Back-Doping Design in AlGaN/GaN Heterostructure Field-Effect Transistors for High-Power Applications
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Back-Doping Design in AlGaN/GaN Heterostructure Field-Effect Transistors for High-Power Applications
Narihiko Maeda, Kotaro Tsubaki, Tadashi Saitoh, and Naoki Kobayashi NTT Basic Research Laboratories, NTT Corporation 3-1 Morinosato Wakamiya, Atsugi-shi, Kanagawa, 243-0198, Japan ABSTRACT A novel doping design has been proposed that yields high two-dimensional electron gas (2DEG) densities in the AlGaN/GaN heterostructure field-effect transistors (HFETs) even when the AlGaN barrier layers are designed to be very thin. In the novel 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, with the help of the negative polarization charges at the heterointerface between the GaN channel and the underlying AlGaN barrier layer. High 2DEG densities can thus be obtained. Moreover, relatively high 2DEG mobilities can be obtained for high 2DEG densities, because back-doped donor atoms are sufficiently remote from the position of the 2DEG so that the 2DEG is less subjected to the ionized impurity scattering due to the relevant donor atoms. By using this back-doping design, a very high 2DEG density of 2.8x1013 cm-2 (2DEG mobility is 850 cm2/Vs) has been obtained at 300 K in the Al0.3Ga0.7N/GaN HFET whose barrier layer (Al0.3Ga0.7N) is as thin as 120 Å. Thus, the back-doping design is effective to obtain high 2DEG densities in the HFETs with thin barrier layers, and 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 [1-11]. 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 [12-14]. On the other hand, reducing the thickness of the AlGaN barrier layer is effective to obtain the high transconductance. However, these two improvements are eventually contradictory, because reducing the AlGaN barrier thickness leads to decrease the total amount of donor atoms in the AlGaN layer and leads to decrease the 2DEG density. Hence, high 2DEG densities become more difficult be obtained as the barrier thickness becomes small. To overcome this contradictory situation, we propose a novel doping design that makes it possible to obtain high 2DEG densities in the HFETs with the thin barrier thickness. EXPERIMENTAL DETAILS AND DISCUSSIONS Back-Doping Design As a solution for the above problem, we propose the back-doping design in the AlGaN/GaN HFET. Figure 1 schematically shows the potential profile of the AlGaN/GaN HFETs with the back-doping design (BD-HFET). In the BD-HFET, an asymmetric double-heterostructure is I12.8.1
Si Doping
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Si Back-Doping
AlGaN GaN AlGaN Figure 1. Potential pr
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