The fabrication and characterization of thin-film silicon bulk resonators processed on glass substrates is described. The microelectromechanical (MEMS) structures consist of surface micromachined disk resonators of phosphorous-doped hydrogenated amorphous silicon (n+-a-Si:H) deposited by radiofrequency plasma enhanced chemical vapour deposition (RF-PECVD). The devices are driven into resonance by electrostatic actuation and the vibrational displacement is detected optically. Resonance frequencies up to 30 MHz and quality factors in the 103-104 range in vacuum were measured. A high density of modes that increases with resonator diameter was observed. Membrane-like vibrational modes show good agreement with finite element simulations. The effect of geometrical dimensions of the disks on the resonance frequency was also studied. When operated in air higher harmonic modes show increasing quality factors.