Mg Doping Gradient Engineering by MOCVD for Threshold Voltage Enhancement in Si-Based p-GaN E-Mode HEMTs

The threshold voltage (Vth) of p-GaN gate enhancement-mode (E-mode) high electron mobility transistors (HEMTs) on silicon substrates grown by metal–organic chemical vapor deposition (MOCVD) is often limited to 1.0–1.5 V. Apart from the low Mg acceptor activation rate, the non-uniform vertical Mg distribution in thin p-GaN layers is also a key bottleneck limiting Vth. This work reveals that the vertical distribution (not only magnitude) of Mg doping fundamentally influences Vth by modulating the charge centroid and electric field coupling to the heterointerface. Through bis(cyclopentadienyl)magnesium (Cp2Mg) flow modulation, surfactant-assisted growth, and growth rate adjustment, the vertical Mg doping uniformity within the 80 nm p-GaN layer was improved while effectively suppressing Mg out-diffusion. A short-cycle gate-first self-aligned process was used to fabricate the devices, and the results showed that the improved Mg vertical distribution led to a significant Vth enhancement by 0.75 V. Technology Computer-Aided Design (TCAD) simulations further demonstrated that the uniform doping profile builds a stronger negative space charge field beneath the gate, raising the energy band and increasing Vth. This work not only presents practical strategies, but also establishes a direct physical link between vertical Mg doping distribution and Vth in Si-based E-mode HEMTs.