AlGaN/GaN Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors (MOSHFETs) with the Delta-Doped Barrier Lay
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AlGaN/GaN Metal-Oxide-Semiconductor Heterostructure Field-Effect Transistors (MOSHFETs) with the Delta-Doped Barrier Layer Z. Y. Fan, J. Li, J. Y. Lin, and H. X. Jianga) Department of Physics, Kansas State University, Manhattan, Kansas 66506-2601, USA Y. Liu, J. A. Bardwell, J. B. Webb, and H. Tang Institute for Microstructural Sciences, National Research Council, Ottawa, ON, K1A 0R6, Canada ABSTRATC: The fabrication and characterization of AlGaN/GaN metal-oxide-semiconductor heterostructure field-effect transistors (MOSHFETs) with the δ-doped barrier are reported. The incorporation of the SiO2 insulated-gate and the δ-doped barrier into HFET structures reduces the gate leakage and improves the 2D channel carrier mobility. The device has a high draincurrent-driving and gate-control capabilities as well as a very high gate-drain breakdown voltage of 200 V, a cutoff frequency of 15 GHz and a maximum frequency of oscillation of 34 GHz for a gate length of 1 µm. These characteristics indicate a great potential of this structure for highpower-microwave applications.
INTRODUCTION With a great potential for applications in the area of high power and high temperature microwave electronics [1, 2], the power and voltage handling ability of AlGaN/GaN heterostructure field-effect transistors (HFETs) has been considerably improved in recent years, with the state-of-art power density of 6.6 W/mm (6 GHz) on sapphire substrates [3], 10.7 W/mm (10GHz) [4] and 5 W/mm (26 GHz) [5] on SiC substrates. However, it has been observed that the Schottky gate of HFET tends to degrade with enhanced gate leakage current and insufficient pinch-off characteristics, especially when the HFETs are operating under high power conditions. The degradation of the gate leads to the premature breakdown and hence a deficient device performance with a reduction of output power, the RF efficiency and noise figure [6]. It is believed that the premature breakdown is caused by the traditional gate-drain diode breakdown as a result of the thermionic emission [7], or the thermal effect as a consequence of the surface hopping conduction of gate leakage current [8]. HFETs with insulated gate structure, especially by incorporating a thin SiO2 or AlN layer under the gate have been employed to tackle the gate leakage problem by increasing the energy barrier, and the leakage current was dramatically reduced by several orders [9,10]. The insulation layer also has the effect to reduce the electrical field in the underlying nitride semiconductor and hence, and increase the gate breakdown voltage [11]. To further reduce the vertical tunneling current [12] and hence increase the gate-drain breakdown voltage, we have proposed to replace the uniformly doping scheme in the AlGaN barrier with a δ-doping profile [13]. With the dopants farther away from the gate, the gate leakage current would be reduced. Moreover, with an optimized separation distance between the
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dopants and the AlGaN/GaN interface, carrier-impurity scattering can be minimized and hence carri
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