Current Gain Simulation of Npn AlGaN/GaN Heterojunction Bipolar Transistors
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Current Gain Simulation of Npn AlGaN/GaN Heterojunction Bipolar Transistors C. Monier a*, S. J. Pearton a, A. G. Baca b, P. C. Chang b, L. Zhang b, J. Han b, R. J. Shul b, F. Ren c, and J. LaRoche c a
Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611 Sandia National Laboratories, Albuquerque, New Mexico 87185 c Department of Chemical Engineering, University of Florida, Gainesville, Florida 32611 *: Current address at Sandia National Laboratories b
ABSTRACT A drift-diffusion model has been used to explore the performance capabilities of Npn AlGaN/GaN heterojunction bipolar transistors. Numerical results have been employed to study the effect of the p-type Mg doping and its incomplete ionization on device performance. The high base resistance induced by the deep acceptor level is found to be the cause of limited current gain values for Npn devices. Several computation approaches have been considered to improve their performance. Reasonable improvement of the DC current gain β is observed by realistically reducing the base thickness in accordance with processing limitations. Base transport enhancement is also predicted by the introduction of a quasi-electric field in the base.
INTRODUCTION Impressive reports in the past few years on AlGaN/GaN high mobility electron transistors have motivated the development of heterojunction bipolar transistors (HBTs) which have demonstrated, in GaAs and InP, improved linearity and more uniform threshold voltages over the field effect transistor counterpart. The first Npn AlGaN/GaN HBTs have been recently reported [1,2]. Initial current gain β values were measured to be as high as 3 at room temperature. Poor conductivity of the p-type GaN base layer, due to the high acceptor ionization energy (>170 meV) is responsible for the current gain limitation [3]. Others factors including Mg memory effect (with substantial hole diffusion to the emitter) or the conflicting effect of a large spontaneous polarization (inherited from typical MOCVD-grown Ga-face heterostructures along the [0001] direction) have also been considered to explain the moderate gain of the devices. Both aspects will result in reduced free hole concentration and artificially larger thickness for the base layer. Solutions involving AlGaN/GaN superlattices or piezoelectric effects associated with modulation-doped heterostructures and emitterup HBT configuration have been suggested to achieve higher hole concentration in the base [4-6]. Because GaN-based electronics are still in their infancy, there has been very little work on simulating HBTs capabilities in these materials [7,8]. In this paper, we report on the simulation results of Npn GaN-based HBTs for use in optimizing the epitaxial multilayer structure and in assessing the factors limiting/maximizing device performance for power applications. Unlike previous works that did not pay too much attention to the Mg ionization effect, simulation results are explored here to evaluate the free hole concentration issue. Solution
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