Delta-doped AlGaN/GaN Heterostructure Field-Effect Transistors with Incorporation of AlN Epilayers
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Y10.23.1
Delta-doped AlGaN/GaN Heterostructure Field-Effect Transistors with Incorporation of AlN Epilayers Z. Y. Fan, M. L. Nakarmi, J. Y. Lin, and H. X. Jianga) Department of Physics, Kansas State University, Manhattan, Kansas 66506-2601, USA
ABSTRATC: The simulation and experiment results of delta-doped AlGaN/GaN heterostructure fieldeffect transistors (HFETs) with the incorporation of highly-resistive AlN epilayer are reported. The high quality AlN epilayer is used as the dislocation filter for the HFET structure growth, and the high resistivity of AlN also removes the parasitic conduction related with the GaN bulk buffer. Delta doping can reduce gate leakage, further more, our simulation and growth results demonstrate that delta-doping in the barrier is more effective than uniform doping scheme to increase the sheet electron density. The influence of spacer layer thickness on the electron mobility and sheet electron density is also presented. The DC characterization of the fabricated devices shows our structure has a very high performance with a maximum current ~ 1 A/mm. INTRODUCTION Due to their intrinsic physical properties such as wide bandgap, high breakdown field, and high saturation electron velocity, AlGaN/GaN HFETs have the potential to achieve outstanding operational characteristics at higher temperatures, voltages, and power levels, and in harsher environments than other semiconductor devices [1, 2]. The nitride HFET devices with the stateof-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 have also been reported in the literature. However, it has been observed that the reliability is a challenging issue for the application of these devices in reality, of which, gate leakage, buffer parasitic conduction, and current collapse are some of the critical issues. Insulated gate has been used to reduce the gate leakage current [6] and we also proposed to replace the uniformly doping scheme in the AlGaN barrier with a delta-doping profile [7] to reduce the vertical tunneling current. In this paper, we will further demonstrate that delta doping can also increase the 2-D electron density in the channel. Moreover, with an optimized separation distance between the dopants and the AlGaN/GaN interface, Coulomb scattering can be minimized and hence carrier mobility would be enhanced. With the optimized doping scheme, the current and the power of the device can be improved. A high performance microwave device requires the GaN bulk layer to be resistive without parasitic conduction and trapping effect. Unfortunately, the nitrogen deficiency tendency in GaN leads to semi-insulator GaN growth impossible without defects introduction [8] or anti-doping [9]. The result is that these intentionally introduced defects in the semi-insulator GaN bulk becomes deep trapping centers, which can trap electrons spilled out from the channel, causing the current collapse phenomenon. To overcome these adverse effects, we substitute GaN bulk with intrinsic
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