FROM IONS TO MOLECULES

A bare ISFET has no target in mind

ISFET + receptors → BioFET

A bare ISFET responds mainly to the pH at its surface. To detect a protein or a DNA strand, the surface needs a recognition layer: molecules that grab one specific target and ignore the rest. That upgrade turns an ISFET into a BioFET.

A receptor layer is what makes a transistor specific.

BIORECEPTORS

Four common kinds

🧬

DNA or RNA probe

A short single strand that pairs with its complementary sequence

🛡️

Antibody

A Y-shaped protein about 10 nm across that binds one antigen

🧵

Aptamer

A short folded strand of DNA or RNA selected to bind a target

⚗️

Enzyme

A catalyst whose reaction makes or uses ions the FET can read

Each one trades off size, stability, and cost.

THE BIOFET

Binding changes the charge at the gate

Receptors sit on the insulator. When a target binds, its charge joins the surface charge, and the threshold moves, just as it did for pH. No fluorescent or enzymatic label is needed, and the reading is electrical and continuous.

The signal is a threshold shift from bound charge: label-free and electrical.

THE SIGNAL

Charge in, threshold out

ΔVth ≈ −Q / C_ox

Bound charge shifts the threshold by the same relation as in any MOSFET: ΔVth ≈ −Q/Cox, where Q is the bound charge per unit area. DNA is a handy example, because every nucleotide carries about one negative charge on its phosphate group.

A negative target raises an NMOS threshold; a positive one lowers it.

A GOOD RECEPTOR

Four things to look for

Specific Binds the target and little else

Strong High affinity, so low concentrations still register

Stable Survives storage and repeated rinsing

Small Keeps the target close to the surface, where the gate can feel it

That last point becomes critical once ions in the sample start screening charge.

UNIT 14 STUDY COMPLETE

🧬

Ready for the Fab Challenge?

You've covered how a recognition layer turns an ISFET into a sensor for molecules.