Heterostructure field effect transistors (HFETs) based on AlGaN / GaN structures have shown good performance as high power and high frequency devices. Theoretical simulations of transport in short channel HFETs show that there are several areas where considerable improvements in mobility, etc. can be made if thin composite structures (as considered in this work) can be utilized. We examine transport in a metal / AlGaN / InN / GaN composite structure. The InN region is very thin (∼ 15 Å) and is introduced to improve the low field transport without significantly impacting the device breakdown properties. Our model is capable of examining any other type of composite structure as well. Our simulation method consists of charge control solution of the heterostructure wave functions, followed by Monte Carlo simulation of scattering and free flight events. We present comparison of results on i) metal / AlGaN / GaN structure, and ii) a metal / AlGaN / GaN structure with a thin InN channel of the order of a few mono-layers. We find that mobility in the channel can improve considerably with very little effect on the mobility - charge product. Indeed, the charge density induced in the thin InN channel region is ∼ 1013cm∼2. While the peak velocities in a metal / AlGaN / InN / GaN structure exhibit an increase of nearly 30% over the values in metal / AlGaN / GaN structures, the low field mobilities are also increased. Low-field mobilities of ∼ 2500 cm2 (V · s)∼ are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface disorder (∼ 70Å), we have found good agreement with experimental Hall mobility data for similar structures. At higher electric fields, we find that most electron population transfers to higher valleys or other sub-bands that lie in AlGaN or GaN. This ensures that high field breakdown of low band gap InN layer is also suppressed.
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This work was supported by grants F001681 (the POLARIS program) and F004815 from the U. S. Office of Naval Research.
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Singh, M., Singh, J. & Mishra, U.K. Design of composite channels for optimized transport in nitride devices. MRS Online Proceedings Library 798, 713–718 (2003). https://doi.org/10.1557/PROC-798-Y7.9