Silicon PIN diode radiation sensors and CMOS readout circuits were designed and fabricated in this study. The PIN diodes were fabricated using a 380-${mu}m$-thick 4-inch n+ Si (111) wafer containing a $2-k{Omega}{cdot}cm$ n- thin epitaxial layer. CMOS readout circuits employed the driving and signal processes in a radiation sensor were mixed with digital logic and analog input circuits. The primary functions of readout circuits are amplification of sensor signals and the generation of the alarm signals when radiation events occur. The radiation sensors and CMOS readout circuits were fabricated in the Institute of Semiconductor Fusion Technology (ISFT) semiconductor fabrication facilities located in Kyungpook National University. The performance of the readout circuit combined with the Si PIN diode sensor was demonstrated.