Sensors are central to engineering applications which are used for process control, monitoring, and safety. Sensors are also important in medicine for diagnostics, monitoring, critical care, and public health. This work is dedicated to the modeling of proposed devices and their applications in engineering systems. This dissertation is divided into four chapters; the first one is reserved to the Sensors and their applications in engineering. In the second chapter, novel designs to deep submicron FET-based chemical sensors are presented. The followed chapter, a new radiation sensitive FET (RADFET) dosimeter design (called the Dual-Dielectric Gate All around DDGAA RADFET dosimeter) to improve the radiation sensitivity performance and its analytical analysis have been proposed. Finally, in the last chapter, Improvement and Implementation of the (DDGAA) RADFET sensor in radioactive environment are presented using genetic Algorithms and artificial neural networks. We hope that this work has succeeded in reaching the fixed goals and satisfying too.