**2. The work principle and simulation results for a p-NOI structure**

Recently, the nothing on insulator (NOI) device, as the succession n-Si/Vacuum nanocavity/ n-Si (nVn) on insulator, was proposed [18] and timely updated [19]. The horizontal variant implementation for the NOI transistor is unknown at the actual technology level, etching a straight cavity in Si from 10 to 20 nm depth of only 2 to 3 nm width, without pipes which seem to be impossible [20].

dots, magnetic nanoparticles, carbon nanotubes, and graphene due to their surface properties, excellent electron transfer, and a large ratio of surface area to volume, making them particularly attractive for use in labels or transducing platforms for optical or electrochemi-

The examples of such biosensors are the organophosphorus pesticides using liposome-based nano-biosensors [3]. Gold nanoparticles for pesticide detection using cyclic voltammetry [4], organophosphorous pesticide (OP) biosensor based on quenching of the fluorescence from CdTe QDs [5]. Acetylcholinesterase action is monitored using a localized surface plasmon resonance (LSPR) fiber optic biosensor [6]. AuNP-AChE conjugates for paraoxon electrochemical biosensor [7]. AuNP-AChE onto chemically reduced graphene nanosheets (cr-Gs) [8], graphene oxide/Nafion (RGON) nanohybrids electrochemical biosensor platform to detect organophosphorus hydrolase as an enzyme for the hydrolysis of Ops [9], pathogen detection

Unfortunately, the pesticides used in this field not only spoil the soil but also infest the entire food chain. Less toxic new generations of pesticides may reduce the risks transmitted to people and environment, especially by water contamination. The pesticides reduce the nitrogen fixation in plants, consequently decrease the biodiversity, destroy habitats, and threaten jeopardized species [12]. Integrated biosensors usually contain onto the same chip of the semiconductor solid-state support, the transducer as an electronic device, and the biological detector

This chapter describes a pesticide biosensors fabricated using nanoporous Si materials to entrap the receptor element, along with the transducer element consisting of an interdigitated capacitive electrodes to detect pesticides, like paraoxon. The novel detection scheme is using interdigitated capacitive electrodes which highlighted a special nanostructure called as the planar nothing on insulator (p-NOI) [15, 16]. The biodetection is based on the hydrolysis under the acetylcholinesterase (AChE) enzyme action, as biosensor-specific receptor [17]. The final product is an integrated biosensor that is constructed by microtechnological processes aided by biotechnological enzyme processing steps, having a nanoporous Si layer coupled to a p-NOI capacitive transducer, which is sensitive to the pesticide

The p-NOI structure that is integrated inside the biosensor transducer has another facet in this work: the first p-NOI structure still exists between top metal on insulator placed on silicon, and the second p-NOI structure is present between two adjacent lateral metal fingers. The first one must accomplish an isolation through the bottom nanoporous material. The second one has the distance between fingers high enough versus a nanometric p-NOI that allows a tunnel current flow [15]. Hence, the tunneling conduction is missing in this case. But the liquid droplet that connects two adjacent fingers by an ionic conductor offers a novel conduction route.

**2. The work principle and simulation results for a p-NOI structure**

Recently, the nothing on insulator (NOI) device, as the succession n-Si/Vacuum nanocavity/ n-Si (nVn) on insulator, was proposed [18] and timely updated [19]. The horizontal variant implementation for the NOI transistor is unknown at the actual technology level, etching a

cal sensors and biosensors.

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in soils using nanobiosensors [10, 11].

as an enzyme [13] or an antibody [14].

concentration.

If the nVn succession is used as the device body, we speak about a NOI (nothing on insulator) transistor [20]. If oxide (O) is used instead of the vacuum (V) cavity and, additionally, a metal is used instead of one semiconductor zone, we speak about a mOn succession as metal/oxide/ n-Si. This structure still conserves the NOI work principle [21]. Also, if oxide is replaced by any insulator (I) placed between two metals placed on a Si wafer surface, we speak about MIM structure [22]. The mOn and MIM structures use the same thin insulator tunneling principle but benefit on materials placed on the front plan of the Si wafer. Both of them are associated with the planar variant of a NOI device, simply noted by p-NOI device [15].

Therefore, a vertical implementation of the p-NOI variant is more suitable for the integration of the biosensor transducer. The insulator can be oxide or sandwich of insulators of 10 nm up to 50 nm thickness to prevent the substrate tunneling [23]. The oxide is grown by the Si planar technology. Therefore, the presented p-NOI structure is a vertical simplified NOI variant, with the advantage to be inherent integrated on the Si wafer during the biosensor metallic electrode configuration on insulator. On the other hand, the Fowler-Nordheim tunneling through the bottom insulator is poor. Hence, more than 50 nm oxide thickness ensures an excellent dielectric insulation that is suitable for the biosensor transducer purposes. The explanation comes from two Fowler-Nordheim tunneling ways in this transducer: (i) the useful one that acts the p-NOI device at the surface of the device and (ii) the parasitic tunneling toward substrate that must be avoided. The transducer successfully interacts with bio-liquid on the top of the wafer, generating the capacitance variation, while efficiently prevents the leakage current toward substrate, for thick-enough oxide layer. However, a principle that must be checked is to simulate an exponential I–V dependence for a vertical p-NOI, to put in agreement the Fowler-Nordheim tunneling principle with the p-NOI conduction mechanisms [24].

**Figure 1** presents the proposed vertical p-NOI structure with substrate as back-gate. The usual anodes play the gate role, and the cathodes are the source or drain. Therefore, the notations are kept as in a transistor configuration. Essentially, in **Figure 1** there are three vertical

**Figure 1.** The basic p-NOI device in the planar configuration.

**Figure 4** shows the distribution of the electron concentration in the p-NOI structure biased at

It can be seen in **Figure 5** that the electric field inside the ultrathin oxide has an increased value

The explanation is still associated to a strong tunneling for the main electrode and weak tunneling for the adjacent electrode, in agreement with the Fowler-Nordheim tunneling theory applied for the NOI device [24]. However, the higher distances between two adjacent metallic fingers inside the next pesticide biosensors foster rather the capacitive effect of p-NOI than the conductive effect. Therefore a capacitive analysis is performed for the extreme case of an ultrathin oxide thickness of 5 nm, when a AC signal is applied to the p-NOI structure (**Figure 6**).

boundary conditions with εSi/εoxide ~ 2.7.

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195

Integrated p-NOI Structures on Nanoporous Material Designed for Biodetection

29 V. There does not seem to be significant carrier depletion.

**Figure 4.** Distribution of the electron concentration in the p-NOI structure at 29 V.

**Figure 5.** The electric field distribution in the p-NOI structure at 29 V.

up to approximative 8 V/cm, in agreement to the Si-SiO2

**Figure 2.** The p-NOI device biased at +29 V.

metal-oxide-semiconductor-metal structures as simultaneous three vertical p-NOI structures, similar to the adjacent fingers included in the next studied biosensor.

In **Figure 1** the p-NOI structure size and doping concentration are presented. On the polysilicon terminal, a voltage of 29 V is applied, and the other metallic contacts are grounded. **Figure 2** shows the potential distribution in the central p-NOI device and the current vectors through the structure, after ATLAS Running. The maximum current density is 39.9A/cm2 . In **Figure 3a** and **3b**, the current-voltage characteristics, IG-VG-type, are shown through the p-NOI structure when the gate voltage has increased from 0 to 29 V. It is demonstrated in this case that a tunnel current arises, after the exponential shape of the curves at linear and logarithmic scale, being a good start-up result.

For a p-NOI structure with adjacent metals, the applied voltage on the polysilicon electrode can be more favorable when the semiconductor is superficially doped with 10<sup>20</sup> cm−3.

**Figure 3.** The simulated characteristics of the vertical p-NOI at scale: (a) linear and (b) logarithmic.

**Figure 4** shows the distribution of the electron concentration in the p-NOI structure biased at 29 V. There does not seem to be significant carrier depletion.

It can be seen in **Figure 5** that the electric field inside the ultrathin oxide has an increased value up to approximative 8 V/cm, in agreement to the Si-SiO2 boundary conditions with εSi/εoxide ~ 2.7.

The explanation is still associated to a strong tunneling for the main electrode and weak tunneling for the adjacent electrode, in agreement with the Fowler-Nordheim tunneling theory applied for the NOI device [24]. However, the higher distances between two adjacent metallic fingers inside the next pesticide biosensors foster rather the capacitive effect of p-NOI than the conductive effect. Therefore a capacitive analysis is performed for the extreme case of an ultrathin oxide thickness of 5 nm, when a AC signal is applied to the p-NOI structure (**Figure 6**).

**Figure 4.** Distribution of the electron concentration in the p-NOI structure at 29 V.

**Figure 5.** The electric field distribution in the p-NOI structure at 29 V.

**Figure 3.** The simulated characteristics of the vertical p-NOI at scale: (a) linear and (b) logarithmic.

metal-oxide-semiconductor-metal structures as simultaneous three vertical p-NOI structures,

In **Figure 1** the p-NOI structure size and doping concentration are presented. On the polysilicon terminal, a voltage of 29 V is applied, and the other metallic contacts are grounded. **Figure 2** shows the potential distribution in the central p-NOI device and the current vectors through

and **3b**, the current-voltage characteristics, IG-VG-type, are shown through the p-NOI structure when the gate voltage has increased from 0 to 29 V. It is demonstrated in this case that a tunnel current arises, after the exponential shape of the curves at linear and logarithmic scale, being a

For a p-NOI structure with adjacent metals, the applied voltage on the polysilicon electrode can be more favorable when the semiconductor is superficially doped with 10<sup>20</sup> cm−3.

. In **Figure 3a**

similar to the adjacent fingers included in the next studied biosensor.

good start-up result.

**Figure 2.** The p-NOI device biased at +29 V.

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the structure, after ATLAS Running. The maximum current density is 39.9A/cm2

of poisoning for humans or animals, due to its simply absorption through teguments in contact with the contaminated water from environment. As pesticide, the parathion is dissolved in water and usually is applied by treatment. It is frequently sprayed to rice and fruits. The usual concentrations are 0.05 and 0.1% [28]. After the rain, the pesticide is accumulated in water and soil. This parathion degradation in time produces multiple

Integrated p-NOI Structures on Nanoporous Material Designed for Biodetection

This section depicts the paraoxon biosensor starting from a Si wafer technology. Some intermediate nanoporous materials are used in the biosensor construction, for the enzyme entrap-

[13], or porous Si still exists [30]. The porous

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197

.

to increase the film

O3

material integration on a silicon wafer is starting by the first metal deposition, followed by subsequent processing steps, in order to convert them into compounds and finally into a

• The next process is the boron ion implantation through the patterned mask in a high dose

• Then, the Si p-type layer is converted in porous Si by anodization in the electrolyte

This porous Si technology provides usual porous Si layers with a porosity of 56, suitable for

These intermediate porous materials augment the capillary, allowing the biomaterials entrapping in a liquid phase, during the pre-deposition technological stage. At the same time, the porous layer must be grown onto the Si wafer in order to be strongly anchored to substrate and in order to avoid accidental detachments. Nanoporous Si can be easily converted from a Si thin upper layer. Having a closer lattice constant with Si, the porous Si stands for an efficient intermediate material for the next technological steps. The nanoporous Si material preparation by anodization is a perfect compatible method with the microelectronics technology. The pore sizes can be simply adapted in respect with the anodization reaction parameters, changing the electrolyte composition. Due to an increased area, offered by the nanoporous Si material against the monocrystalline Si, an enhanced miniaturization with capacitive electrodes can be performed. Therefore, the porous Si was selected as intermediate layer for the AChE enzyme entrapping. This solution is also in agreement with the

O with 180:60:60 ratios, at current density more than 2 mA/cm2

and 850°C in N2

[29], Al2

• The Si start wafer is n-type, <100> orientation, and 2–9 Ωcm resistivity.

• The first process is a thermal oxidation that allows the pattern configuration.

porous matrix. The main steps of porous Si layer formation are:

on the front wafer to convert the upper Si layer into p-type.

• The last process consists in annealing at 550°C in H2

water-soluble products.

HF:CH<sup>3</sup>

stability.

nowadays tendency.

COOH:H2

the enzyme entrapping process [30].

ping. Among these materials, TiO2

**3.2. Nanoporous materials for enzyme entrapping**

**Figure 6.** The CV analysis for a AC sweep for VG between −10 V and +10 V for two adjacent metal fingers like source and gate of a p-NOI structure with 5 nm film thickness and similar size as in **Figure 1**, for low frequency (100 Hz) and high frequency (1 THz).

Obviously, the capacitive range is in agreement with the sizes of the p-NOI structure form (**Figure 1**) and varies between 6 × 10−17 and 1.2 × 10−16 F/μm (**Figure 6**).
