Quaternary nitride enhancement mode HFET with 260 mS/mm and a threshold voltage of +0.5 V

The demand for robust and fail-safe GaN-based enhancement mode (e-mode) heterostructure field effect transistors (HFET) is high. Several different approaches for e-mode HFET fabrication have been demonstrated already [1, 2]. Most approaches deplete the channel by either manipulating the surface potential or the thickness of the barrier layer. Here, we demonstrate a new approach for the heterostructure design following the idea to reduce the interface charge itself by applying a quaternary barrier layer with rather low polarization. The e-mode HFET presented consist of a GaN buffer and a quaternary barrier layer, whose composition and thickness have been chosen carefully to result in an e-mode device. Special attention in the design process has been paid to the barrier-inherent polarization to reduce the total amount of polarization-induced interface charges. Additionally, a capping layer for carrier concentration enhancement in the access region has been applied to reduce the series resistance of the devices.