# Application of Single-Electron Transistor to Biomolecule and Ion Sensors
## 6 Summary and Future Perspectives  
In this review, the feasibility of the ultrahigh-sensitive detection of ions and biomolecules using a Si SET biosensor was clearly proven through systematic study. Silicon structures are important for large-scale integration of SET sensors and the peripheral circuits. The mechanism for the ultrahigh-sensitive detection of ions/biomolecules based on an SET sensor was explained and compared with the detection mechanism using a nanowire FET sensor. Nanowire FET sensors can decrease the percolation effect by decreasing the channel width; however, it cannot increase \(g_{m}\) because of the narrowness of the channel width. On the other hand, SET sensors can increase \(g_{m}\) while keeping the channel width narrow, which leads to a decrease in the percolation effect. This was confirmed by theoretical and experimental analyses. Therefore, in principle, the sensitivity in an SET sensor can be made higher than that in a nanowire FET sensor by increasing \(V_{g}\). Here, the \(V_{g}\) values with the larger \(g_{m}\) in the SET can be made to appear near 0 V by adjusting the dopant concentration in the channel and the connecting regions between the channel and the source/drain. From the requirement for room-temperature SET operation, the width of the island must be around 10 nm or less. SET biosensors have the clear advantage over nanowire FET biosensors in detecting the biomolecules with a size of around 10 nm or less. In the detection of biomolecules much larger than 10 nm, highly sensitive detection also can be carried out by using the nanowire FET sensor with a large channel width (the same width as the size of the biomolecule), because the \(g_{m}\) of such FET sensors are very large. In contrast, nanowire barriers with a large width cannot be used for an SET in the detection of biomolecules much larger than 10 nm, because the quantum confinement effect does not occur and room-temperature SET operations are not possible with such wide nanowire barriers. Consequently, the \(g_{m}\) of an SET cannot be increased by enlarging the width of nanowire barrier when detecting biomolecules much larger than 10 nm. The only means of increasing the \(g_{m}\) of the SET in this case is to increase \(V_{g}\). By increasing \(V_{g}\), the resistance of nanowire barrier decreases. This leads to an increase in the current flowing in the nanowire barriers and the \(g_{m}\) of SET. However, there is an upper limit of \(V_{g}\) applied for SET operation. When the resistance of a nanowire barrier becomes smaller than the resistance quantum (25.813 k\(\Omega\)) by increasing \(V_{g}\), the quantum fluctuation in the electron number in the island due to tunneling through the barrier becomes large, and this makes SET operation impossible [29]. Therefore, nanowire FET biosensors have greater sensitivity than SET biosensors in detecting extremely large biomolecules. However, even in this case, the actual size of the biomolecule may be much smaller than its nominal size due to the Debye shielding in the solution, which may make the sensitivity of SET greater. Experimental and theoretical research is necessary to clarify this. The room-temperature operation of Si SETs is inevitably necessary for ion sensors and biosensors because these sensors use buffer solutions. Because a serially connected multiple-island channel structure is advantageous to room-temperature operation of SET, a pH sensor and biomolecule sensors  
Figure 12: Drain current (\(I_{d}\))-gate voltage (\(V_{g}\)) characteristics of an SET sensor for the detection of PSA. Reproduced with permission from [22], Copyright 2013, AIP Publishing LLC. for the detection of streptavidin have been developed based on the SETs with the channel structure. The detection of PSA at the cut-off concentration (4 ng/mL) has been also achieved by the SET sensor, which leads to practical use.