The effect of resonant cavity structure on the performance operation of InAs/GaAs quantum ring inter-subband photodetector is studied for detection of terahertz radiations range. In order to confinement of optical field within active region and consequently enhancement in responsivity of device, two periods of Al2O3/GaAs distributed bragg reflectors are used as bottom dielectric mirror and a thin layer of Au material as top mirror of device. For further improvement in detectivity, Al0.3Ga0.7As/ In0.3Ga0.7As resonant tunneling barriers are included in absorption layers to reduce dark current of device. Proposed photodetector shows a peak responsivity of about 0.4 (A/W) and quantum efficiency of 1.2% at the wavelength of 80μm (3.75THz). Furthermore, specific detectivity (D*) of device is calculated and results are compared to conventional quantum ring inter-subband photodetector. Results predict a D* of ~1011(cm.Hz1/2/W) for device at T=80K and V=0.4V which is two orders of magnitude higher than that of conventional QRIPs.