Threshold Voltage Engineering in GaN-Based HFETs: A Systematic Study With the Threshold Voltage Reaching More Than 2 V

One of the key challenges for the adoption of gallium nitride (GaN)-based heterostructure field effect transistors (HFETs) in power-switching applications is obtaining enhancement mode behavior. A large variety of methods have been applied to shift the threshold voltage V_th of HFETs. However, most of the time, approaches were discussed individually, neglecting the effects of combinations. Hence, in this paper, a comprehensive study of four different approaches to shift Vth well into the positive range is presented. We show the effects of different gate metallizations, of a backbarrier, of a gate oxide, and of a gate recess. Each approach is discussed individually, and special focus is on the insulator/semiconductor interface, which is apparently different with and without gate recess. The final device exhibits a Vth of +2.3 V, which is shown to be stable when applying OFF-state stress during dynamic characterization.