xtended gate-AlGaN / GaN high electron mobility transistor for bioassay applications

i-Lab, Suzhou Institute of Nano-Tech and Na Suzhou 215125, People’s Republic of dmwu2008@sinano.ac.cn Department of Applied Physics, College 313000, People’s Republic of China. E-mail: State Key Laboratory of Applied Optics, Cha and Physics, Chinese Academy of Sciences, China University of Chinese Academy of Sciences, Electronic Technology, Beijing 100049, Peop Division of Nano-devices and Materials, S Bionics, Chinese Academy of Sciences (CAS China NANO-X, Suzhou Institute of Nano-Tech Sciences, Suzhou 215125, People’s Republic School of Nano Technology and Nano Bionic China, Hefei, 230026, People’s Republic of C Cite this: RSC Adv., 2017, 7, 55835


Introduction
Over the past few years, the eld of biosensors based on ionsensitive eld effect transistors (ISFETs) has developed rapidly, which may assist humans in making point-of-care testing (PoCT) a reality. 1,22][3] On the other hand, the development of new materials broadens the diversity of the ISFET eld.Recent studies of AlGaN/GaN HEMTs have explored their possible application in bioassays, 4 on account of their high electron sheet carrier concentration, biocompatibility, lack of ion diffusion, and so forth. 5Plenty of work has been done to detect various biological analytes, such as cells, 6 deoxyribonucleic acid, 7 antigens, 8,9 virus inhibitors, 10 mercury ions (Hg 2+ ), 11 2,4,6-trinitrotoluene (TNT), 12 etc.
4][15] To overcome these issues, the extended gate eld effect transistor (EG FET) was proposed as a solution in 1983. 16This structure enables people to pursue a better electrical performance of the ampli-cation component, independently of the tailored sensing component, and vice versa.Additionally, a much simpler encapsulation solution can be used to make the amplication component free from most of the undesirable interferences like light and temperature.Since then, this novel structure has proved quite useful for different elds like genetic analysis, 17 immunoassay, 18 urinalysis, 19 and so on.However, to the best of our knowledge, this structure has not been demonstrated with AlGaN/GaN HEMTs, and so whether the EG-AlGaN/GaN HEMT will achieve a comparable level of electrical performance compared with the Au gate AlGaN/GaN HEMT 8 or the biomolecule gate AlGaN/GaN HEMT 9 needs to be studied.
In this work, we have investigated the feasibility of developing the EG-AlGaN/GaN HEMT for the detection of prostate specic antigen (PSA), which is considered to be the best biomarker for prostate cancer. 20,21A schematic illustration and plan view photomicrograph are shown in Fig. 1.The core of the EG-AlGaN/GaN HEMT is separated into two parts: the extended sensing region and the AlGaN/GaN HEMT.The area of the sensing region is as large as 700 mm Â 700 mm, which will be shown to be quite useful for the signal amplication later.

Sensor fabrication
To begin with, the nitride heterostructure used in this study consists of a 1.5 mm thick GaN buffer layer, an 18 nm thick AlGaN barrier layer and a 1.5 nm thick GaN cap layer, which were grown by metal organic chemical vapor deposition (MOCVD) on the sapphire substrate.The wafer was then precleaned using piranha solution (H 2 SO 4 and H 2 O 2 with a volumetric ratio of 3 : 1) and dried with nitrogen to ensure that it is clean.Inductively coupled plasma (ICP) etching with Cl 2 /BCl 3 was performed to form the mesa isolation.The mesa height is around 40 nm to make sure that the etching surpasses the 2DEG layer.In order to remove the native oxide layer, the wafer was then dipped into 10% (by volume) hydrochloric acid for 60 seconds.Ohmic contacts were formed with the typical multilayer of Ti/Al/Ni/Au by e-beam evaporation and annealed by a rapid thermal processing system under a ow of ambient nitrogen at 880 C for 45 seconds.To achieve an easier connection to the printed circuit board, the Ti/Ni/Au combination multilayer was evaporated overlapping above the ohmic contact electrode.A sputtering process was implemented to fabricate the gold extended gate.The wafer was encapsulated with SU8 except for the sensing region and connecting regions.
Aer dicing, the separate devices were mounted and electrically connected to the printed circuit boards.

Sensor modication
Aer the devices were fabricated and encapsulated, surface modication was implemented.The sensing region, namely the gold extended gate, was cleaned in an ozone/UV chamber for 30 minutes in the rst instance.Shortly aer cleaning, the gold extended gate was immersed in deoxygenated cysteamine aqueous solution for 6 hours at room temperature, to form ubiquitous thiol-gold bonds, and then rinsed with deionized   water.Glutaraldehyde solution with a concentration of 1.25% (by volume) was added into the lab-made reservoir for 2 hours to form a Schiff base between the aldehyde group and the amino group.Subsequently the device was rinsed with deionized water before the immobilization of 10 mg mL À1 PSA monoclonal antibody.The device was nally incubated at 4 C for 24 hours, ready for the following measurements.The cysteamine and glutaraldehyde were purchased from Sigma-Aldrich Co. LLC.(Shanghai, China).The gold nanoparticle solution was purchased from Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China).All of the measurements were performed in phosphate-buffered saline solution (PBS).

Characterization
It has been well elucidated and demonstrated that there is a stable bond between amines and the surface of gold nanoparticles (Au NPs).This may stem from both the electrostatic adsorption between protonated amine groups and AuCl 4À / AuCl 2À , and a related complex of the form [AuCl(NH 2 R)]. 22ence, Au NPs can be utilized to demonstrate the presence of cysteamine.Fig. 2 shows images resulting from scanning electron microscopy (SEM).The Au NPs were uniformly distributed upon the surface of the gold platform aer being rinsed over ve times and sonicated, as shown in Fig. 2A, whilst adsorbed Au NPs are hardly observed with the absence of cysteamine, as shown in Fig. 2B.The corresponding diameter of the gold nanoparticles is around 20 nm.

PSA detection
The device was nally connected to a Keithley 2636A instrument with a constant bias of 100 mV for the data acquisition.As shown in Fig. 3, the current response was quite steady in the rst hundred seconds with the addition of 18 mL PBS, which indicates the stability of the device.Then, the current showed an instant abrupt decrease and then recovery to a steady level.This was due to the mechanical disturbance right aer each step of titration by hand.For each cycle, within 60 seconds, the current achieved another steady level aer completion of the antibody-antigen reaction.The obvious drop between every two titrations is around 100 nA.The red dashed lines in Fig. 3 depict the calculated average current during the steady stage, and the bold black line gives the tted result of the real-time detection.At the end of the detection, a drop of PBS was titrated to verify the specicity of the device.The current response quickly returned to its previous level (i.e. at a PSA concentration of 100 ng mL À1 ).The device also shows good linearity with an R 2 of 0.9934 in the inset, and the corresponding relationship is: where DI and DI max represent the current change due to a certain concentration of PSA ([PSA]), and the maximum current change during the whole test, respectively.Table 1 summarizes the PSA biosensors based on AlGaN/GaN HEMTs.
Compared to those of previous reports and our former work, the EG-AlGaN/GaN HEMT showed a relatively wider linear range and retained a low limit of detection.

Discussion of the sensing area
It was reported that the presence of a gold layer may degrade the performances of AlGaN/GaN HEMT transducers, 23 however based on the above results, it appears to be less inuential than we expected for the EG-AlGaN/GaN HEMT.The main factor of the EG-AlGaN/GaN HEMT which may enhance the device performance in this situation, we suppose, is the sensing area.This is because it was reported that more surface receptors lead to a better electrical parameter of sensitivity, 24,25 and a larger sensing area has more receptors.In order to elucidate the relationship between the sensing area of the EG-AlGaN/GaN HEMT and its electrical characteristics, three different areas of the sensing pads were designed and modied (400 mm Â 400 mm, 500 mm Â 500 mm and 700 mm Â 700 mm).The rest of the parameters of the EG-AlGaN/GaN HEMT are identical.With the exact same conditions of modication, target PSA with different concentrations in buffer solutions, including 0.1 pg mL À1 , 1 pg mL À1 , 10 pg mL À1 , 100 pg mL À1 , 1 ng mL À1 , 10 ng mL À1 , and  100 ng mL À1 , were detected.The current changes (DI) of those three different sensing areas of the EG-AlGaN/GaN HEMT transducers for PSA detection were obtained.In order to minimize the inuences caused by the modication, each group of experiments was repeated 3 times.As shown in Fig. 4, the statistical average results give a quite explicit conclusion, that a larger area of the extended gate leads to a larger current change in our transducer.Based on the above observation, we hypothesize that the larger area of the extended gate gives a larger total amount of surface charge which causes a larger change of the gate voltage due to the capacitance coupling.This explains why our EG-AlGaN/GaN HEMT exhibits good performance with a gold layer.

Conclusions
In summary, we have developed a highly sensitive EG-AlGaN/ GaN HEMT with a low limit of detection (0.1 pg mL À1 ).Furthermore, our transducer exhibits an outstanding electrical performance, including excellent linearity (R 2 ¼ 0.9934) with a wide range from 0.1 pg mL À1 to 100 ng mL À1 .The device also shows stability and specicity in the presence of PBS.We also prove that a larger sensing area of the extended pad leads to a better electrical performance.It should also be noted that the extended gate can be tailored to various platforms for the corresponding detection of analytes, as long as the gate electrode is conductive.It is reasonable to believe that the EG-AlGaN/GaN HEMT will be a promising diagnostic tool for further PoCT applications.

Fig. 1
Fig.1The structure of the EG-AlGaN/GaN HEMT: (A) a schematic illustration of the device, and (B) a plan view photomicrograph.

Fig. 2
Fig. 2 Characterization of the device: (A) the adsorption of gold nanoparticles in the presence of successfully modified cysteamine, and (B) non-adsorption in the absence of cysteamine.

Fig. 3
Fig.3The real-time detection of PSA (the red dashed lines are the average values and the bold black line is a fitted curve).

Fig. 4
Fig. 4 The results of the current changes of three transducers with different sensing areas of the extended pads.The transducers were modified and measured with exactly the same conditions, and the experiments were repeated 3 times.(A) 400 mm Â 400 mm, (B) 500 mm Â 500 mm, and (C) 700 mm Â 700 mm.

Table 1
Comparison of the electrical parameters of PSA biosensors based on AlGaN/GaN HEMTs