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When a Si PIN diode is exposed to fast neutrons, displacement damage to the Si lattice structure of the diode occurs. Defects induced from structural dislocation become effective recombination centers for carriers which pass through the base of the PIN diode. Hence, increasing the resistivity of the diode decreases the current for the applied forward voltage. This paper describes the development of a neutron sensor based on the phenomena of the displacement effect induced by neutron exposure. Multi PIN diode arrays with various intrinsic layer thickness and cross sections were fabricated. Irradiation tests, using an on-line-electronic-dosimetry system, have been shown that the increase in their intrinsic layer thickness along with the decrease in their cross-section area improves their detection sensitivity. The best neutron sensitivity was achieved when their intrinsic layer thickness was similar to the length of a side of the rectangular cross-section of the layer. The diodes showed a good linearity up to 1,000 cGy(Tissue). Its neutron sensitivity of up to 13mV/cGy was achieved at a 5 mA current pulse. It is three times higher sensitivity than that of similar commercial neutron diodes. Along with a good stability in their long-term-annealing performance, the newly developed PIN diodes show less dependency on neutron-beam direction than diodes with different geometry.