Charge-Modulated Underlap I-MOS Transistor as a Label-Free Biosensor: A Simulation Study

The possibility of fast label-free detection of biomolecules using electronic devices has resulted in an increased focus on developing such biosensing devices and optimizing their structure for enhanced performance. One of the main factors that determine biosensor´s performance is its sensitivity. A high sensitivity in detecting biomolecules is achieved with FET-based biosensors by biasing the device in the subthreshold regime. Recent works have shown the possibility of significantly increased sensitivity in detecting biomolecules using emerging steep subthreshold slope FETs as label-free biosensors. A lower subthreshold swing, achievable with steep subthreshold slope devices, also results in a faster response time for the biosensor. The impact-ionization MOS (I-MOS) transistor can achieve very steep subthreshold slopes as it is turned ON abruptly due to the impact ionization phenomenon. In this paper, we propose an underlap I-MOS (UI-MOS) transistor sensor for the label-free detection of the charged biomolecules. The UI-MOS is a three-terminal device with an underlap in the middle of the gate electrode serving as the location for immobilization by biomolecules. The gate electrode of the UI-MOS offers the advantage of individual addressability of biosensors in a sensor array. The compatibility of the UI-MOS with the CMOS process enables monolithic integration with the measurement circuitry. Using the TCAD simulation, we demonstrate that a biosensor based on the UI-MOS is highly sensitive to the presence of charged biomolecules. The UI-MOS, when operating in the linear regime, also experiences a large threshold voltage (V_T) shift due to the presence of biomolecules. The performance of UI-MOS is found to be less sensitive to gate-oxide thickness variations, and shows high sensitivity for a range of gate and underlap lengths. The very high sensitivity of the UI-MOS biosensor and its compatibility with the existing CMOS processes make it an exciting alterna- ive to the conventional FET-based biosensors.