Bio-FET (Bio sensor MOSFET)

Most people tend to perceive electronics and its applications primarily limited to multimedia entertainment, security systems, drones, autonomous systems and other component-based devices. However, these overlook the extensive applications electronics has to biology and biological systems. 

The intermingling of biology and electronics is so expansive today that there are subspecialties that range from biomolecular (i.e Bioelectronics) to the gross integration of robots, biology and electronics (Biomechatronics). 

One such application we intend to briefly touch upon today is Bio-FET.

A field-effect-transistor-based biosensor, also referred to as Bio-FET are field-effect transistors that are gated by changes within the surface potential induced by the binding of molecules. When charged biomolecules bind to the FET gate made from dielectric material they will change the charge distribution of the underlying semiconductor material leading to a change in conductance of the FET channel. A Bio-FET consists of two main components: one is that the biological recognition element and also the other is that the FET.

These transistors have previously been used in chemoreceptive applications like pH detection in electrolyte solutions. A semiconductor has a net surface charge when interacting with an electrolyte solution due to the addition and removal of protons of functional groups on the solid/liquid interface which affects the effective gate voltage and subsequently the charge-carrier flow through channel. A FET device can therefore translate biochemical binding or ionic concentration changes on its surface into a measurable signal associated with the surface properties of the gate input. A field-effect-transistor-based biosensor, also referred to as Bio-FET are field-effect transistors that are gated by changes within the surface potential induced by the binding of molecules. When charged biomolecules bind to the FET gate made from dielectric material they will change the charge distribution of the underlying semiconductor material leading to a change in conductance of the FET channel. A Bio-FET consists of two main components: one is that the biological recognition element and therefore the other is that the FET. 

Mechanism of operation

Figure 1: Typically an electrically and chemically insulating layer (like Silica) separates the analyte solution from the semiconducting device. A polymer layer chemically links surface to the receptor. Upon binding of the analyte, changes in electrostatic potential at the surface of the electrolyte-insulator layer occur, in turn resulting in electrostatic gating effect and a measurable change in current between drain and source electrodes.

Bio-FETs couple a transistor device with a bio-sensitive layer which will specifically detect bio-molecules like nucleic acids and proteins. A Bio-FET system consists of a semiconducting FET that acts as a transducer separated by an insulator layer (eg. SiO2) from the biological recognition element (e.g. receptors or probe molecules) which are selective to focus on molecule called analyte. 

Once the analyte binds to the recognition element, the charge distribution at the surface changes with a corresponding change within the electrostatic surface potential of the semiconductor. this alteration within the surface potential of the semiconductor acts as a gate voltage would during a traditional MOSFET, i.e. changing the quantity of current which will flow between the source and drain electrodes. this alteration in current (or conductance) are often measured, thus the binding of the analyte are often detected. The precise relationship between the present and analyte concentration depends upon the region of transistor operation.

In our next article, we'll be covering the fabrication techniques and advantages of Bio-FETs.

-Sagar Mahajan