Application of junctionless nanowire transistors as ultrasensitive biosensors

Junctionless nanowire transistors (JNTs) have been recently proposed as a disruptive alternative of the conventional metal-oxide-semiconductor field-effect transistors (MOSFETs) [1]. They are gated resistors where the source, channel and drain have the same type of doping without any junctions and dopant concentration gradient. Thus JNT is the simplest possible transistor structure and probably the most scalable of all FET structures. It is easier to fabricate than standard MOSFETs and has also a number of performance advantages over them [1,2]. Therefore, JNTs have quickly attracted a vast interest. However, their application as sensors, although very appealing, has not yet been extensively studied.

Here we report on the fabrication of silicon JNTs and their implementation as chemical and biological sensors. The devices have been fabricated by a top-down approach mainly based on electron beam lithography and reactive ion etching (see Fig 1) [3]. The nanowire surfaces have been appropriately functionalised for the respective analytes of interest. A series of experiments (see Fig. 2) for sensing the ionic strength (see Fig. 3) and the pH value of buffer solutions have proven the excellent sensitivity of the fabricated sensors [3,4]. Moreover, sensing of the protein streptavidin at a concentration as low as 580 zM has been observed (see Fig. 4), which is by far the lowest concentration of this protein ever detected and corresponds to detection in the range of only few molecules.

To explain the ultrahigh sensitivity of JNT sensors, we will discuss in detail two advantages of JNTs over the classical MOSFETs [1-4], which are especially important for their application as sensors:

1. The current flow in JNTs is not controlled by a reverse biased p-n junction as in standard MOSFETs but entirely by the gate potential modulating the carrier density in the channel. Thus they are more sensitive to any change in the electrostatic potential on the channel surface acting as a gate potential.

2. JNTs demonstrate bulk conductance near the centre of the channel, in contrast to the conductance in a thin surface inversion or accumulation layer near the gate in the classical inversion mode or accumulation mode transistors, which leads to higher drive currents. Moreover, this fact makes the modulation of depletion and the conduction in JNTs less affected by the noise-inducing parasitic surface states than in the case of conventional MOSFETs, which is very important for achieving high signal-to-noise ratio and hence low detection limit [6,7].

The ultrahigh sensitivity of JNT sensors, combined with their very simple structure and relaxed fabrication process, makes them promising candidates for cheap mass production by the conventional microelectronic technology. This can enable their numerous applications in various fields where fast, low-cost, label-free, low-volume and real-time detection of chemical and biological species at low detection levels is required.

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[2] J. P. Colinge, A. Kranti, R. Yan, C. W. Lee, I. Ferain, R. Yu, N. D. Akhavan, P. Razavi. Solid State Electron. 65-66 (2011) 33-37.
[3] Y.M. Georgiev, N. Petkov, B. McCarthy, R. Yu, V. Djara, D. O'Connell, O. Lotty, A. M. Nightingale, N. Thamsumet, J. C. deMello, A. Blake, S. Das, J. D. Holmes. Microelectron. Eng. 118 (2014) 47-53.
[4] Y. M. Georgiev, R. Yu, N. Petkov, O. Lotty, A. M. Nightingale, J. C. deMello, R. Duffy, J. D. Holmes. Silicon and Germanium Junctionless Nanowire Transistors for Sensing and Digital Electronics Applications, in A. Nazarov, F. Balestra, V. Kilchytska, D. Flandre, eds., Functional Nanomaterials and Devices for Electronics, Sensors and Energy Harvesting, (Springer International Publishing AG, Cham, Switzerland, Ch. 17, 2014) 367-388.
[5] Y. M. Georgiev, N. Petkov, R. Yu, A. M. Nightingale, E. Buitrago, O. Lotty, J. De Mello, A. M. Ionescu, J. D. Holmes, Nanotechnology (2019), accepted, https://doi.org/10.1088/1361-6528/ab192c
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[7] K. Bedner, V. A. Guzenko, A. Tarasov, M. Wipf, R. L. Stoop, S. Rigante, J. Brunner, W. Fu, C. David, M. Calame, J. Gobrecht, C. Schönenberger. Sensor. Actuat. B-Chem. 191 (2014) 270.