Biological sensitive FETs (BioFETs) are an emerging technology that is going to replace today's analyzing and sensing facility in bio-chemistry. These devices are conventional MOSFETs, which are equipped with a biofunctionalized surface that replaces the normal gate design. This surface is sensitive to special bonds in an aqueous solution that contains an analyte. This analyte specifically binds to the biofunctionalized surface layer and changes the charge distribution and surface potential, which in turn influences the conductivity of the transducer. The small change in the conductivity enables a label-free detection within minutes. Exchanging the optical detection mechanism in state-of-the-art devices with an electrical signal has several advantages. It enables the integration of amplifying and analyzing circuits on the same chip, thus making expensive reading devices superfluous, and offers the possibility of outdoor measurement of diseases and toxins without the necessity of a lab. By substituting the type of surface layer, a wide variety of applications is possible. Sensors based on conventional silicon structures and silicon nanowires have been developed, enabling large scale production. BioFETs which have the ability to detect single-stranded DNA and tumor markers have been produced. Although great progress has been achieved, the theoretical framework necessary for a quantitative understanding of the experiments is still lacking. The combination of the semiconductor, the surface chemistry, and the transport mechanisms in the aqueous solution create a complex system, which needs a consistent method of analysis. Due to the underlying multi-physics, it is crucial to describe the system with an adequate multi-physics model. These requirements are met via a drift-diffusion equation in the semiconductor, while the analyte obeys the Poisson-Boltzmann equation. Hence, a self-consistent simulation of the whole device is vital to quantitatively understanding and engineering these novel devices.