Hall Effect sensors are usually preferred compared to other sensors like optical sensors although the later might be superior in terms of speed or accuracy. This is because Hall sensors are generally more robust against environment contaminations and temperature variations. Among various types of magnetic sensors, Hall-effect sensors receive special attention by industry because of their predictable behavior, ability to sense field direction and being cost effective. As Hall sensors are supposed to be employed in some complex applications, it is necessary to integrate the sensor element with all signal conditioning circuits such as bias circuit, amplifier, filter etc. on a single chip to achieve an economic fully integrated sensor. Magneto sensitive Field Effect Transistor, called MAGFET, due to its compatibility with CMOS fabrication process, is one of the most popular components that help to realize such idea. In this research, first the Hall-effect phenomenon is described and then MAGFET is introduced and characterized in order to find important factors affecting the MAGFET sensitivity. Temperature, geometrical shape and dimensions, MAGFET operating point and substrate type are the main parameters that affect the MAGFET sensitivity. In addition, several MAGFET structures, including rectangular (with different dimensions), two and four-blade sector shapes, different gate oxide thicknesses, and some other structures were designed and fabricated. The different MAGFET parameters such as linearity, electrical characteristic and sensitivity are measured for the fabricated samples. The effect of oxide thickness, drain gap and metal coverage on MAGFET sensitivity is studied. The samples were fabricated in a 0.18 m CMOS technology and the maximum sensitivity was measured 2.5472%/T for a two-blade sector MAGFET implemented by thick gate oxide.