Bio-FET Fabrication & Advantages

To summarize our previous article about Bio-FETs, Biologically active field-effect transistors (BioFETs) are miniaturized silicon-based electrochemical biosensors that can detect biomolecular interactions occurring at the gates of MOSFETs. Compared to the conventional optical and mass spectrometry detection methods, BioFETs provide fast, label-free, high sensitivity and direct electronic signal readout. FET-based biosensors have the potential for developing high-throughput label-free microarrays for applications in clinical diagnostics, drug discovery, food monitoring, and bioterrorist security inspection.

The following steps are involved in the fabrication of a Bio-FET system: 

1.  Finding a substrate suitable for serving as a FET site, and forming a FET on the substrate. 
2.  Exposing an active site of the FET from the substrate. 
3.  Providing a sensing film layer on the active site of FET. 
4. Providing a receptor on the sensing film layer in order to be used for ion detection
5. Removing a semiconductor layer, and thinning a dielectric layer
6. Etching the remaining portion of the dielectric layer to expose an active site of the FET
7. Removing the photoresist, and depositing a sensing film layer followed by the formation of a photoresist pattern on the sensing film
8. Etching the unprotected portion of the sensing film layer, and removing the photoresist

Advantages of Bio-FETs

Bio-FET devices are based on detecting changes in electrostatic potential due to the binding of the analyte. This is the same mechanism of operation as glass electrode sensors which also detect changes in surface potential but were developed as early as the 1920s. Due to the small magnitude of the changes in surface potential upon binding of biomolecules or changing pH, glass electrodes require a high impedance amplifier which increases the size and cost of the device. 

In contrast,  the advantage of Bio-FET devices is that they operate as an intrinsic amplifier, converting small changes in the surface potential to large changes in current (through the transistor component) without additional circuitry. This means BioFETs have the capability to be much smaller and much more affordable than glass-electrode based biosensors. If the transistor is operated in subthreshold region, then an exponential increase in current is expected for a unit change in surface potential. 

Bio-FETs can be used for medical diagnostics, biological research, environmental protection and food analysis. Conventional measurements like optical, spectrometric can also be used to analyze biological molecules. However, these methods are relatively time-consuming and expensive, involving multi-stage processes and also not compatible with real-time monitoring. 

BioFETs are low weight, low cost for mass production, small size and compatible with commercial planar processes for large-scale circuitry.

In conclusion, BioFETs and their potential for medical diagnosis is an interesting crossroad between electronics and biology, of which we have merely explored a small fraction. 

-Krishna Londhe