Methods and devices integrating circuitry including both III-N (e.g., GaN) transistors and Si-based (e.g., Si or SiGe) transistors. In some monolithic wafer-level integration embodiments, a silicon-on-insulator (SOI) substrate is employed as an epitaxial platform providing a first silicon surface advantageous for seeding an epitaxial III-N semiconductor stack upon which III-N transistors (e.g., III-N HFETs) are formed, and a second silicon surface advantageous for seeding an epitaxial raised silicon upon which Si-based transistors (e.g., Si FETs) are formed. In some heterogeneous wafer-level integration embodiments, an SOI substrate is employed for a layer transfer of silicon suitable for fabricating the Si-based transistors onto another substrate upon which III-N transistors have been formed. In some such embodiments, the silicon layer transfer is stacked upon a planar interlayer dielectric (ILD) disposed over one or more metallization level interconnecting a plurality of III-N HFETs into HFET circuitry.