S. Ueda, R. Tomita, Y. Kawada, K. Horio
Shibaura Institute of Technology,
Japan
Keywords: GaN, HEMT, breakdown voltage, high-k passivation layer, buffer layer
Summary:
AlGaN/GaN HEMTs are now receiving great interest for application to high-power microwave devices and high power switching devices. To improve the breakdown voltage, the introduction of field plate is shown to be effective, but it may increase the parasitic capacitance, leading to degrading the high-frequency performance. In a previous work, as another method to improve the breakdown voltage, we proposed a structure including a high-k passivation layer, and showed that the breakdown voltage increased significantly. We assumed an undoped semi-insulating buffer layer where a deep donor compensates a deep acceptor. Recently, Fe- and C-doped semi-insulating buffer layers are often adopted and they acts as deep acceptors. Therefore, in this work, we analyze AlGaN/GaN HEMTs with a buffer layer including only a deep acceptor, and studied how the breakdown voltage is influenced by its density and the gate-to-drain distance. Here, we adopt the Fe-doped semi-insulating buffer layer and the deep-acceptor density N_{DA} is varied between 10^{17} cm^{-3} and 3x10^{17} cm^{-3}. In all cases, the breakdown voltage increases as the relative permittivity of the buffer layer e_r increases, because the electric field of the drain edge of the gate is reduced. When N_{DA} is 10^{17} cm^{-3}, the breakdown voltage is determined by impact ionization of carriers when e_r is low, but it saturates when e_r > 30 because the buffer leakage current becomes to determine the breakdown voltage. On the other hand, when N_{DA} is 2x10^{17} cm^{-3} or 3x10^{17} cm^{-3}, the breakdown voltage is determined by impact ionization of carriers until e_r = 60 and it reaches 500 V, which corresponding to an average electric field of 3.3 MV/cm between the gate and the drain. Also, the breakdown voltage increase as the gate-to-drain distance increases when e_r is higher than 30. When e_r is 60 and the gate-to-drain distance is 5 um, the breakdown voltage becomes about 1350V which corresponds to an average electric field of 2.7 MV/cm between the gate and the drain.