AlGaN/GaN Heterostructure Field-Effect Transistors with Back-Doping Design for High-Power Applications: High Current Den
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AlGaN/GaN Heterostructure Field-Effect Transistors with Back-Doping Design for High-Power Applications: High Current Density with High Transconductance Characteristics Narihiko Maeda1, Kotaro Tsubaki2, Tadashi Saitoh1, Takehiko Tawara1, and Naoki Kobayashi1 1 NTT Basic Research Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-0198, Japan 2 Department of Electrical & Electronic Engineering, Toyo University 2100 Kujirai, Kawagoe-shi, Saitama, 350-8585, Japan ABSTRACT Electron transport properties and DC device characteristics have been examined in the AlGaN/GaN heterostructure field-effect transistors (HFETs) with back-doping design that makes it possible to obtain high two-dimensional electron gas (2DEG) densities even for the devices with thin AlGaN barrier layers. In the back-doping design, an asymmetric double-heterostructure is employed, and donor atoms are doped not only in the surface-side AlGaN layer but also in the underlying AlGaN layer. In this structure, electrons are efficiently supplied also from the back-doped AlGaN barrier layer to the GaN channel and merged into a single 2DEG layer, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. By using back-doping design, very high 2DEG densities around 3x1013 cm-2 has been achieved in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is designed to be as thin as 120 Å. An HFET with the gate-length of 1.5 µm has exhibited a high current density of 1.2 A/mm and a high transconductance of 200 mS/mm, which is ascribed to high 2DEG densities and thin barrier layers in these devices. HFETs with the back-doping design are thus promising for high-power applications. INTRODUCTION AlGaN/GaN heterostructure field-effect transistors (HFETs) have recently emerged as the attractive transistors suitable for high-temperature and high-power microwave applications [1-12]. For further improving high-power device performance, increasing the two-dimensional electron gas (2DEG) density is effective, which can be attained by using the unrelaxed HFETs with high Al compositions [13-16]. On the other hand, reducing the thickness of the AlGaN barrier layer is effective to obtain the high transconductance. In this paper, we point out the problem with the conventional doping design that the 2DEG density decreases when the AlGaN barrier thickness is reduced. As a solution for this problem, we propose the back-doping doping design that makes it possible to obtain high 2DEG densities in the HFETs with the thin barrier thickness. We examine the electron transport properties and DC device characteristics of the HFETs with the back-doping design. EXPERIMENTAL DETAILS AND DISCUSSIONS Problem with Conventional Modulation Doping accompanied by Thinning Barrier Layer
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To examine the 2DEG density in modulation doped HFETs with thin AlGaN layers, we have grown modulation doped HFETs whose barrier thickness is as thin as 120 Å, and, for comparison, we have als
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