Origin of Soft-Switching Output Capacitance Loss in Cascode GaN HEMTs at High Frequencies

Output capacitance (COSS) loss (E-DISS) is produced when the COSS of a power device is charged and discharged, which ideally should be a lossless process. This loss was recently revealed to be a crucial concern for GaN high electron mobility transistors (HEMTs) in high-frequency soft-switching applications. Among various GaN devices, the composite-type, cascode GaN HEMT was reported to show the largest E-DISS with a voltage dependence distinct from discrete GaN HEMTs. However, the physical origins of the E-DISS in cascode GaN HEMTs remain unclear. This article fills this gap by identifying three loss components and, for the first time, experimentally quantifying them in the multi-MHz resonant switching. These loss components include: a) the avalanche loss of Si mosfet; b) the intrinsic E-DISS of GaN HEMT; and c) the Si avalanche-induced GaN turn-on loss. The last component was found to dominate E-DISS at high voltage. By eliminating the Si avalanche and the associated loss components (a) and (c), the E-DISS of cascode GaN HEMTs can be reduced by up to 75% at the price of an increase in output charge and switching transition time. These results provide new physical insights and practical guidelines to trim the soft-switching loss of cascode GaN HEMTs in high-frequency applications.