In this work, the effect of Germanium mole fraction variation in the source region of a dielectrically modulated Silicon Tunneling Field Effect Transistor (DMTFET) based biosensor has been investigated with the help of extensive device-level simulation. Results show that the increasing germanium mole fraction significantly reduces the DMTFET sensitivity towards the bio-molecules, and the degree of this sensitivity degradation has strong dependence on the properties of bio-molecule namely dielectric constant and charge density. The increasing Germanium mole fraction reduces the effect of gate fringing field at the source region and consequently the conduction band lowering being diminished in this region, resulting in the sensitivity degradation in the DMTFET biosensor. This study offers a fair design level understanding over the use of Silicon-Germanium source in DMTFET based biosensor.