A CMOS biosensor with a folded floating-gate is created to detect charged biochemical molecules. It contains a FET, a control-gate and a sensing area. The floating-gate spans the whole device, allowing the sensing area to be placed on top of the FET, resulting in a decrease of the device's total area. The device is sensitive to the polarity and quantity of charged poly amino acids and could be used for electronic recognition of temporal and spatial migration of charges, such as in biological phenomena.