Investigation of electrolyte gated negative capacitance vertical TFET pH sensor based on biomolecule position

In an electrolyte-gated negative capacitance vertical Tunnel Field Effect Transistor (TFET) pH sensor, the position of biomolecules within the sensing region significantly impacts the device's electrical characteristics and overall sensitivity. The closer the biomolecules are to the gate dielectric-semiconductor interface, the stronger their gating effect and thus the higher the sensor's response.

Impact of Biomolecule Position Gating Effect: The core principle of these biosensors is that charged biomolecules (or changes in pH causing charge variations) within the electrolyte/nanocavity area modulate the surface potential and channel conductance. The closer the biomolecules are to the active channel region, the more effectively they can modulate the tunneling barrier width, leading to a greater change in drain current (

Idscap I sub d s end-sub𝐼𝑑𝑠) and enhanced sensitivity.
Nanocavity Design: Current research often employs structures with specific nanocavities (sensing areas) near the source/channel junction to capture biomolecules. The geometry and dimensions of this nanocavity, including its thickness and the position of the biomolecules within it, are critical design parameters.
Fill-in Factor and Diffusion: The "fill-in factor" (how much of the sensing area is occupied by biomolecules) and the transport mechanism (e.g., diffusion-limited vs. rapid mixing) within the nanocavity are directly tied to their effective position and concentration.Higher Fill-in Factor: A higher fill-in factor, effectively meaning a more concentrated presence of molecules closer to the gate region, results in a more significant change in threshold voltage (

ΔVthcap delta cap V sub t h end-subΔ𝑉𝑡ℎ) and a larger spread of drain current in the sub-threshold region.
Diffusion Mechanisms: The dynamics of how quickly the biomolecules move and bind (diffusion-limited process) versus a static, uniform distribution (rapid mixing) also affect the performance parameters, such as current and voltage sensitivity.
Sensitivity Enhancement with Negative Capacitance: The integration of negative capacitance (NC) using ferroelectric (FE) materials in the gate stack further amplifies the input signal at low gate voltages. This effect magnifies the sensitivity to the changes in surface potential caused by the positioned biomolecules, allowing the sensor to surpass the conventional Nernstian limit of 59.2 mV/pH.
Optimized Performance: The ability to tune sensitivity via geometric parameters and biomolecule position suggests that engineered vertical TFET structures can provide ultra-high detection capabilities for various biomolecules and pH levels.

In summary, optimal performance is achieved when the device structure and biomolecule position allow for the maximum electrostatic control over the TFET's channel, an effect that is further enhanced by the negative capacitance mechanism.

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