Design of composite channels for optimized transport in nitride devices
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Design of composite channels for optimized transport in nitride devices Madhusudan Singh and Jasprit Singh Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, MI, 48109
Umesh K. Mishra Department of Electrical and Computer Engineering, University of California, Santa Barbara, CA 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 ˚ and is introduced to improve the low field structure. The InN region is very thin (∼ 15 A) 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 ∼ 1013 cm−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)−1 are predicted along with high sheet charges for low interface disorder for the structure with a thin InN layer. For higher degree of interface ˚ we have found good agreement with experimental Hall mobility data for disorder (∼ 70A), 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.
I.
INTRODUCTION
In electronic devices such as field effect transistors (FETs), there are regions (under device operating conditions) where electric fields are small (for instance, in the source-gate region) and other regions where electric fields are very high (such as the gate-drain region). For optimum performance, one would need a high mobility (low effective mass and low band gap) material for low field regions and a high saturation velocity and high breakdown field material for the high field regions. In this paper, we examine a GaN / ˚ The motivation for the AlGaN HFET with a very thin InN intermediate region (∼ 15A). study is the expectation that at low fields, the electron distribution will be in the InN region and at high fields, there will be essentially no overla
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