Impact of Gate Metal on Surface States Distribution and Effective Surface Barrier Height in AlGaN/GaN Heterostructures
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Impact of Gate Metal on Surface States Distribution and Effective Surface Barrier Height in AlGaN/GaN Heterostructures Nitin Goyal and Tor A. Fjeldly Department of Electronics and Communication, Norwegian University of Science and Technology, Trondheim, Norway ABSTRACT A physics based model is presented to describe the surface donor density distribution for metal/AlGaN/GaN structures. This model partly relies on experimental observations to describe the reduction that takes place in surface donor density when the metal gate is deposited. This new model is based on our previous work on the bare surface barrier height for both unrelaxed and partially relaxed barrier layers. The model predictions are consistent with reported experimental data.
INTRODUCTION GaN based High Electron Mobility Transistors (HEMTs) have shown great promise for future high-power and high frequency applications due to their wide band gap and high electron mobility. Wide bandgap III-V based HEMTs also possess excellent thermal properties, which make them suitable candidates for high-temperature applications. The most interesting feature of these devices is the presence of a high mobility, two-dimensional electron gas (2DEG) with a sheet density of the order of 1013 cm-2 at the AlGaN/GaN interface, even in the absence of both AlGaN barrier layer doping and a gate metal (bare surface). This phenomenon is attributed to the strong piezoelectric as well as spontaneous polarization effects in the structure[1]. In the last few years, considerable efforts have been made to explain the mechanism and source of electrons for the formation of this 2DEG. It has been shown that the presence of distributed donor states in the forbidden gap of the barrier AlGaN surface is the main source of the 2DEG [2-6]. We have previously shown how the bare surface (Schottky) barrier height (SBH) relates to polarization, thickness, and Al content of the barrier, as well as to the surface donor state distribution. This model applies well to bare surface AlGaN/GaN [7,8], but the understanding of what happens when a gate metal is deposited on top AlGaN layer is still lacking. Understanding of metal contacts on AlGaN/GaN heterostructures is important, since these contacts are required for an efficient gate control in practical HEMTs. A large SBH implies low leakage current as well as high breakdown voltage. THEORY The SBH of a metal-semiconductor contact is defined as the energy difference between the conduction band minimum (CBM) and the Fermi level EF at the interface. The SBH has been a constant topic of research for several decades. In their basic model, Schottky and Mott assumed ideal interfaces with no surface states, where SBH is given by qΦb = qΦm - χ, where Φm is the
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metal work function and ɖ is the electron affinity of semiconductor [9] . Bardeen later introduced a model based on the assumption of a very high density of surface states in order to explain an observed pinning of the Fermi level. A part of this problem is that the metal can give rise to socalled m
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