Progress in Materials Science 55 (2010) 1–59 Contents lists available at ScienceDirect Progress in Materials Science journal homepage: www.elsevier.com/locate/pmatsci Recent advances in wide bandgap semiconductor biological and gas sensors S.J Pearton a,*, F Ren b, Yu-Lin Wang b, B.H Chu b, K.H Chen b, C.Y Chang b, Wantae Lim a, Jenshan Lin c, D.P Norton a a Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA c Department of Electrical and Computer Engineering, University of Florida, Gainesville, FL 32611, USA b a r t i c l e i n f o Article history: Received 10 June 2009 Received in revised form 27 August 2009 Accepted 28 August 2009 a b s t r a c t There has been significant recent interest in the use of surfacefunctionalized thin film and nanowire wide bandgap semiconductors, principally GaN, InN, ZnO and SiC, for sensing of gases, heavy metals, UV photons and biological molecules For the detection of gases such as hydrogen, the semiconductors are typically coated with a catalyst metal such as Pd or Pt to increase the detection sensitivity at room temperature Functionalizing the surface with oxides, polymers and nitrides is also useful in enhancing the detection sensitivity for gases and ionic solutions The wide energy bandgap of these materials make them ideal for solar-blind UV detection, which can be of use for detecting fluorescence from biotoxins The use of enzymes or adsorbed antibody layers on the semiconductor surface leads to highly specific detection of a broad range of antigens of interest in the medical and homeland security fields We give examples of recent work showing sensitive detection of glucose, lactic acid, prostate cancer and breast cancer markers and the integration of the sensors with wireless data transmission systems to achieve robust, portable sensors Ó 2009 Elsevier Ltd All rights reserved Contents Introduction Gas sensing * Corresponding author E-mail address: spear@mse.ufl.edu (S.J Pearton) 0079-6425/$ - see front matter Ó 2009 Elsevier Ltd All rights reserved doi:10.1016/j.pmatsci.2009.08.003 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 2.1 H2 sensing 2.2 O2 sensing 2.3 CO2 sensing 2.4 CH4 sensing UV photodetectors 3.1 UV photoresponse of single ZnO nanowires Sensor functionalization pH measurement Exhaled breath condensate Heavy metal detection Biotoxin sensors 8.1 Botulinum Biomedical applications 9.1 Prostate cancer detection 9.2 Kidney injury molecule detection 9.2.1 Breast cancer 9.2.2 Lactic acid 9.2.3 Chloride ion detection 9.2.4 Pressure sensing 9.3 Traumatic brain injury 10 Wireless sensors 11 Summary and conclusions Acknowledgments References 12 13 15 17 18 20 23 27 27 32 33 34 40 41 43 45 47 49 50 51 54 55 55 Introduction Chemical sensors have gained in importance in the past decade for applications that include homeland security, medical and environmental monitoring and also food safety A desirable goal is the ability to simultaneously analyze a wide variety of environmental and biological gases and liquids in the field and to be able to selectively detect a target analyte with high specificity and sensitivity In the area of detection of medical biomarkers, many different methods, including enzyme-linked immunsorbent assay (ELISA), particle-based flow cytometric assays, electrochemical measurements based on impedance and capacitance, electrical measurement of micro-cantilever resonant frequency change, and conductance measurement of semiconductor nanostructures gas chromatography (GC), ion chromatography, high density peptide arrays, laser scanning quantitative analysis, chemiluminescence, selected ion flow tube (SIFT), nanomechanical cantilevers, bead-based suspension microarrays, magnetic biosensors and mass spectrometry (MS) have been employed [1–9] Depending on the sample condition, these methods may show variable results in terms of sensitivity for some applications and may not meet the requirements for a hand-held biosensor For homeland security applications, reliable detection of biological agents in the field and in real time is challenging During the anthrax attack on the World Bank in 2002, field tests showed 1200 workers to be positive, and all were sent home One hundred workers were provided antibiotics However, confirmatory testing showed zero positives False positives and false negatives can result due to very low volumes of samples available for testing and poor device sensitivities Toxins such as ricin, botulinum toxin or enterotoxin B are environmentally stable, can be mass-produced and not need advanced technologies for production and dispersal The threat of these toxins is real This is evident from the recent ricin detection from White House mail facilities and a US senator’s office Terrorists have already attempted to use botulinum toxin as a bio-weapon Aerosols were dispersed at multiple sites in Tokyo, and at US military installations in Japan on at least occasions between 1990 and 1995 by the Japanese cult Aum Shinrikyo [10] Four of the countries listed by the US government as ‘‘state sponsors of terrorism” (Iran, Iraq, North Korea, and Syria) [10] have developed, or are believed to be developing, botulinum toxin as a weapon [11,12] After the 1991 Persian Gulf War, Iraq admitted to S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 the United Nations inspection team to having produced 19,000 L of concentrated botulinum toxin, of which approximately 10,000 L were loaded into military weapons This toxin has not been fully accounted for and constitutes approximately three times the amount needed to kill the entire current human population by inhalation [10] A significant issue is the absence of a definite diagnostic method and the difficulty in differential diagnosis from other pathogens that would slow the response in case of a terror attack This is a critical need that has to be met to have an effective response to terrorist attacks Given the adverse consequences of a lack of reliable biological agent sensing on national security, there is a critical need to develop novel, more sensitive and reliable technologies for biological detection in the field [13,14] Some specific toxins of interest include Enterotoxin type B (Category B, NIAID), Botulinum toxin (Category A NIAID) and ricin (Category B NIAID) While the techniques mentioned above show excellent performance under lab conditions, there is also a need for small, hand-held sensors with wireless connectivity that have the capability for fast responses The chemical sensor market represents the largest segment for sales of sensors, including chemical detection in gases and liquids, flue gas and fire detection, liquid quality sensors, biosensors and medical sensors Some of the major applications in the home include indoor air quality and natural gas detection Attention is now being paid to more demanding applications where a high degree of chemical specificity and selectivity is required For biological and medical sensing applications, disease diagnosis by detecting specific biomarkers (functional or structural abnormal enzymes, low molecular weight proteins, or antigen) in blood, urine, saliva, or tissue samples has been established Most of the techniques mentioned earlier such as ELISA possesses a major limitation in that only one analyte is measured at a time Particle-based assays allow for multiple detection by using multiple beads but the whole detection process is generally longer than h, which is not practical for in-office or bedside detection Electrochemical devices have attracted attention due to their low cost and simplicity, but significant improvements in their sensitivities are still needed for use with clinical samples Micro-cantilevers are capable of detecting concentrations as low as 10 pg/ml, but suffer from an undesirable resonant frequency change due to the viscosity of the medium and cantilever damping in the solution environment Nano-material devices have provided an excellent option toward fast, label-free, sensitive, selective, and multiple detections for both preclinical and clinical applications Examples of detection of biomarkers using electrical measurements with semiconductor devices include carbon nanotubes for lupus erythematosus antigen detection [4], compound semiconducting nanowires and In2O3 nanowires for prostate-specific antigen detection [5], and silicon nanowire arrays for detecting prostate-specific antigen [9] In clinical settings, biomarkers for a particular disease state can be used to determine the presence of disease as well as its progress Semiconductor-based sensors can be fabricated using mature techniques from the Si chip industry and/or novel nanotechnology approaches Silicon based sensors are still the dominant component of the semiconductor segment due to their low cost, reproducible and controllable electronic response However, these sensors are not suited for operation in harsh environments, for instance, high temperature, high pressure or corrosive ambients Si will be etched by some of the acidic or basic aqueous solutions encountered in biological sensing By sharp contrast, GaN is not etched by any acid or base at temperatures below a few hundred degrees Therefore, wide band-gap group III nitride compound semiconductors (AlGaInN materials system) are alternative options to supplement silicon in these applications because of their chemical resistance, high temperature/high power capability, high electron saturation velocity and simple integration with existing GaN-based UV light-emitting diode, UV detectors and wireless communication chips A promising sensing technology utilizes AlGaN/GaN high electron mobility transistors (HEMTs) HEMT structures have been developed for use in microwave power amplifiers due to their high two-dimensional electron gas (2DEG) mobility and saturation velocity The conducting 2DEG channel of AlGaN/GaN HEMTs is very close to the surface and extremely sensitive to adsorption of analytes HEMT sensors can be used for detecting gases, ions, pH values, proteins, and DNA The GaN materials system is attracting much interest for commercial applications of green, blue, and UV light-emitting diodes (LEDs), laser diodes as well as high speed and high frequency power devices Due to the wide-bandgap nature of the material, it is very thermally stable, and electronic devices can be operated at temperatures up to 500 °C The GaN-based materials are also chemically stable, and no known wet chemical etchant can etch these materials; this makes them very suitable for operation in S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 chemically harsh environments Due to the high electron mobility, GaN material based high electron mobility transistors (HEMTs) can operate at very high frequency with higher breakdown voltage, better thermal conductivity, and wider transmission bandwidths than Si or GaAs devices [15–17] An overlooked potential application of the GaN HEMT structure is sensors The high electron sheet carrier concentration of nitride HEMTs is induced by piezoelectric polarization of the strained AlGaN layer in the hetero-junction structure of the AlGaN/GaN HEMT and the spontaneous polarization is very large in wurtzite III-nitrides This provides an increased sensitivity relative to simple Schottky diodes fabricated on GaN layers or field effect transistors (FETs) fabricated on the AlGaN/GaN HEMT structure The gate region of the HEMT can be used to modulate the drain current in the FET mode or use as the electrode for the Schottky diode A variety of gas, chemical and health-related sensors based on HEMT technology have been demonstrated with proper surface functionalization on the gate area of the HEMTs, including the detection of hydrogen, mercury ion, prostate-specific antigen (PSA), DNA, and glucose [18–58] In this review, we discuss recent progress in the functionalization of these semiconductor sensors for applications in detection of gases, pH measurement, biotoxins and other biologically important chemicals and the integration of these sensors into wireless packages for remote sensing capability Gas sensing 2.1 H2 sensing There is great interest in detection of hydrogen sensors for use in hydrogen-fueled automobiles and with proton-exchange membrane (PEM) and solid oxide fuel cells for space craft and other long-term sensing applications These sensors are required to detect hydrogen near room temperature with minimal power consumption and weight and with a low rate of false alarms Due to their low intrinsic carrier concentrations, GaN- and SiC-based wide band gap semiconductor sensors can be operated at lower current levels than conventional Si-based devices and offer the capability of detection to $600 °C [18–36] Unlike conventional sensors where the changes of the sensing material’s conductivity or resistivity are used to detect the gas concentration, by integrating the gas sensing material such as Pd or Pt metal on the gate electrode of the HEMTs, the change of the sensing material’s conductivity can be amplified through Schottky diode or FET operation It is generally accepted that H2 is dissociated when adsorbed on Pt and Pd at room temperature The reaction is as follows: H2adsị ! 2Hỵ ỵ e Dissociated hydrogen causes a change in the channel and conductance change This makes the integrated semiconductor device based sensors extremely sensitive and the sensors have a broad dynamic range of the sensing concentration A surface covered reference semiconductor device can also be easily fabricated side-by-side with sensing device to eliminate the temperature variation of the ambient and the fluctuation of the supplied voltage to the sensors [20] Fig shows a schematic of a Schottky diode hydrogen sensor on AlGaN/GaN HEMT layer structure and a photograph of packaged sensors The layer structure included an initial lm thick undoped GaN buffer followed by a 35 nm thick unintentionally doped Al0.28Ga0.72N layer Mesa isolation was achieved by using an inductively coupled plasma system with Ar/Cl2 based discharges The Ohmic contacts were formed by lift-off of sputtered Ti/Al/TiB2/Ti/Au, followed by annealing at 850 °C for 45 s under a flowing N2 ambient [21] A thin (100 Å) Pt Schottky contact was deposited by e-beam evaporation for the Schottky metal The final step was deposition of e-beam evaporated Ti/Au interconnection contacts The individual devices were diced and wire-bonded to carriers The sensor carrier was then placed in our test chamber Mass flow controllers were used to control the gas flow through the chamber, and the devices were exposed to either 100% pure N2, or 1% H2 in nitrogen Fig shows the linear (top) and log scale (bottom) forward current–voltage (I–V) characteristics at 25 °C of the HEMT diode, both in air and in a 1%H2 in air atmosphere For these diodes, the current increases upon introduction of the H2, through a lowering of the effective barrier height The data was fit to the relations for thermionic emission and showed decreases in Schottky S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 Fig Cross-sectional schematic of completed Schottky diode on AlGaN/GaN HEMT layer structure (top) and plan-view photograph of device (bottom) Fig Time response of differential GaN sensor to introduction of 1% H2 in air ambient barrier height UB of 30–50 meV at 50 °C and larger changes at higher temperatures The decrease in barrier height is completely reversible upon removing the H2 from the ambient and results from diffusion of atomic hydrogen to the metal/GaN interface, altering the interfacial charge The H2 catalytically decomposes on the Pt metallization and diffuses rapidly to the interface where it forms a dipole layer The changes in forward diode current upon introduction of the hydrogen into the ambient is $1 mA over the diode bias voltage above V, as shown in Fig (left) As illustrated in the Fig (right), the change of the diode current was significantly lower when the bias voltage was below V This low current operation offers the ability of lower power hydrogen detection Fig shows time response of diode forward current at a fixed bias of 0.6 V when switching back and forth between the ambient from N2 to 1%H2 balanced with nitrogen The change in forward current for the diode was in the micro-amp range with a bias voltage of 0.6 V, which was corresponding to a power consumption of 3.6 lW The response time was less than s It took sometime to purge out the hydrogen from the gas chamber, therefore the recovery time was longer than the response time Using the same layer structure, the sensor can also be fabricated as a field effect transistor Excellent response time and repeatability were also achieved, as illustrated in Fig (bottom) To achieve the goal of detecting reactions due to hydrogen only and excluding other changes caused by variables such as temperature and moisture, a differential detection interface was used Several kinds of differential devices have been fabricated and each of its performance has been evaluated to select the most effective solution These differential devices have two sensors integrated on the same chip The two sensors are identical except one is designed to react to hydrogen whereas the 7.2 7.1 7.0 6.9 6.8 6.7 50 100 150 200 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 other one is covered by dielectric protection layer and not exposed to ambient gas Fig 1(bottom) shows the die photo of a differential sensor device with a reference diode One sensor reacted promptly with the exposure of hydrogen while the other, the reference diode, had no significant response as expected, proving the functionality of the differential sensor W/Pt contacted GaN Schottky diodes also show forward current changes of >1 mA at low bias (3 V) in the temperature range 350–600 °C when the measurement ambient is changed from pure N2 to 10%H2/90%N2 We have found that use of a metal–oxide-semiconductor(MOS) diode structure with Sc2O3 gate dielectric and the same W/Pt metallization show these same reversible changes in forward current upon exposure to H2-containing ambients over a much broader temperature range (90 to >625 °C) The increase in current in both cases is the result of a decrease in effective barrier height of the MOS and Schottky gates of 30–50 mV 10%H2/90%N2 ambients relative to pure N2 and is due to catalytic dissociation of the H2 on the Pt contact, followed by diffusion to the W/GaN or Sc2O3/ GaN interface The presence of the oxide lowers the temperature at which the hydrogen can be detected and in conjunction with the use of the high temperature stable W metallization enhances 12 10 Current(mA) Nitrogen 10% Hydrogen o T = 500 C 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.0 1.2 1.4 Bias Voltage(V) 12 10 Current(mA) Nitrogen 10% Hydrogen o T = 500 C 0.0 0.2 0.4 0.6 0.8 Bias Voltage(V) Fig Change in current in W/Pt GaN and AlGaN/GaN diodes at 500 °C when the ambient is switched from N2 to 10% H2 in N2 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 the potential applications of these wide bandgap sensors Fig shows that the relative change in current is larger with the MOS structure SiC Schottky diodes with Pd or Pt contacts are also sensitive to the presence of hydrogen in the ambient, as shown in Fig The advantage of the nitride system relative to SiC is the availability of a heterostructure and the strong piezoelectric and polarization fields present in the nitrides that enhance their capability for chemical sensing We have also found that nanostructured wide bandgap materials functionalized with Pd or Pt are even sensitive than their thin film counterparts because of the large surface-to-volume ratio [30,31] 1-D semiconductor nanomaterials, such as carbon nanotubes (CNTs), Si nanowires, GaN nanowires, and ZnO nanowires are good candidates to replace 2-D semiconductors due to several advantages First, 1-D structure has large surface-to-volume ratio which means that a significant fraction of the atoms can participate in surface reactions Second, the Debye length (kD) for 1-D nanomaterial is comparable to their radius over a wide temperature and doping range, which causes them more sensitive than 2-D thin film Third, 1-D nanostructure is usually stoichiometrically controlled better than 2-D thin film, and has a greater level of crystallinity than the 2-D thin-film With 1-D structures, common defect problems in Fig Pt/SiC Schottky diode (top) and change in current at fixed bias of 1.5 V when the ambient is switched from air to 10% H2 in air S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 2-D semiconductors could be easily solved Fourth, further decreasing the diameter, onset of quantum effects is expected to be appeared Finally, low cost and low power consumption, and high compatibility with microelectronic processing make 1-D nanostructure potential and practical materials of sensors Impressive results have been demonstrated with GaN, InN and ZnO nanowires or nanobelts that are sensitive to hydrogen down to approximately 20 ppm at room temperature As an example, Fig (top) shows scanning electron microscopy (SEM) micrographs of as-grown nanowires A layer of 10 nmthick Pd was deposited by sputtering onto the nanowires to verify the effect of catalyst on gas sensitivity The bottom of Fig shows the measured resistance at a bias of 0.5 V as a function of time from Pd-coated and uncoated multiple GaN nanowires exposed to a series of H2 concentrations (200– 1500 ppm) in N2 for 10 at room temperature Pd-coating of the nanowires improved the sensitivity to ppm level H2 by a factor of up to 11 The addition of Pd appears to be effective in catalytic dissociation of molecular hydrogen Diffusion of atomic hydrogen to the metal/GaN interface alters the surface depletion of the wires and hence the resistance at fixed bias voltage [59] The resistance change depended on the gas concentration but the variations were small at H2 concentration above Fig SEM images of as-grown GaN nanowires (top) and measured resistance at an applied bias of 0.5 V as a function of time from Pd-coated and uncoated multiple GaN nanowires exposed to a series of H2 concentrations (200–1500 ppm) in N2 for 10 at room temperature 10 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 1000 ppm The resistance after exposing the nanowires to air was restored to approximately 90% of initial level within [30,31] Similar results can be obtained with InN nanostructures The hydrogen sensing characteristics of multiple InN nanobelts grown by Metalorganic Chemical Vapor Deposition have been reported previously [29,60] Pt-coated InN sensors could selectively detect hydrogen at the tens of ppm level at 25 °C while uncoated InN showed no detectable change in current when exposed to hydrogen under the same conditions Upon exposure to various concentrations of hydrogen (20–300 ppm) in N2 ambient, the relative resistance change increased from 1.2% at 20 ppm H2 to 4% at 300 ppm H2, as shown in Fig (Top) X-ray diffraction spectrum of MOCVD-grown InN nanobelts (the inset shows SEM images of the nanobelts) and change in current at fixed bias for switching from 20 to 300 ppm H2 in air to pure air (bottom) S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 45 Fig 35 Drain current of an AlGaN/GaN HEMT over time for c-erbB-2 antigen from 0.25 lg/ml to 17 lg/ml (top) and change of drain current versus different concentrations from 0.25 lg/ml to 17 lg/ml of c-erbB-2 antigen Clinically relevant concentrations of the c-erbB-2 antigen in the saliva and serum of normal patients are 4–6 lg/ml and 60–90 lg/ml, respectively For breast cancer patients, the c-erbB-2 antigen concentrations in the saliva and serum are 9–13 lg/ml and 140–210 lg/ml, respectively Our detection limit suggests that HEMTs can be easily used for detection of clinically relevant concentrations of biomarkers Similar methods can be used for detecting other important disease biomarkers and a compact disease diagnosis array can be realized for multiplex disease analysis 9.2.2 Lactic acid Lactic acid can also be detected with ZnO nanorod-gated AlGaN/GaN HEMTs Interest in developing improved methods for detecting lactate acid has been increasing due to its importance in areas such as clinical diagnostics, sports medicine, and food analysis An accurate measurement of the concentration of lactate acid in blood is critical to patients that are in intensive care or undergoing surgical operations as abnormal concentrations may lead to shock, metabolic disorder, respiratory insufficiency, and heart failure Lactate acid concentration can also be used to monitor the physical condition of athletes or of patients with chronic diseases such as diabetes and chronic renal failure In the food industry, lactate level can serve as an indicator of the freshness, stability and storage quality For the reasons 46 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 above, it is desirable to develop a sensor capable of simple and direct measurements, rapid response, high specificity, and low cost Recent researches on lactate acid detection mainly focus on amperometric sensors with lactate acid specific enzymes attached to an electrode with a mediator [171,212–219] Examples of materials used as mediators include carbon paste, conducting copolymer, nanostructured Si3N4 and silica materials Other methods of detecting lactate acid include utilizing semiconductors [220] and electro-chemiluminescent materials [221] A ZnO nanorod array, which was used to immobilize lactate oxidase oxidase (LOx), was selectively grown on the gate area using low temperature hydrothermal decomposition (Fig 36, top) The array of one-dimensional ZnO nanorods provided a large effective surface area with high surface-to-volume ratio and a favorable environment for the immobilization of LOx The AlGaN/GaN HEMT drain-source current showed a rapid response when various concentrations of lactate acid solutions were introduced to the gate area of the HEMT sensor The HEMT could detect lactate acid concentrations from 167 nM to 139 lM Fig 36 (bottom) shows a real time detection of lactate acid by measuring the HEMT drain current at a constant drain-source bias voltage of 500 mV during exposure of HEMT Fig 36 Schematic cross sectional view of the ZnO nanorod gated HEMT for lactic acid detection (top) and plot of drain current versus time with successive exposure to lactate acid from 167 nM to 139 lM (bottom) S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 47 sensor to solutions with different concentrations of lactate acid The sensor was first exposed to 20 ll of 10 mM PBS and no current change could be detected with the addition of 10 ll of PBS at approximately 40 s, showing the specificity and stability of the device By contrast, a rapid increase in the drain current was observed when target lactate acid was introduced to the device surface The sensor was continuously exposed to lactate acid concentrations from 167 nM to 139 lM As compared with the amperometric measurement based lactate acid sensors, our HEMT sensors not require a fixed reference electrode in the solution to measure the potential applied between the nano-materials and the reference electrode The lactate acid sensing with the HEMT sensor was measured through the drain current of HEMT with a change of the charges on the ZnO nanorods and the detection signal was amplified through the HEMT Although the time response of the HEMT sensors is similar to that of electrochemical based sensors, a significant change of drain current was observed for exposing the HEMT to the lactate acid at a low concentration of 167 nM due to this amplification effect In addition, the amount of sample, which is dependent on the area of gate dimension, can be minimized for the HEMT sensor due to fact no reference electrode is required Thus, measuring lactate acid in the exhaled breath condensate (EBC) can be achieved as a non-invasive method 9.2.3 Chloride ion detection Chlorine is widely used in the manufacture of many products and items directly or indirectly, i.e in paper product production, antiseptic, dye-stuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products It is used to kill bacteria and other microbes in drinking water supplies and waste water treatment Excess chlorine also reacts with organics and forms disinfection by-products such as carcinogenic chloroform, which is harmful to human health Thus, to ensure the safety of public health, it is very important to accurately and effectively monitor chlorine residues, typically in the form of chloride ion concentration, during the treatment and transport of drinking water [222–230] In addition, the chloride ion is an essential mineral for humans, and is maintained to a total body chloride balance in body fluids such as serum, blood, urine, exhaled breath condensate, etc., by the kidneys Variations in the chloride ion concentration in serum may serve as an index of renal diseases, adrenalism, pneumonia and, thus, the measurement of this parameter is clinically important [231–235] HEMTs with a Ag/AgCl gate are found to exhibit significant changes in channel conductance upon exposing the gate region to various concentrations of chorine ion solutions The Ag/AgCl gate electrode, prepared by potentiostatic anodization, changes electrical potential when it encounters chorine ions This gate potential changes lead to a change of surface charge in the gate region of the HEMT, inducing a higher positive charge on the AlGaN surface, and increasing the pizeo-induced charge density in the HEMT channel These anions create an image positive charge on the Ag gate metal for the required neutrality, thus increasing the drain current of the HEMT The HEMT source-drain current showed a clear dependence on the chorine concentration [231] Fig 37 shows the time dependence of Ag/AgCl HEMT drain current at a constant drain bias voltage of 500 mV during exposure to solutions with different chlorine ion concentrations The HEMT sensor was first exposed to DI water and no change of the drain current was detected with the addition of DI water at 100 s This stability was important to exclude possible noise from the mechanical change of the NaCl solution By sharp contrast, there was a rapid response of HEMT drain current observed in less than 30 s when target of  10À8 M NaCl solution was switched to the surface at 175 s The abrupt current change due to the exposure of chlorine in NaCl solution stabilized after the chlorine thoroughly diffused into the water to reach a steady state When Ag/AgCl gate metal encountered chorine ions, the electrical potential of the gate was changed, inducing a higher positive charge on the AlGaN surface, and increased the pizeo-induced charge density in the HEMT channel  10À7 M of NaCl solution was then applied at 382 s and it was accompanied with a larger signal corresponding to the higher chlorine concentration Further real time tests were carried out to explore the detection of higher ClÀ ion concentrations The sensors was exposed to 10À8 M, 10À7 M, 10À6 M, 10À5 M and 10À4 M solutions continuously and repeated five times to obtain the standard deviation of sourcedrain current response for each concentration The limit of detection of this device was  10À8 M chlorine in DI water Between each test, the device was rinsed with DI water These results suggest 48 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 Fig 37 Schematic cross-sectional view of a Ag/AgCl gated HEMT (top) and time dependent drain current of a Ag/AgCl gated AlGaN/GaN HEMT exposed to different concentrations of NaCl solutions (bottom) that our HEMT sensors are recyclable with simple DI water rinse The presence of the Ag/AgCl gate leads to a logarithmic dependence of current on the concentration of NaCl Real time detection of chloride ion detection with AlGaN/GaN high electron mobility transistors (HEMTs) with an InN thin film in the gate region has also been demonstrated The sensor, shown schematically in Fig 38, exhibited significant changes in channel conductance upon exposure to various concentrations of NaCl solutions The InN thin film, deposited by Molecular Beam Epitaxy, provided fixed surface sites for reversible anion coordination The potential change in the gate area induced a change of the piezo-induced charge density in the electron channel in the HEMT The sensor was tested over the range of 100 nM–100 lM NaCl solutions Fig 38 also shows the results of real time detection of ClÀ ions by measuring the HEMT drain current at a constant drain bias voltage of 500 mV during exposure to solutions of different chloride ion concentrations The HEMT sensor was first exposed to DI water and no change of the drain current was detected with the addition of DI water at 100 s The small spike in the current is due to mechanical disturbance of the HEMT surface when the water was added By sharp contrast, a rapid response of HEMT drain current was observed in less than 20 s when target of 100 nM NaCl solution was exposed to the surface at 200 s The abrupt current change stabilized after the sodium chloride solution thoroughly diffused into water and reached a steady state When the InN gate metal encountered chloride ion, the electrical potential of the gate was changed and resulted in the increase the pizeo-induced charge density in the HEMT channel A S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 49 Fig 38 Cross section schematic of the InN-gated HEMT (top) and real time source-drain current at a constant bias of 500 mV as different concentrations of Cl-ions were added (bottom) larger signal change was observed when lM of NaCl solution was applied at 300 s The sensor was exposed to higher ClÀ ion concentrations of 10 lM and 100 lM sequentially for a further real time test The test was repeated with the same sensor for five times to obtain the standard deviation of source-drain current response for each concentration The sensor can be reusable by washing it with DI water and drying with nitrogen gas The limit of detection of this device was 100 nM chloride ions in DI water The presence of the InN gate leads to a logarithmic dependence of current on the concentration for NaCl 9.2.4 Pressure sensing Piezoelectric materials are used widely as sensitive pressure sensors and piezoelectric gauges are typically fabricated with materials such as PZT, lithium niobate and quartz [236–240] In 1969, Kawai found a very high piezoelectric effect in polarized polyvinylidene fluoride (PVDF) [241] Since then, polarized PVDF has also become an important piezoelectric material due to its flexibility, low density, low mechanical impedance and easy fabrication as a ferroelectric Because of its versatility, PVDF has many applications in low cost and disposable pressure sensors [241] HEMTs with a polarized polyvinylidene difluoride (PVDF) film coated on the gate area exhibited significant changes in channel conductance upon exposure to different ambient pressures The PVDF thin film was deposited on the gate region with an inkjet plotter Next, the PDVF film was polarized with an electrode located mm above the PVDF film at a bias voltage of 10 kV and 70 °C A schematic of the HEMT is shown in 50 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 Fig 39 Schematic of HEMT sensor coated with polarized PVDF (top) and drain current of a PVDF gated AlGaN/GaN HEMT as a function of pressure The PVDF was polarized by grounding the copper chuck holding the sample and the copper wire electrode was applied with 10 kV Fig 39 Variations in ambient pressure induced changes in the charge in the polarized PVDF, leading to a change of surface charges on the gate region of the HEMT Changes in the gate charge were amplified through the modulation of the drain current in the HEMT By reversing the polarity of the polarized PVDF film, the drain current dependence on the pressure could be reversed Our results indicate that HEMTs have potential for use as pressure sensors For the pressure sensing measurement, the HEMTs sensor were mounted on a carrier and put in a pressure chamber N2 gas was used for pressurizing the chamber and a constant drain bias voltage of 500 mV was applied to the drain contact of the sensor Fig 39 also shows the real time pressure detection with the polarized PVDF gated HEMT The drain current of HEMT sensor showed a rapid decrease in less than s when the ambient pressure was changed to 20 psig [242–247] A further decrease of the drain current for the HEMT sensor was observed when the chamber pressure increased to 40 psig These abrupt drain current decreases were due to the change of charges in the PVDF film upon a shift of ambient pressure A HEMT sensor without the PDVF coating was loaded in the pressure chamber and there was no change of drain current observed 9.3 Traumatic brain injury TBI is one of the most frequent causes of morbidity and mortality on the modern battlefield US casualties in Iraq are suffering a greater percentage of brain injuries than in previous wars One of the S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 51 Fig 40 Time dependent current signal when exposing the HEMT to 2–188 lg/ml BA0127 TBI antigen in PBS buffer contributing factors is the proliferation of the use of Improvised Explosive Device (IED) against US warfighters [249,250] Recent assessments have indicated that about 65% of casualties are correlated with brain injuries Traumatic brain injury, including concussion are also growing medical problem among civilians, with almost million cases in the US each year The development of a fast response and portable TBI sensor can have tremendous impact in early diagnosis, and proper management of TBI Accurate and early diagnosis of a soldier’s health in acute care environments can significantly simplify decisions about situation management For example, decisions need to be made about whether to admit or discharge injured soldiers or to transfer other facility with advanced diagonal system, such as computer tomography (CT) and magnetic resonance imaging (MRI) scans The capability to detect, in real time, markers in body fluids of soldiers can result in better patient outcomes especially in the battlefield or remote areas, where complicated and expensive CT and MRI scans are not available For example, traumatic brain injury (TBI) is one of the most frequent causes of morbidity and mortality on the modern battlefield [248,249] US casualties in Iraq are suffering a greater percentage of brain injuries than in previous wars Recent assessments have indicated that about 65% of casualties are correlated with brain injuries, and concussion is a growing medical problem The development of a fast response and portable TBI sensor can have tremendous impact in early diagnosis, and proper management of TBI Preliminary results show the TBI antibody can be functionalized on the HEMT surface and fast response of TBI antigen was achieved The detection of limit of detection (LOD) was in the 10th of lg/ml range, however this is not low enough for practical use The typical TBI antigen concentration in the TBI patient’s serum is in the range of ng/ml We have used HEMT sensors to detect the kidney injury molecules and prostate-specific antigen and achieved the LOD in the range of 1–10 pg/ml range The reason for higher LOD for the TBI antigen detection was due to the much smaller size of the TBI antigen Smaller antigens carry less charges, thus provide less effect on the drain current of the HEMT sensor Based on the promising biomarker and device data, we have recently used HEMTs for detecting a biomarker UCH-L1 (BA0127) antigen involved in Traumatic Injury Molecule The gate region was functionalized with a specific antibody to traumatic brain injury antigen The HEMT current showed a decrease as a function of TBI antigen concentration in PBS buffer (Fig 40) This shows the time dependent current change in BA0127 (UCH-L1) antibody modified HEMTs upon exposure to lg/ ml, and then to 16.9, 80, and 188 lg/ml of BA0127 (UCH-L1) in PBS buffer The response time is around s The preliminary limit of detection (LOD) was found to be 20 lg/ml, demonstrating the potential for TBI detection with accurate, rapid, non-invasive, and high throughput capabilities 10 Wireless sensors With many of the sensor applications, it is desirable to have the detected signal transmitted wirelessly to a central location This could be part of an unmanned system for biotoxin detection or part of 52 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 a personal medical monitoring system in which a patient could breathe into a hand-held device that then transmits the encrypted signal to a doctor’s office This would allow for less numerous visits to doctor’s offices and less problems with false positive tests because data could be accumulated over an extended period and a more reliable baseline established A prototype of a remote hydrogen sensing system was installed in a Ford dealership in Orlando (Greenway Ford), which is a test site for a hydrogen-fueled vehicle program supported by Florida State Government, since August 2006 The hydrogen-fueled buses and cars are stored and have maintenance performed on them in a large work area at the Ford dealership Our hydrogen sensing system includes four on-site sensors, power management subsystem, wireless transmitter and receiver connect to a computer An intelligent monitoring software developed by our team is used to control data logging and tracking of each individual sensor as well as defining and implementing the monitoring states, transitions, and actions of the hydrogen sensor network It can trigger an alarm and/or send messages to computers, cell phones or PDAs, when a preset hydrogen threshold level is detected Also, the software is able to warn the users of potential sensor failure, power outages and network failures through cell phone network and Internet Currently, the cost of electronic parts including wireless transceiver and detection circuits is less than $20 in small quantity In large quantity, it can be lower than $10 If the complete wireless transceiver and detection circuit are designed and integrated on a custom IC for mass production, the cost should be in the range of $5–8, similar to Bluetooth wireless chips The sensors themselves can be mass-produced for 5–10 cents each according to Nitronex, Inc A schematic block diagram of sensor module and wireless network server is shown in Fig 41 As shown in Fig 42, we have also developed a pen-sized portable, re-configurable wireless transceiver integrated with pH sensor has been designed and fabricated The wireless transmitter and receiver pair was designed to acquire EBC data and transmit it wirelessly This system is able to interface multiple different sensors and consists of a transmitter and receiver pair The transmitter was designed such that it is the size of a marker-pen so that it could be used as an ultra-portable lightweight hand-held device The transmitter is designed to be operated on an ultra-low-power mode The transmitter is also equipped with an on-board recharging circuit, which can be powered by using a standard mini-USB cable The transmitter consumes on average 80 lA The transmitter and receiver pair is designed to operate at 2.4 GHz with range of up to 20 ft line-of-sight The receiver has USB 2.0 connectivity, which relays EBC data from the transmitter to a PC while powering the receiver The transmitter is designed to integrate with various different sensors through a connector The transmitter can be reset for the required input signal range to trigger the alarm through the bi-directional wireless communication for a different sensing application Thus this system is re-configurable over-the-air The wireless circuits only consume a power level around lW If the sensor consumes a similar power Fig 41 Schematic of remote data transmission system for sensors S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 53 Fig 42 Photographs of integrated pH sensor (top) and receiver/transmitter pair (center) and connection to computer (bottom) level, the battery installed on the transmitter package can last more than one month This EBC sensing pair of devices can be mass-produced cost effectively well below $100 each pair The sensor occupies the tip of the pen-shaped layout in Fig 42 and runs off a 75 mA Li ion polymer rechargeable battery Fig 42 illustrated the package sensors mounted on a circuit board containing the detection circuit and microcontroller and the wireless transmitter for data collection The sensor module is fully integrated on an FR4 PC board and is packaged with battery The dimension of the sensor module package is: 4.500  2.900  200 The maximum line-of-sight range between the sensor module and the base station is 150 m The base station of the wireless sensor network server is also integrated in a single module (3.000  2.700  1.100 ) and is ready to be connected to a laptop by a USB cable The base station draws its 54 S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 power from the laptop’s USB interface, thus not require any battery or wall AC transformer, which reduces its form factor The PC is used to record the sensing data, send the data to internet, and take actions when the hydrogen is detected A client program has also been developed to receive the sensor data remotely The remote client can get a real-time log of the system for 10 via the client program In addition, a full data log will be obtained via accessing the server using a ftp client as the server program incorporates a full data logging functionality When the current of any of the sensors exceeds a preset level, the server program will automatically execute the phone-dialing program, reporting the emergency to relevant personnel A web server was developed using MATLAB (Mathworks Inc.) to share the collected sensor data via the Internet The data display time range can be chosen real-time, 85 min, 15 h and day 11 Summary and conclusions We have summarized recent progress in AlGaN/GaN HEMT sensors These devices can take advantage of the advantages of microelectronics, including high sensitivity, possibility of high-density integration, and mass manufacturability The goal is to realize real-time, portable and inexpensive chemical and biological sensors and to use these as hand-held exhaled breath, saliva, urine, or blood monitors with wireless capability Frequent screening can catch the early development of diseases, reduce the suffering of the patients due to late diagnoses, and lower the medical cost For example, a 96% survival rate has been predicted in breast cancer patients if the frequency of screening is every three months This frequency cannot be achieved with current methods of mammography due to high cost to the patient and invasiveness (radiation) There are many possible applications, including:  Diabetes/glucose testing – The population of diabetics is large and growing There are in excess of 150 million diabetics in the world and some believe this number will double by 2010 Although the frequent monitoring of glucose levels is strongly encouraged by health professionals most of the glucose testing products currently on the market are uncomfortable for the user and dissatisfaction is high Less invasive products are also less effective and have been unable to gain market share Conditions are favorable for the introduction of an effective, non-invasive product  Hydrogen sensors – Because the market for hydrogen sensors is highly dependent upon that of hydrogen fuels the market for hydrogen sensors is not very large Because the accurate detection of hydrogen leaks is extremely important, however, a number still smaller niche markets have developed and competition within them remains high Further development of the ‘‘hydrogen economy” in the near future could increase demand for hydrogen sensors  Breastcancer testing – The market size for breast cancer testing is vast – nearly 200,000 women and 1700 men were diagnosed in 2006 alone Although lucrative, competition in this industry is strong Growth potential is possible, however, as the most effective and widely used diagnostic exam for breast cancer, the mammogram, is potentially harmful due to radiation exposure Other, less popular, exams that not involve radiation tend to be both invasive and expensive  Asthma testing – Asthma testing products are increasingly in demand One in 20 Americans suffers from asthma and this number stands to increase in the near future Despite this, the market for preattack testing materials is undersaturated Only one product has reached the marketplace and although inexpensive this product is also relatively inaccurate Other potential competition may be more accurate but has yet to reach the market  Prostate testing – Because in men will be diagnosed with prostate cancer during his lifetime, the market for testing products is also quite large While two tests currently possess the bulk of the market, they either inaccurate or invasive or both Because of this relatively weak competitive market new entry possibilities are strong  Narcotics testing – Toxicology screens are the most common narcotics testing products currently on the market Used regularly by law enforcement agencies, medical facilities, and corporate businesses tests are inexpensive and effective More advanced technology has also been developed to in an effort to monitor discrete drug levels in an individual’s system S.J Pearton et al / Progress in Materials Science 55 (2010) 1–59 55 HEMT sensors show promising results for protein, DNA, prostate cancer, kidney injury molecules, pH values of solutions, mercury ions as well glucose in the exhaled breath condensate The method relies on an amplification of small changes in antibody-structure due to binding to antigens The characteristics of these sensors include fast response (liquid phase – 5–10 s and gas phase – millisecond), digital output signal, small device size (less than 100  100 lm2) and chemical and thermal stability Given the ever increasing incidence of diabetes in both the United States and abroad the market for diabetes testing and supplies is large and growing Moreover, prevalent market-wide dissatisfaction with current testing alternatives – due to discomfort, inaccuracy, and/or cost – leads us to believe that the diabetes testing market is by far the most promising one for this technology Although possible concerns include a difficult government certification process and insurance coverage, a survey of possible competition shows similar products to be either nonexistent or in an embryonic stage of development, meaning possible market entry can be made carefully and strategically The gas sensor market, on the other hand, is significantly less attractive Although the product offers significant advantages over existing technology these features are limited to what is currently a small niche market within the hydrogen gas and fuel market which itself low growth prospects and a high level of competition Because general gas detection outside of this market is vastly simpler and even more competitive the product does not offer significant advantages to the consumer, but possible advantages could be introduced to achieve production economies of scale and allow for market entry In either case any attempts at mass market entry should be made quickly as still more competition is already in preliminary development Like the market for diabetes testing, the market for breast cancer testing is also highly promising Although numerous testing alternatives exist the market size is huge and growing – in 2005 the testing market in the United States alone was worth well in excess of $1 billion – and the most common diagnostic methods involve some level of discomfort and/or exposure to radiation giving it the same kind of patient dissatisfaction found among diabetes Because testing is performed less frequently, however, its larger market may prove less lucrative and market entry is highly dependent upon government regulations and insurance coverage Barring these difficulties market entry (if made carefully) should be relatively easy There are still some critical issues First, the sensitivity for certain antigens (such as prostate or breast cancer) needs to be improved further to allow sensing in body fluids other than blood (urine, saliva) Second, a sandwich assay allowing the detection of the same antigen using two different antibodies (similar to ELISA) needs to be tested Third, integrating multiple sensors on a single chip with automated fluid handling and algorithms to analyze multiple detection signals, and fourth, a package that will result in a cheap final product is needed Fourth, the stability of surface functionalization layers in some cases is not conducive to long-term storage and this will limit the applicability of those sensors outside of clinics There is certainly a need for detection of multiple analytes simultaneously However, there are many such approaches and acceptance from the clinical community is generally slow for many reasons, including regulatory concerns Acknowledgments This work is supported by the ONR funded Center for Sensor Materials and Technologies, the State of Florida funded Center for Nano-Bio Sensors, NSF and ARO The collaborations with T Lele, 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