Periodic pillars of semiconductor in sub-wavelength size can serve multiple roles as diffracting, trapping and absorbing light for effective photoelectric conversion which has been intensively studied in the visible range. Here, we design and fabricate the micro-pillar arrays of AlGaAs/GaAs multi quantum wells(QWs) for high performance detection of long wavelength infrared light. Compared to its planar counterpart, the array offers 5.1 times intensified absorption at peak wavelength of 8.7 µm with 4 times shrinked electrical area. It's illustrated by simulation that the normal incident light is guided in the pillars by HE11 resonant cavity mode to form strengthened Ez electrical field, which enables the inter-subband transition of n-type QWs. Moreover, the thick active region of dielectric cavity that contains 50 periods of QWs with fairly low doping concentration will be beneficial to the optical and electrical merits of the detectors. This study demonstrates an inclusive scheme to substantially raise the signal to ratio of infrared detection with all-semiconductor photonic structures.