S. Akiyama, M. Kondo, L. Wada, K. Horio
Shibaura Institute of Technology,
Japan
Keywords: GaN, HEMT, breakdown voltage, field plate, buffer layer, deep acceptor
Summary:
AlGaN/GaN HEMTs are now receiving a great attention for applications to high-power microwave devices and high-power switching devices. It is well known that the introduction of field plate enhances the breakdown voltage of AlGaN/GaN HEMTs. This is because the electric field at the drain edge of the gate is reduced. Recently, C-doped and Fe-doped semi-insulating buffer layers are often adopted and C or Fe acts as a deep acceptor. So, it is interesting how the deep-acceptor density in the buffer layer affects the breakdown voltage of the HEMTs. Therefore, in this work, we have made a two-dimensional analysis of　field-plate AlGaN/GaN HEMTs with a Fe-doped semi-insulating buffer layer, and studied how the deep-acceptor density and the field-plate length affect the breakdown characteristics of AlGaN/GaN HEMTs. The deep-acceptor density is varied between 10^{17} cm^{-3} and 3x10^{17} cm^{-3}, and its energy level is set 0.5 eV below the bottom of conduction band. The deep acceptors act as electron traps. The field-plate length is varied between 0 and 1 um. The calculated off-state breakdown characteristics show that the drain current usually increases steeply due to impact ionization of carriers, resulting in breakdown. The buffer leakage current is larger for lower acceptor density, and the breakdown voltage becomes higher for higher acceptor density in the buffer layer. It is also shown that the breakdown voltage takes a maximum value at the field-plate length of 0.2-0.3 um when the gate-to-drain distance is 1.5 um, and that the average electric field between the gate and the drain becomes 3.2 MV/cm when the acceptor density is 3x10^{17} cm^{-3}.