge of the transport parameters of a material. Though the Monte Carlo model suffers from some basic uncertainties, these mainly being in the details of the deformation potentials and the concomitant high energy scattering rates, it is nonetheless presently the most reliable simulation tool for investigating high field transport. The full details of our Monte Carlo model have been presented elsewhere [9]. In addition, the details of how our Monte Carlo model has been merged with a Poisson solver to enable device simulation have also been discussed elsewhere [6]. For brevity we will not repeat these details here. This study is comparative in nature. The study is not meant to project the ultimate limits of potential performance of GaN MESFETs. Instead, it is our goal to examine devices with the same geometry and doping concentrations but made using the two different polytypes of GaN, wurtzite and zi
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