This paper describes the design, fabrication and experimental characterization of photonic crystal microbeam cavity resonators for the terahertz band implemented using suspended dielectric rectangular waveguide (DRW) in high resistivity silicon. Electrical quality factors of up to 11,900, combined with small modal volumes of 0.28 mm3 and 0.077 mm3, are demonstrated for devices operating at 100 and 200 GHz, respectively. The devices are found to be extremely light-sensitive, opening up new opportunities for light-controlled switching devices at terahertz frequencies. It is shown that the quality factor of the resonator can be tuned and the resonance extinguished through photo-illumination with an infrared light-emitting diode (IR LED). Additionally, the questions of thermal tunability and thermal stability of the resonators are examined. The demonstrated resonators are inherently suited to integration with DRW and by silicon bulk micromachining represent an attractive approach for realizing microphotonic integrated circuits for terahertz systems-on-a-substrate.