Highly Sensitive Detection of Deoxyribonucleic Acid Hybridization Using Au-Gated AlInN/GaN High Electron Mobility Transistor-Based Sensors^*

The cross discipline research of microelectronic technology and biomedical science has recently become a hot issue and made great progress. Biosensing of deoxyribonucleic acid (DNA) hybridization using one single strand probe is crucial in diagnosing genetic diseases, collecting the physical evidence left behind the criminal scene, and detecting certain infectious or inherited diseases as well as cancers.^{[1, 2]}

Gallium nitride (GaN) based materials are attractive candidates for fabrication of the FET-based biosensors, due to their superior advantages such as non-toxicity to the living cells, high chemical and thermal stability, high sensitivity, wide energy bandgap, etc.^[3] Recently, AlGaN/GaN high electron mobility transistors (HEMTs) with high mobility and high density of two-dimensional electron gas (2DEG), which is induced by the piezoelectric and spontaneous polarization effects, have been studied for a variety of applications in detecting hydrogen, carbon monoxide (CO), polar liquids, halide ions, mercury ion, prostate-specific antigen (PSA), DNA and so on.^[4–16] Compared with AlGaN/GaN HEMTs, AlInN/GaN HEMTs are more suitable for these detections, since AlInN/GaN HEMTs with thinner barrier thickness can realize the same concentration of 2DEG as AlGaN/GaN HEMTs. Therefore, the 2DEG in the former case is closer to the surface and hence more sensitive to the accumulation of analytes on the gate surface. On the other hand, AlInN can be grown in lattice matched to GaN, thus the stress and piezoelectric effects are absent, potentially improving the stability of the heterostructure.^[17] Nonetheless, only a small number of works have been reported on AlInN/GaN sensor so far.^[18–20]

In this study, we fabricate both Au-gated AlInN/GaN HEMT and AlGaN/GaN HEMT biosensors for the detection of DNA hybridization. The AlInN/GaN HEMT biosensor is found to be more sensitive to the DNA hybridization when compared with the AlGaN/GaN HEMT biosensor.

The AlInN/GaN and AlGaN/GaN heterostructures for biosensors were grown on a 2-inch c-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD).^{[21, 22]} Figure 1 shows the cross-sectional schematic diagram of the GaN HEMT-based sensor with a metal gate (Fig. 1(a)) and the plan view photomicrograph of the device (Fig. 1(b)). The HEMT structure consists of a 2-${\rm{\mu }}$ m-thick undoped GaN buffer layer, a 1 nm AlN interlayer, either an 8 nm In ${}_{0.17}$Al${}_{0.83}$N or a 22 nm Al ${}_{0.26}$Ga${}_{0.74}$N barrier layer with no intentional doping. The indium composition of AlInN and the aluminum composition of AlGaN were measured by XRD. The 2DEG was located at the interface between the GaN layer and the Al(In)GaN layer. The electron mobility and 2DEG density of AlInN/GaN and AlGaN/GaN heterostructures are 1112 cm²/V $\cdot$ s, $2.1\times {10}^{13}$ cm ${}^{-2}$ and 1774 cm²/V $\cdot$ s, $8.2\times {10}^{12}$ cm ${}^{-2}$, respectively, as obtained by the Hall measurement at room temperature.

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After the epitaxial growth, the wafer was cleaned thoroughly and processed into HEMT devices. The ohmic contact metals (Ti/Al/Ti/Au) were sputtered as the source and drain electrodes of the devices and patterned by a lift-off process, followed by rapid thermal annealing at a high temperature of 870°C under ambient N₂ for 30 s. The $20\times 100\,{\rm{\mu }}$ m² ohmic contacts of source and drain were separated with a gap of 20 ${\rm{\mu }}$ m. Mesa isolation was performed using an inductively coupled plasma (ICP) etching system with a BCl₃/Cl₂-based plasma gas mixture. A 15-nm-thin Au film was deposited as the gate metal to immobilize the thiol-modified oligonucleotides, with the length/width of 8 ${\rm{\mu }}$ m/100 ${\rm{\mu }}$ m. A 200 nm silicon nitride (Si₃N₄) passivation layer was deposited by plasma-enhanced chemical vapor deposited as a stable encapsulant, and then the Si₃N₄ layer on top of the Au gate area for liquid solution and pads of the electrodes for measurement were removed by ICP dry etching.

The source–drain current voltage characteristics of the device were measured before and after the surface functionalization,^{[15, 23]} using an Agilent B2900A precision source. The immobilized thiol-modified probe DNA sequence was ${5}^{\prime}$-HS-(CH₂)₆-ATACCAGCTTATTCAATT-${3}^{\prime}$. The tested complementary target DNA was ${5}^{\prime}$-AATTGAATAAGCTGGTAT-${3}^{\prime}$. These biomolecules were synthesized and purified by Sangon Biotech Co., Ltd. (Shanghai, China). When the DNA immobilized surface was dropped of 10 ${\rm{\mu }}$ l 1 ${\rm{\mu }}$M matched single stranded DNA (ssDNA) in the TE buffer solution (10 mmolL ${}^{-1}$ Tris-HCl, 1 mmolL ${}^{-1}$ EDTA, 0.1 molL ${}^{-1}$ NaCl, pH=7.40, purchased from Sigma Aldrich) with a micropipette, the thiol probes could hybridize with the ssDNA according to the strict complementary nature of the base pairs. Figure 2 shows the detection procedure of DNA hybridization on the device sensor region. To provide enough time for the DNA hybridization, we left 1 h before the next measurement for the I–V characteristics of the device.

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Figure 3 shows the ${I}_{\mathrm{DS}}$–${V}_{\mathrm{DS}}$ characteristics of the Au-gated HEMT before gate functionalization, after functionalizing with 10 ${\rm{\mu }}$M thiol-modified probe ssDNA, and after hybridizing with 1 ${\rm{\mu }}$M matched ssDNA solution. Here ${I}_{\mathrm{DS}}$ of the sensor acts as a function of ${V}_{\mathrm{DS}}$ from 0 V to 0.5 V. The change in the I–V characteristics of the device confirms that the thiol-modified probe DNA was immobilized to the Au gate area successfully, and the hybridization between the immobilized DNA and target DNA had indeed occurred. In comparison with the AlGaN/GaN sensor, a much greater shift of ${I}_{\mathrm{DS}}$–${V}_{\mathrm{DS}}$ is observed in the AlInN/GaN sensor, indicating that the thinner barrier can make GaN-based sensors more sensitive to the change of the surface charge.

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Figure 4 illustrates a real time detection of DNA hybridization with the two HEMT sensors at ${V}_{\mathrm{DS}}=0.5$ V. The concentration and volume of thiol probe DNA solution (10 ${\rm{\mu }}$M, 10 ${\rm{\mu }}$ l) and target DNA solution (1 ${\rm{\mu }}$M, 10 ${\rm{\mu }}$ l) added to both the sensors are the same. From figs. 3 and 4 we can clearly observe that the current change of the AlInN/GaN sensor is greater than that of the AlGaN/GaN sensor. Its drain–source current shows a decrease of 69 ${\rm{\mu }}$A after DNA hybridization while that for the AlGaN/GaN sensor is 38 ${\rm{\mu }}$A. Several groups of the two sensors had been tested in our study with similar results. This is strong evidence that the AlInN/GaN HEMT is more sensitive for DNA hybridization compared with the AlGaN/GaN HEMT. Thus the AlInN/GaN HEMT sensors are better potential candidates for DNA hybridization detection.

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The reasons for the source–drain current change of the nitride HEMT sensors can be explained in Fig. 5. Figure 5(a) represents the initial state of the device prior to immobilizing the thiol-modified probe ssDNA. When the thiol-modified DNA in TE buffer solution was dropped on the Au gate sensing area, the Au–S bonds would form quickly because of a strong interaction between Au and the thiol group. Because sulfur atoms more easily obtain electrons than Au atoms, they pull the electrons of Au atoms to the surface of the Au film,^[24] as shown in Fig. 5(b). This effect is equivalent to the introduction of additional positive charges on the Au gate surface, thus increasing the concentration of 2DEG at the interface of Al(In)GaN/GaN heterojunction, which is also equivalent to applying a positive potential to the gate, as illustrated in figs. 5(c) and 5(d), respectively. Due to the high mobility of the GaN-based HEMT, the sensor is very sensitive to the potential changes at the gate. An increase of the drain current was observed in the experiment. In contrast, the DNA molecules are negatively charged in the aqueous solution, when the matched target ssDNA was added on the gate area, a negative potential was introduced to the gate where hybridization happened, and leading to a decrease of the drain–source current.

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In summary, both Au-gated AlInN/GaN HEMT and AlGaN/GaN HEMT biosensors have been fabricated for the detection of DNA hybridization. Moreover, an explanation for the source–drain current change of the GaN-based HEMT sensor is proposed. According to the experimental results, the AlInN/GaN HEMT sensors with thin barriers exhibit higher sensitivity than the traditional AlGaN/GaN HEMT sensors, indicating that AlInN/GaN HEMT biosensor with a thinner barrier has a greater potential in the detection of DNA hybridization in the medical field.

We are grateful to Professor Weikun Ge for his critical reading of the manuscript.