Biosensors are used to detect biomolecules by utilising their dielectric constant. This work is based on the oxide thickness variation where the sensor is simulated by varying the oxide thickness. The proposed sensor structure is a double gate junctionless MOSFET with a recessed channel using a high-k dielectric material that helps the device with the gate tunnelling current since there is no p–n junction. The sensor is then investigated by using different dielectric constants of biomolecules and obtaining results for threshold voltage, subthreshold slope, ON current and ON current to OFF current ratio. These parameters are simulated using the SILVACO TCAD tool. The sensitivity of threshold voltage was also investigated and analysed and is observed that the sensitivity of an oxide thickness of 6 nm is better as compared to an oxide thickness of 3 nm. This biosensor is better suitable for sensing biomolecules with high sensitivity.