Biomolecular turn-ons

Peter Allen

Transistors can make good biosensors. The usual approach is to functionalize the gate oxide of a field-effect transistor with receptors for specific, charged biomolecules. When these are captured, they modulate the conductance of the channel, which can be measured. Deblina Sarkar and Kaustav Banerjee of the University of California, Santa Barbara have now predicted that tunnel field-effect transistors could be used to make biosensors that are even better.

Both tunnel and conventional field-effect transistors use a gate electrode to control the barrier to carrier flow. In a conventional device, carrier injection over this barrier is thermally assisted, restricting turn-on speeds to 60 mV of gate voltage for every factor of 10 increase in current at room temperature. In a tunnel transistor, however, carriers tunnel from band to band at a rate that is independent of temperature. This leads to faster turn-on.

Moreover, a transistor-based biosensor is most sensitive while it is turning on because in that regime, the change in current for a given change in gate voltage is greatest. This means faster turn-on translates to better performance. Sarkar and Banerjee calculate that a tunnel transistor's fast turn-on translates to an increase in biomolecule sensitivity of four orders of magnitude. The response time also improves by one order of magnitude.

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