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A novel circular CMOS MAGFET (Magnetic Field Effect Transistor) design is introduced and a novel device geometry design methodology is proposed to optimize the magnetic particle detection sensitivity of such devices. In order to optimize the signal to noise ratio, it was determined that the geometry of the MAGFET is required to have specific ratios, where its sector angle θ and its inner and outer radii r1 and r2 are optimized when θ/ln(r2/r1) = 1.3 . Compared to the more traditional rectangular MAGFET, the circular MAGFET has compatible SNR peak performance with rectangular MAGET. However, when the size of the MAGFET is scaled down in order to detect smaller magnetic particles, the proposed circular MAGFET has more robust SNR performance, design flexibility and tolerance to processing variations.