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INEOS OPEN, 2026, 9 (4), 115–118  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2605a

Electrode Fabrication Techniques for Creating a Multisensor Chip
Based on an Array of Electrolyte-Gated Field-Effect Transistors

E. Yu. Poimanova,^a B. S. Kuleshov,^a V. A. Pavlov,^a A. A. Abramov,^a
K. A. Prusakov,^b D. A. Basmanov,^b and E. V. Agina*^a,c

^a Enikolopov Institute of Synthetic Polymeric Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia
^b Lopukhin Federal Research and Clinical Center of Physical-Chemical Medicine, Federal Medical-Biological Agency, Center of Technologies and Microfabrication, ul. Malaya Pirogovskaya 1a, Moscow, 119435 Russia
^c Department of Fundamental Physical and Chemical Engineering, Lomonosov Moscow State University, Leninskiye Gory 1/51, Moscow, 119991 Russia
 

Corresponding author: E. V. Agina, e-mail: agina@ispm.ru
Received 3 October 2025; accepted 12 April 2026

Abstract

This communication presents the effect of an architecture of electrodes and techniques of their deposition to a quartz wafer on the operation of 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene-based electrolyte-gated field-effect transistor array fabricated for the creation of a liquid multisensor chip.

Key words: electrolyte-gated field-effect transistor, multisensor device, electrode fabrication.

 

Introduction

An architecture of electrolyte-gated field-effect transistors (EGOFETs) includes a substrate (silicon or quartz wafer) with deposited contacts source and drain, an organic semiconductor (OSC) located between the source and drain electrodes, an electrolyte solution placed on the OSC surface and a gate electrode immersed in the electrolyte or contacted with the electrolyte surface in the case of solid electrolyte [1–3]. The OSC layer works as an active channel of the device. Upon application of a gate bias, two electric double layers (EDLs) are formed: one at the OSC/electrolyte and another at the electrolyte/gate interfaces. Due to high capacitance of these EDLs, a significant amount of the charge carriers appears at the OSC/electrolyte interface under a gate bias as low as 1 V, which results in well-measurable field-effect current under applied drain-source bias [4]. It allows for biosensing of various biological analytes or ions without any side reactions, including water electrochemical decomposition.

Besides the choice of the substrate and semiconducting material, considerable attention should be drawn to the material and topology of the source and drain electrodes as well as the fabrication techniques of their deposition due to high importance of charge injection processes for overall EGOFET performance.

Gold is the most commonly used material for the fabrication of drain and source contacts owing to its high electrochemical stability, excellent electrical conductivity and low contact resistance when employed with p-type semiconductors [5]. To improve the mechanical adhesion between the electrodes and the substrate, a thinner chromium [6, 7] or titanium [8] adhesion layer is often plated under the gold layer. Platinum is usually employed as a gate electrode material [6]. Other materials were also used for the fabrication of the gate electrodes. For instance, gold was utilized in some experiments [9, 10] because it can be easily modified with organic molecules for the creation of biorecognizing layers.

Interdigitated drain–source contacts are very promising architectures for creating electrolyte-gated organic transistors because interdigitated circuits allow for a significant increase in the channel width (W) at a fixed channel length (L). According to the Schockley equation (1, vide infra), I_ds is proportional to the W/L ratio. Hence, interdigitated circuits make it possible to increase the current in the transistor cannel and reduce the relative error of measurement. The interdigitated contacts enable a significant increase in I_ds, while remaining the leakage current I_gs unchanged.

The estimation of EGOFET operation quality requires the measurement of several main parameters, namely, threshold voltage (V_th), on- and off-current in the transistor channel (I_on, I_off), and transconductance (g_m). These characteristics can be calculated from the Shockley equation (1) [11]:

where µ is the charge carrier mobility, W is the channel width, L is the channel length, and С_i is the specific capacitance.

The transconductance (g_m) can be determined at constant V_DS using equation (2) [12]:

Highly doped silicon wafers coated with a layer of thermally grown silicon dioxide, quartz wafers and polymer films are usually used as substrates for the EGOFET fabrication. Small-molecule organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C₈-BTBT-C₈) is one of the most widely used semiconductors for the EGOFET fabrication owing to its excellent electrochemical, thermal and thermo-oxidative stability combined with high mobility of charge carriers. C₈-BTBT-C₈ is one of the most prominent materials for EGOFETs [13–16] and is applied in EGOFET-based chemo- and biosensors [17–21].

This report describes the effect of an architecture of electrodes and techniques for their deposition to a quartz wafer on the operation of 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene-based electrolyte-gated field-effect transistor array fabricated for the creation of a liquid multisensor chips.

Results and discussion

Various EGOFETs architectures were examined in order to find the optimal contact deposition technique as well as single EGOFET emplacement on a sensor chip for multiplex analysis.

Vector images of the substrate topology with transistors arrays were created for photolithography. The model was designed taking into account the necessity to create a large aspect ratio (W/L) of the transistor channel, which could be easily achieved using an interdigital arrangement of contacts. It is believed that such an arrangement will result in the creation of significantly more sensitive sensors compared to the sensors based on linear unstructured contacts. The first approach to fabrication of the device array was carried out by thermal vacuum evaporation of gold electrodes using standard linear or interdigitated topology (Figs. 1a–c), the second one—by thermal vacuum evaporation of metals (chromium and gold) followed by photolithography using a designed topology (Figs. 1d,e), and the third approach was carried out by screen printing of contacts using special inks (pastes) containing nanoparticles of electrode metal (Fig. 1f). In this manner, the devices with both linear and interdigital contacts were obtained using various electrode materials and fabrication techniques as well as device emplacement topologies on the substrate. C₈-BTBT-C₈/polystyrene mixtures were used for fabrication of a semiconducting layer and were deposited onto the fabricated substrates by the doctor blade coating under identical conditions. Smooth thin films with morphology typical for C₈-BTBT-C₈ deposited by the doctor blade coating were obtained for all explored devices. The atomic force microscopy (AFM) images of the film surface topology in the center of the transistor channel show that the quality of C₈-BTBT-C₈ film does not depend on the method of the electrode deposition for thermally deposited through shadow masks or lithographically etched contacts (Figs. 1a–e). However, the OSC film in the transistor channel of screen-printed devices seems to be crumpled (Fig. 1f), which means that the film has non-uniform morphology with numerous defects, probably, due to the large height of silver-printed contacts that prohibit the blade uniform movement and film alignment typical for other samples.

Figure 1. C₈-BTBT-C₈-based EGOFETs with different types of the electrode materials, deposition techniques and architecture—real photograph of the chip, POM-image of the Source/Drain electrode area, AFM image of the C₈-BTBT-C₈-film in the transistor channel: Au, linear with W/L = 33, thermal deposition (a); Au, interdigitated with W/L = 200, thermal deposition (b); Au, interdigitated with W/L = 365, thermal deposition (c); Au/Cr, interdigitated with W/L = 3000, lithography technique (d); Au/Cr, interdigitated (circle) with W/L = 3000, lithography technique (e); Ag, linear with planar gate, screen printing with W/L = 20 (f).

The current–voltage characteristics were measured in the linear (V_sd = –0.1 V) and saturated (V_sd = –0.5 V) EGOFET modes (Fig. 2 and Fig. S1 in the Electronic supplementary information (ESI)). Typical output and transfer characteristics of fabricated EGOFET with interdigitated thermally deposited contacts are presented on Fig. 2, for other devices—on Fig. S1 in the ESI. The electrical characteristics of EGOFETs fabricated by different electrode deposition techniques are presented in Table 1.

Figure 2. Typical output and transfer characteristics of EGOFETs fabricated with
the interdigitated contacts thermally deposited through shadow mask.

Table 1. Electrical characteristics of EGOFETs fabricated by different electrode deposition techniques

Electrode material, type, deposition method	W/L	Ion, µA	Ioff, nA	Vth, mV	gm·106, A1/2·V–1	Ion/Ioff
Au, linear, thermal deposition	33	0.6 ± 0.3	1.0 ± 0.4	39 ± 4	1.3 ± 0.4	660 ± 70
Au, interdigitated, thermal deposition	200	1.0 ± 0.5	2.8 ± 0.3	97 ± 7	0.38 ± 0.04	475 ± 47
Au, interdigitated, thermal deposition	365	19 ± 1	15 ± 1	100 ± 9	2.8 ± 0.6	124 ± 8
Au/Cr, interdigitated, lithography	3000	6 ± 2	4 ± 1	67 ± 3	1.0 ± 0.3	1505 ± 204
Au/Cr, interdigitated, lithography circle	3000	7 ± 1	1.5 ± 0.5	85 ± 4	0.6 ± 0.1	4823 ± 682
Ag, linear with planar gate, screen printing	20	–	–	–	–	–

It is obviously seen that the method of thermal vacuum evaporation of metal contacts through shadow masks offers good reproducibility of electrodes height and shape, as well as uniform surface morphology of organic semiconductor deposited over the contacts. However, it is quite expensive due to material cost and an architecture of the resulting substrates is determined by shadow masks, which are available from a very limited number of manufacturers. Furthermore, for interdigitated contacts architectures, it is quite difficult to achieve a well-aligned deposition of chromium and gold adhesive layers through shadow masks. At the same time, the presence of an adhesive layer is critical for preventing electrode delamination during EGOFET operation. This problem could be solved using lithography, which, however, also has its own drawbacks and requires strict adherence to metal etching and photoresist cleaning protocols because even the slightest deviations from the procedure can lead to non-reproducible surface properties of the substrate and non-uniform deposition of the organic semiconducting film.

Stencils for contacts screen printing were manufactured specifically for EGOFET devices and allow measurements using a planar gate electrode, i.e., an electrode located on a substrate in the same plane as the drain–source contacts (Fig. 1f). Screen-printed contacts were conductive and could have been used for further EGOFETs fabrication. However, high leakage and a virtually complete absence of the field effect were detected under measuring the electrical characteristics of the devices fabricated by the deposition of C₈-BTBT-C₈/polystyrene film on the top of the screen-printed contacts (Fig. S1e in the ESI) This may be due to the fact that contacts fabricated by screen printing are significantly thicker than those deposited by thermal evaporation, and the semiconducting film does not completely isolate the contacts from interactions with the electrolyte, which is indirectly confirmed by the crumpled film on the AFM image (Fig. 1f) and results in high leakage.

Operating devices with all studied electrodes show low hysteresis and small threshold voltage (Table 1). The highest I_on values were obtained for interdigitated thermally deposited electrodes while the most significant parameter of EGOFET for sensor measurements, the ratio I_on/I_off, has the highest value for the interdigitated lithographical electrodes. That is why the most suitable electrodes for fabricating an EGOFET multisensor chip are the interdigitated lithographical electrodes, coping with the problem of delamination and providing important electrical parameters for sensing.

Experimental section

The synthesis of the organic semiconductor 2,7-dioctyl-benzothieno[3,2-b][1]benzothiophene, used in the formation of an active layer of the EGOFET device, was performed according to the published procedure [22]. Toluene was purchased from Acros Organics and used as received. Polystyrene (PS, M_w = 280000) was purchased from Sigma Aldrich and used as received. Ultrapure deionized water with a resistance of 18.2 MΩ was obtained using an Akvilon deionizer D-301 (Russia). Acetone, isopropanol (IPA) (both A.C.S. grade, EKOS-1, Russia) and deionized (DI) water were used to rinse a plate and photomask. The lithography square and circle interdigitated electrodes (with W/L = 3000) were formed from a gold (40nm)/chromium (10nm) film that covered the whole glass plate according to the procedure described in Ref. [19]. For another deposition method, the source (S) and drain (D) gold electrodes (with W/L = 33 for the linear electrodes and 200 or 365 for the interdigitated counterparts) were thermally evaporated through appropriate shadow masks. For better charge injection the S/D contacts were modified by a layer of 2,3,4,5,6-pentafluorobenzothiophene (PFBT) according to the earlier published procedure [14]. Deionized water was used as an electrolyte. The gate electrode was a platinum wire immersed in the electrolyte.

CRSN2442 silver paste (Sun Chemical), consisting of nanosized silver particles in a solvent, was used as the screen printing inks. Silver nanoink was chosen due to its commercial availability and the possibility to print contacts with good adhesion to the substrate [23]. The glass substrate with the printed contacts was annealed at 150 °C in a vacuum oven for 2 h to completely remove traces of the ink solvent and ensure good sintering of the silver nanoparticles. Linear silver source and drain electrodes with the planar gate electrode with W/L = 20 were fabricated by screen printing.

The C₈-BTBT-C₈/PS mixture with a mass ratio of 4:1 was prepared by dissolving C₈-BTBT-C₈ in a toluene solution of polystyrene in order to obtain a total concentration of 18 mg·mL^–1. The blend deposition was made with a Coatmaster 510 Doctor Blade coater Film Applicator (Erichsen, Hemer, Germany) at 85 °C as described in Ref. [14] and resulted in a crystalline film of the organic semiconducting layer.

The semiconducting layers were characterized by an NT-MDT Solver Next atomic force microscope (Russia) in a tapping mode under ambient conditions using NT-MDT HA-FM silicon probes with a resonant frequency of 77 kHz. Polarizing optical microscopy was conducted at all stages of the experiment using a Carl Zeiss Axioscop A40Pol instrument (Germany).

The electrical measurements were performed using a Keithley 2634B (Keithley, USA) source-meter under an ambient environment using a PS-100 probe station (Printeltech, Russia). A platinum wire (Ø 0.5 mm) was used as a gate electrode during measurements in the drop on EGOFET surface. A delay of 100 ms between the drain/gate voltage sourcing and current measurement was set to avoid capacitive effects. Every curve was measured in direct and reverse scans, which consisted of 41 points in each direction. The electrical values were extracted by fitting the transfer characteristics with a Shockley's gradual-channel model in the saturation mode (V = –0.5 V) according to equations 1 and 2.

Conclusions

Summarizing the results obtained, it can be concluded that the electrode fabrication technique applied for creating a multisensor chip based on an array of electrolyte-gated field-effect transistors plays the key role in the EGOFET electrical performance which determines its sensory properties. In order to fabricate the EGOFET array for multisensor chip aiming biosensing in liquid media, it is more suitable to use interdigitated lithographical electrodes, with the highest value of W/L ratio resulting in the highest I_on/I_off ratio, which is the main EGOFET parameter well-correlated with the high and reproducible sensor response in liquids.

Acknowledgements

This work was supported by the Russian Science Foundation (project no. 23-73-00103) and performed at ISPM RAS using the equipment of the Collaborative Access Center "Center for Polymer Research", while lithography of transistor contacts on glass wafers was carried out within the framework of the state assignment "Amalthea-1", state registration number R&D 124031500113-3 and performed at Lopukhin Federal Research and Clinical Center of Physical-Chemical Medicine of Federal Medical-Biological Agency.

Electronic supplementary information

Electronic supplementary information (ESI) available online: the electrical characteristics of the fabricated devices. For ESI, see DOI: 10.32931/io2605a.

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