Herein, epitaxially regrown n⁺ (In)GaN source/drain layers for radio frequency high electron mobility transistors, addressing material and electrical characterization, are reported. A range of n⁺ GaN and n⁺ InGaN layers with indium 4–12 at% and silicon 0–5.1 × 10²⁰ cm⁻³ are evaluated. The active carrier concentration in n⁺ InGaN exceeds 2 × 10²⁰ cm⁻³. The layers exhibit so‐called V‐defects, observed by atomic force microscope and transmission electron microscope (TEM), which are associated with local composition changes. In addition to the high Si doping levels, nitrogen vacancies are also considered to contribute toward their net carrier concentration. Due to its relevance for device processing, selectivity analysis is performed, and the optimal process conditions for selective regrowth are identified. Regrowth under high temperature (800 °C) is found to be conducive to improved selectivity. However, a high thermal budget negatively impacts the overall regrowth process, as reported here for In_0.17Al_0.83N and Al_0.26Ga_0.74N barriers: whereas the InAlN barrier suffers from intermixing, the AlGaN barrier demonstrates high‐temperature stability. The impact of intermixing is studied from complementary TEM and DC electrical measurements. A low overall contact resistance of 75 Ω μm is obtained with the regrown n⁺ InGaN layers.