Split Gate Bulk-Planar Junctionless FET-Based Biosensor for Label-Free Detection of Biomolecules
The dielectric-modulated field-effect transistor (DM-FET) has emerged as a powerful biosensing technology capable of detecting neutral and charged biomolecules with high precision. Bulk-planar junction-less field-effect transistors (BP-JLFET)-based biosensors offer a cost-effective and simpler alternative to DM-FETs by overcoming challenges like fabrication complexity, high thermal budgets, and random doping fluctuations.
BP-JLFETs detect these biomolecules by monitoring drain current changes caused by variations in dielectric constants or capacitance. This current change occurs when biomolecules are immobilized within the device’s cavity.
The split gate concept further improves sensing by reducing the transit time for the charge by shortening transport time and lowering power dissipation. Despite extensive theoretical studies, the split-gate BP-JLFET (SG-BPJLFET) remains untested experimentally for biosensing.
The researchers present an SG-BPJLFET fabricated on a p-type bulk silicon substrate with a 2 µm channel length and a cavity designed for biomolecule immobilization, such as streptavidin and biotin. Simulations demonstrated its potential to detect protein biomolecules, even without a biosensing setup.
The fabrication process involves cleaning a p-type silicon wafer, removing the native oxide with HF, and depositing an n+ active layer using spin-on dopant (SOD). A 20 nm HfO2 gate oxide layer is then deposited and etched to form windows for source and drain contacts.
A mask is used to create the split gate structure with two gate electrodes separated by a 2 µm gap, forming the sensing cavity. The device is then annealed in nitrogen, and electrical characteristics are measured, revealing subthreshold conduction around 4 V with a Debye's length of 6.45 nm, critical for mitigating charge effects in high ionic strength solutions.
Simulations with ATLAS software showed that the drain current varied when the cavity was filled with biomolecules like streptavidin-biotin, demonstrating the SG-BPJLFET’s suitability for detecting protein complexes. Sensitivity increased by 75% with streptavidin-biotin binding due to their strong noncovalent interaction.
Sensitivity was also influenced by buffer solutions, where pH variations impacted the interface charge density at the HfO2/Si interface. The cavity’s thickness and length also played crucial roles; thicker cavities reduced charge carrier movement, while more extended cavities increased current but decreased biomolecule interaction, impacting sensitivity.
Temporal analysis revealed the SG-BPJLFET’s faster response time for streptavidin-biotin (345 ms) compared to biotin alone (425 ms), with minimal signal distortion. The signal-to-noise ratio was higher for streptavidin-biotin (~100 dB) than biotin (~80 dB), enhancing sensitivity and selectivity.
The biosensor demonstrated a low limit of detection of 0.12 µV, indicating its ability to detect low biomolecule concentrations. The positioning of biomolecules and steric hindrance also affected performance, with higher sensitivity when biomolecules were specifically bound or when the cavity was fully filled.
Compared to other FET-based devices, the SG-BPJLFET offered higher sensitivity, lower costs, and better scalability, making it ideal for point-of-care biomedical applications. With a high on-and-off (ION/IOFF) ratio of approximately 10^7 and a Debye’s length of 6.45 nm, the SG-BPJLFET holds excellent potential for future biosensing technologies, offering a scalable solution for disease detection with fast response time, high sensitivity, and low fabrication costs.