InGaN-Channel FETs - Growth, Technology and Characteristics
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InGaN-Channel FETs – Growth, Technology and Characteristics E. Kohn*, I. Daumiller*, M. Neuburger*, M. Seyboth°, C. Kirchner°, M. Kamp+ * Dept. of Electron Devices and Circuits, University of Ulm, D-89081 Ulm, Germany ° Dept. of Optoelectronics, University of Ulm, D-89081 Ulm, Germany + Global Light Industries, 47475 Kamp – Lintfort, Germany
ABSTRACT Making use of the polar nature of III-nitride heterostructures, a new FET device concept is proposed. The structure contains an InGaN QW channel sandwiched in between two GaN barrier layers. The charge in this structure is mainly generated by the strain field in the InGaN layer and is an electron/hole dipole sheet charge located at the opposite InGaN/GaN interfaces. To obtain nchannel characteristics the hole charge at the rear interface (for Ga-face oriented material) is compensated by donor doping of the channel or by modulation doping from the real GaN barrier layer. Growth, processing technology and characteristics of first fabricated devices is discussed. INTRODUCTION AlGaN/GaN heterostructure field effect transistors have demonstrated highest microwave power densities and high efficiencies due to the materials high breakdown field, high channel sheet charge density and 2DEG properties [1,2]. III-nitrides are also highly polar materials generating image charges of the bonded polarization charges in the lattice on the surfaces. Since these image charges are dipole charges in nature, they are positioned at opposite planes, namely for AlGaN/GaN FET structures at the substrate/GaN backplane, the AlGaN/GaN interface and at the surface of the AlGaN layer (fig.1a). As can be seen from fig.1a for Ga-face orientation the positive image charge is found on the top surface and the negative one on the bottom. +
+ AlGaN
-
2DEG GaN buffer -
Figure 1a. Schematic distribution of polar image charge components in AlGaN/GaN FET heterostructures for Ga-face orientation. All dipoles point in the same direction, especially the differential spontaneous polarization image charges of the AlGaN and GaN lattices and the piezo image charge of the strained layer system. Thus, a 2DEG channel is formed at the AlGaN/GaN interface. The nature of the image charges on E3.1.1
the backplane and on the surface is not well identified yet. The positive surface image charge can be electronic, ionic or molecular in nature, one suggestion being that of a surface donor [3]. The amount of the AlGaN/GaN 2DEG interface image charge as function of the heterostructure composition is shown in fig.1b. At an Al-content of 20% for example, 60% of the interface charges stem from the difference in spontaneous polarization of the GaN and AlGaN lattice and 40% from the piezo polarization caused by the strained AlGaN layer. 0 Piezo 13
2
P/q [1/cm ]
-2x10
13
-4x10
Spontaneous 13
-6x10
AlGaN/GaN interface
13
-8x10
14
-1x10
0.0
0.2
0.4 0.6 Aluminium content x [%]
0.8
1.0
Figure 1b. Image charge densities as function of heterostructure composition for Ga-face orientation. Data after [8,9] The surface h
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