Fluorine-Implanted Enhancement-Mode Transistors

Abstract

GaN-based heterojunction devices, in the form of high electron mobility transistors (HEMTs) and metal–insulator–semiconductor HEMTs (MIS-HEMTs), are capable of delivering superior performance in high-frequency power amplifiers and high-voltage power switches. Conventional Ga-face C-plane GaN-based heterostructures offer record-high 2DEG (two-dimensional electron gas) density without any intentional doping because of the strong intrinsic charge polarization. High-density 2DEG channel inevitably results in depletion-mode (D-mode) GaN transistors with negative threshold voltage (V th). However, enhancement-mode (E-mode) HEMTs are highly desirable for their inherent fail-safe operation and simple circuit configurations. The key to a feasible normally off HEMT technology is a post-epitaxy threshold voltage control technique that allows localized conversion from D-mode to E-mode or vice versa, since a high-performance E-mode HEMT requires a gate-controlled E-mode channel and D-mode regions (for low access resistance). Gate recess, p-type cap (AlGaN or GaN), and fluorine plasma ion implantation are the three commonly used approaches to fabricating normally off HEMTs. In this chapter, a comprehensive discussion on the underlying physical mechanisms of the fluorine implantation is presented, including atomistic simulation and experimental studies. Further development of the F-implant technique and its integration with other advanced techniques such as gate recess and AlN passivation is described. Finally, the robustness of the F-implant technique is further illustrated with the demonstration of various mixed-signal circuits fully integrated with GaN power devices.