NANOWIRE FETS

Thin channels, big response

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Tiny cross-section

A silicon nanowire is tens of nanometers across

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High surface-to-volume

Nearly the whole channel is close to the surface, so bound charge affects all of it

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An early landmark

Nanowire sensors detected proteins and DNA at very low concentrations in the early 2000s

The channel is so thin that surface charge controls all of it.

GRAPHENE AND 2D FETS

Atomically thin channels

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One atom thick

Graphene's channel is entirely surface

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Ambipolar

It conducts both electrons and holes, with a minimum at the Dirac point

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Shift readout

Binding shifts the Dirac point, much as it shifts Vth

A one-atom channel has nowhere for charge to hide.

EXTENDED GATE

Keep the transistor dry

In an extended-gate design, the sensing pad sits in the liquid and is wired to the gate of a standard transistor placed away from it. The transistor stays dry and can be an ordinary, mass-produced MOSFET.

Separating the sensor from the transistor makes both easier to build.

CMOS ARRAYS

Millions of sensors on one chip

ISFET array → sequencing

Because ISFETs can be built in standard CMOS, millions can share a chip. Semiconductor DNA sequencers use this: when a nucleotide joins a growing strand, it releases a hydrogen ion, and an ISFET under each well reads the pH change.

The ISFET pH sensor became a genome-reading tool.

CHOOSING A FORMAT

Match the device to the job

Planar FET Well understood and easy to make in a fab

Nanowire Highest charge sensitivity per molecule

Graphene An atomically thin channel, with a different fabrication path

Extended gate A cheap, replaceable sensing pad

CMOS array Thousands to millions of sensors read in parallel

There is no single best format: the application decides.

UNIT 24 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered beyond the planar FET: the device designs used for biosensing.