This article introduces a millimeter-wave electromagnetic (EM) bandgap (EBG) resonator, commonly referred to as a photonic crystal (PC) resonator (PCR), designed for use in a 45-GHz ultralow phase noise MMIC oscillator. The novel 2-D slab PCR design utilizes a higher order resonant mode, which has been engineered to have a reduced dielectric filling factor (FF) of 47%. These improvements demonstrate a factor of two improvement in unloaded Q -factor (122 000 at 297 K) and halving of the resonant frequency temperature sensitivity (21 ppm/K) compared to a conventional L5 PCR design. The PCR is fabricated using bulk micromachining of high-resistivity silicon (HRS), which has been neutron transmutation doped (NTD) leading to an additional factor of two improvement in quality ( Q -) factor over previous works featuring nonirradiated HRS. Additionally, it is shown that unloaded Q -factors in excess of 200 000 are achievable when the temperature is reduced to below 274 K. The PCR benefits from a planar geometry, integrated waveguides, and fabrication using a mature silicon micromachining process.