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
b
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
article info
Article history:
Received 10 June 2009

Contents lists available at ScienceDirect
Progress in Materials Science
journal homepage: www.elsevier.com/locate/pmatsci
2.1. H
2.
sensing . 4
2.2. O
2
sensing . 12
2.3. CO
2
sensing 13
2.4. CH
4
sensing 15
3. UV photodetectors . . . . . . . . . 17
3.1. UV photoresponse of single ZnO nanowires . . . . . . . . . . . . 18
4. Sensor functionalization . . . . 20
5. pH measurement . . . . . . . . . . 23
6. Exhaled breath condensate . . 27
7. Heavy metal detection . . . . . . 27
8. Biotoxin sensors . . . . . . . . . . . 32
8.1. Botulinum . 33
9. Biomedical applications . . . . . 34
9.1. Prostate cancer detection . . . . . . . . 40
9.2. Kidney injury molecule detection . 41
9.2.1. Breast cancer. . . . . . 43
9.2.2. Lactic acid . . . . . . . . 45
9.2.3. Chloride ion detection . . . . . . . . . . . . . . . . . 47
9.2.4. Pressure sensing . . . 49

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
2 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 ac-
counted 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 secu-
rity, 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 nat-
ural 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, dis-
ease 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 2 h, which is not practical for in-office
or bedside detection. Electrochemical devices have attracted attention due to their low cost and sim-
plicity, but significant improvements in their sensitivities are still needed for use with clinical sam-
ples. 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, la-

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
3
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.
2. Gas sensing
2.1. H
2.
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 min-
imal 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].

contacts were formed by lift-off of sputtered Ti/Al/TiB
2
/Ti/Au, followed by annealing at 850 °C for
45 s under a flowing N
2
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 inter-
connection 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 N
2
,or1%H
2
in nitrogen. Fig. 2 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%H
2
in air atmosphere. For these
diodes, the current increases upon introduction of the H
2
, through a lowering of the effective barrier
height. The data was fit to the relations for thermionic emission and showed decreases in Schottky
4 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
barrier height
U
B
of 30–50 meV at 50 °C and larger changes at higher temperatures. The decrease in
barrier height is completely reversible upon removing the H
2
from the ambient and results from dif-
fusion of atomic hydrogen to the metal/GaN interface, altering the interfacial charge.

uated 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
0 50 100 150 200
6.7
6.8
6.9
7.0
7.1
7.2
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 re-
sponse 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 N
2
to
10%H
2
/90%N
2
. We have found that use of a metal–oxide-semiconductor(MOS) diode structure with
Sc
2
O
3
gate dielectric and the same W/Pt metallization show these same reversible changes in forward
current upon exposure to H
2
-containing ambients over a much broader temperature range (90 to

o
C
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
2
4
6
8
10
12
Current(mA)
Bias Voltage(V)
Nitrogen
10% Hydrogen
T = 500
o
C
Fig. 4. Change in current in W/Pt GaN and AlGaN/GaN diodes at 500 °C when the ambient is switched from N
2
to 10% H
2
in N
2
.
S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
7
the potential applications of these wide bandgap sensors. Fig. 4 shows that the relative change in cur-
rent 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. 5. The advantage of the nitride system rel-
ative to SiC is the availability of a heterostructure and the strong piezoelectric and polarization fields

ity to ppm level H
2
by a factor of up to 11. The addition of Pd appears to be effective in catalytic dis-
sociation 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 H
2
concentration above
Fig. 6. 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 H
2
concentrations (200–1500 ppm) in N
2
for
10 min at room temperature.
S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
9
1000 ppm. The resistance after exposing the nanowires to air was restored to approximately 90% of
initial level within 2 min [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 previ-
ously [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 N
2
ambient,
the relative resistance change increased from 1.2% at 20 ppm H
2
to 4% at 300 ppm H
2

0.01
H
2
H
2
H
2
H
2
ZnO nanorod with Pd
Air
Air
Air
Air
O
2
500ppm
250ppm
100ppm
10ppm
N
2
Delta R/R (Sensitivity)
Time(min)
Fig. 8. Schematic of ZnO nanowire sensor (top), SEM of completed device (center) and change in resistance as a function of time
when switching to H
2
-containing ambients.
S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
11

2
. The O
2
measurement is also not a complete measure of circulatory sufficiency. If there is
insufficient blood flow or insufficient hemoglobin in the blood (anemia), tissues can suffer hypoxia de-
spite high oxygen saturation in the blood that does arrive. The current oxide-based O
2
sensors can
operate at very high temperatures, such as the commercialized solid electrolyte ZrO
2
(700 °C) or the
semiconductor metal oxides such as TiO
2
,Nb
2
O
5
, SrTiO
3
, and CeO
2
(>400 °C). However, it remains
important to develop a low operation temperature and high sensitivity O
2
sensor to build a small, por-
table and low cost O
2
sensor system for biomedical applications.
Oxide-based materials are widely used and studied for oxygen sensing because of their low cost
and good reliability. The commercialized solid electrolyte ZrO

generated during oxide growth. Typically, the higher the concentration of oxygen vacancies in the
oxide film, the more conductive is the film. InZnO (IZO) films have been used in fabricating thin film
transistors and the conductivity of the IZO is also found to depend on the oxygen partial pressure dur-
ing the oxide growth [73–75]. The IZO is a good candidate for O
2
sensing applications.
The schematic of the oxygen sensor is shown at the top of Fig. 9. The bottom part of the figure
shows the device had a strong response when it was tested at 120 °C in pure nitrogen and pure oxygen
alternately at Vds = 3 V. When the device was exposed to the oxygen, the drain-source current de-
creased, whereas when the device was exposed to nitrogen, the current increased. The IZO film pro-
vides a high oxygen vacancy concentration, which makes the film readily sense oxygen and create a
potential on the gate area of the AlGaN/GaN HEMT. A sharp drain-source current change demonstrates
the combination of the advantage of the high electron mobility of the HEMT and the high oxygen va-
cancy concentration of the IZO film. Because of these advantages, this oxygen sensor can operate with
a high sensitivity at a relatively low temperature compared to many oxide-based oxygen sensors
which operate from 400 °C to 700 °C.
In summary, it is clear that through a combination of IZO films and the AlGaN/GaN HEMT structure,
a low operation temperature and low power consumption oxygen sensor can be achieved. The sensor
12 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
can be either used in the steady-state or in the annealed mode which provide flexibility in various
applications. This device shows promise for portable, fast response and high sensitivity oxygen
detectors.
2.3. CO
2
sensing
The detection of carbon dioxide (CO
2
) gas has attracted attention in the context of global warming,
biological and health-related applications such as indoor air quality control, process control in fermen-
tation, and in the measurement of CO

13
impinging upon it. It is generally considered that the NDIR technology is limited by power consump-
tion and size.
In recent years, monomers or polymers containing amino-groups, such as tetrakis(hydroxy-
ethyl)ethylenediamine, tetraethylene-pentamine and polyethyleneimine (PEI) have been used for
CO
2
sensors to overcome the power consumption and size issues found in the NDIR approach [84–
88]. Most of the monomers or polymers are utilized as coatings of surface acoustic wave transducers.
The polymers are capable of adsorbing CO
2
and facilitating a carbamate reaction. PEI has also been
used as a coating on carbon nanotubes for CO
2
sensing by measuring the conductivity of nanotubes
upon exposing to the CO
2
gas. For example, CO
2
adsorbed by a PEI coated nanotube portion of a NTFET
(nanotube field effect transistor) sensor lowers the total pH of the polymer layer and alters the charge
transfer to the semiconducting nanotube channel, resulting in the change of NTFET electronic charac-
teristics [89–92].
A schematic cross-section of the device is shown in Fig. 10. The interaction between CO
2
and amino
group-containing compounds with the influence of water molecules is based on an acid–base reaction.
The purpose of adding starch into the PEI in our experiment was to enhance the absorption of the
water molecules into the PEI/starch thin film. Several possible reaction mechanisms have been sug-
gested. The key reaction was that primary amine groups, –NH

the 2DEG in the AlGaN/GaN HEMTs.
Fig. 10 (bottom) shows the drain current of PEI/starch functionalized HEMT sensors measured ex-
posed to different CO
2
concentration ambients. The measurements were conducted at 108 °C and a
fixed source-drain bias voltage of 0.5 V. The current increased with the introduction of CO
2
gas. This
was due to the net positive charges increased on the gate area, thus inducing electrons in the 2DEG
channel. The response to CO
2
gas has a wide dynamic range from 0.9% to 50%. Higher CO
2
concentra-
tions were not tested because there is little interest in these for medical related applications. The re-
sponse times were on the order of 100 s. The signal decay time was slower than the rise time and was
due to the longer time required to purge CO
2
out from the test chamber.
The effect of ambient temperature on CO
2
detection sensitivity was investigated. The drain current
changes were linearly proportional to the CO
2
concentration for all the tested temperatures. However,
the HEMT sensors showed higher sensitivity for the higher testing temperatures. There was a notice-
able change of the sensitivity from the sensors tested at 61 °C to those tested at 108 °C. This difference
is likely due to higher ambient temperature increasing the reaction rate between amine groups and
CO
2

) on the same chip
can be used to obtain this result. Another prime focus should be the thermal stability of the detectors,
since they are expected to operate for long periods at elevated temperature [64,96–102]. MOS diode-
based sensors have significantly better thermal stability than a metal-gate structure and also sensitiv-
ity than Schottky diodes on GaN. In this work, we show that both AlGaN/GaN MOS diodes and Pt/ZnO
bulk Schottky diodes are capable of detection of low concentrations(10%) of ethylene at temperatures
between 50–300 °C (ZnO) or 25–400 °C (GaN).
Fig. 11 (top) shows a schematic of the completed AlGaN/GaN MOSHEMT and at bottom the differ-
ence in forward diode current at 400 °C of the Pt/Sc
2
O
3
/AlGaN/GaN MOS-HEMT diode both in pure N
2
relative to a 10% C
2
H
4
/90%N
2
atmosphere. At a given forward bias, the current increases upon intro-
duction of the C
2
H
4
. In analogy with the detection of hydrogen in comparable SiC and Si Schottky
diodes, a possible mechanism for the current increases involves atomic hydrogen which is either
decomposed from C
2
H

a lowering of the effective barrier height. One of the main mechanisms is once again the catalytic
decomposition of the C
2
H
4
on the Pt metallization, followed by diffusion to the underlying interface
with the ZnO. In conventional semiconductor gas sensors, the hydrogen forms an interfacial dipole
layer that can collapse the Schottky barrier and produce more Ohmic-like behavior for the Pt contact.
The recovery of the rectifying nature of the Pt contact was many orders of magnitude longer than for
Pt/GaN or Pt/SiC diodes measured under the same conditions in the same chamber. For measurements
over the temperature range 50–150 °C, the activation energy for recovery of the rectification of the
contact was estimated from the change in forward current at a fixed bias of 1.5 V. This was thermally
activated through a relation of the type I
F
= I
O
exp(ÀE
a
/kT) with a value for E
a
of $0.22 eV, comparable
for the value of 0.17 eV obtained for the diffusivity of atomic deuterium in plasma exposed bulk ZnO
[104,105]. This suggests that at least some part of the change in current upon hydrogen gas exposure
Fig. 11. Schematic of AlGaN/GaN MOS diode and change in current at fixed bias when ethylene is introduced into the test
chamber with the sensor held at different temperatures.
16 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
is due to in-diffusion of hydrogen shallow donors that increase the effective doping density in the
near-surface region and reduce the effective barrier height.
The changes in current at fixed bias or bias at fixed current were larger for the ZnO diodes than for
the AlGaN/GaN MOS diodes because of this additional detection mechanism. Note that the changes in

detectors.
3. The fabrication approach developed previously for ZnO nanowire gas sensors allows for a simple,
low-cost, single-step approach to realizing robust UV detectors.
4. ZnO nanowire UV detectors can be readily integrated with on-chip wireless circuits to provide data
transmission to a central monitoring location. Thus, it is possible to have either single detectors or
arrays of detectors that operate at very low power levels and do not need constant monitoring by
humans.
5. The versatility of substrates also makes it possible to utilize 3D stacking technology developed for
silicon substrates for data intensive applications. Devices stacked with overlying ZnO sensors
would permit maximum sensor density and higher levels of integration with silicon or gallium
arsenide electronics.
UV detectors have application in space exploration by providing imaging and spectroscopic data of
nearby galaxies. Because the universe is expanding, UV radiation emitted by distant galaxies is red-
shifted and reaches our galaxy as visible or infrared radiation. Galaxies closer to the Milky Way can
be analyzed with UV radiation and comparisons can be made to visible and infrared images to ascer-
tain how the universe formed and changes with time. Launch of the GALEX (Galaxy Evolution Ex-
plorer) in 2003 has provided complementary information to the Hubble Telescope, Extreme UV
Explorer (EUVE) and some UV imaging performed on space shuttle missions. The GALEX mission is un-
ique in a number of ways, including the large field of view of the system that will enable an all-sky
ultra-violet map of the universe beyond the Milky Way galaxy. Extensive imaging and spectroscopy
of the universe in the far UV (135–174 nm) and near UV (175–280 nm) has been performed by GALEX.
Two micro-channel plate detectors are used on board the GALEX in either imaging or spectroscopic
mode. These detectors are sealed arrays (4096 Â 4096 pixels, 65 mm diameter) with incident photons
generating electrons in a photocathode layer. These are subsequently accelerated across a gap (700 V)
to a delay line grid and time resolved pulses generate images with accompanying electronics. The
detectors have a resolution of approximately 25
l
m and gain of 10
7
.

Fig. 13 shows the change in current at fixed bias of the nanowires in the dark and under illumina-
tion from 366 nm light. The conductivity is greatly increased as a result of the illumination, as evi-
denced by the higher current. No effect was observed for illumination with below bandgap light.
Transport measurements show that the ideality factor of Pt Schottky diodes formed on the nanowires
exhibit an ideality factor of 1.1, which suggests that there is little recombination occurring in the
Fig. 13. SEM micrograph of ZnO nanowire and time dependence of photocurrent as the 366 nm light source is modulated.
S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
19
nanowire. It also exhibits the excellent Ohmicity of the contacts to the nanowire, even at low bias. On
blanket films of n-type ZnO with carrier concentration in the 10
16
cm
À3
range we obtained contact
resistance of 3–5 Â 10
À5
X
cm
À2
for these contacts. In the case of ZnO nanowires made by thermal
evaporation, the I–V characteristics were rectifying in the dark and only became Ohmic during above
bandgap illumination. The conductivity of the nanowire during illumination with 366 nm light was
0.2
X
cm.
The photoresponse of the single ZnO nanowire at a bias of 0.25 V under pulsed illumination from a
366 nm wavelength Hg lamp in Fig. 13 shows the photoresponse is much faster than that reported for
ZnO nanowires grown by thermal evaporation from ball-milled ZnO powders and likely is due to the
reduced influence of the surface states seen in that material. The generally quoted mechanism for the
photoconduction is creation of holes by the illumination that discharge the negatively charged oxygen

tion often incorporates moving fluids. For example, sensors must sample a stream of air or water to
interact with the specific molecules they are designed to detect.
The general approach to detecting biological species using a semiconductor sensor involves func-
tionalizing the surface (e.g. the gate region of an ungated field effect transistor structure) with a layer
or substance which will selectively bind the molecules of interest. In applications requiring less spe-
cific detection, the adsorption of reactive molecules will directly affect the surface charge and affect
the near-surface conductivity. In their simplest form, the sensor consists of a semiconductor film pat-
terned with surface electrodes and often heated to temperatures of a few hundred degrees Celsius to
enhance dissociation of molecules on the exposed surface. Changes in resistance between the elec-
trodes signal the adsorption of reactive molecules. It is desirable to be able to use the lowest possible
20 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59

and transmit, unlike fluorescence detection methods which need human inspection and are difficult to
precisely quantify and transmit the data.
One drawback of HEMT sensors is a lack of selectivity to different analytes due to the chemical
inertness of the HEMT surface. This can be solved by surface modification with detecting receptors.
Sensor devices of the present disclosure can be used with a variety of fluids having environmental
and bodily origins, including saliva, urine, blood, and breath. For use with exhaled breath, the device
may include a HEMT bonded on a thermo-electric cooling device, which assists in condensing exhaled
breath samples.
In our HEMT devices, the surface is generally functionalized with an antibody or enzyme layer. The
success of the functionalization is monitored by a number of methods. Examples are shown in Figs. 14
and 15. The firsttestis a change in surface tension when thefunctional layer is in place and the changein
surface bonding can in some cases be seen by X-ray Photoelectron Spectroscopy. Typically, a layer of Au
is deposited on the gate region of the HEMT as a platform to attach a chemical such as thioglycolic acid,
whose S-bonds readily attach to the Au. The antibody layer can then be attached to the thioglycolic acid.
Fig. 15. Example of successful functionalization of HEMT surface-the device is no longer sensitive to water when the surface is
completely covered with the functional layer.
22 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
When the surface is completely covered by these functional layers, the HEMT will not be sensitive to

tivity is greatly increased as a result of the illumination, as evidenced by the higher current. No effect
was observed for illumination with below bandgap light. The photoconduction appears predominantly
to originate in bulk conduction processes with only a minor surface trapping component. The adsorp-
tion of polar molecules on the surface of ZnO affects the surface potential and device characteristics.
The current at a bias of 0.5 V as a function of time from nanorods exposed for 60s to a series of solu-
tions whose pH was varied from 2 to 12 was reduced upon exposure to these polar liquids as the pH is
Table 1
Summary of surface functional layers used with HEMT sensors.
Detection Mechanism Surface functionalization
H
2
Catalytic dissociation Pd,Pt
Pressure change Polarization Polyvinylidene difluoride
Botulinum toxin Antibody Thioglycolic acid/antibody
Proteins Conjugation/hybridization Aminopropylsilane/biotin
pH Adsorption of polar molecules Sc
2
O
3
, ZnO
Hg
2+
Chelation Thioglycolic acid/Au
KIM-1 Antibody KIM-1 antibody
Glucose GO
x
immobilization ZnO nanorods
Prostate-specific antigen PSA antibody Carboxylate succimdyl ester/PSA antibody
Lactic acid LO
x

the nanorod as a function of the pH of the solution flowed across it is shown at the bottom of the figure.
24 S.J. Pearton et al. / Progress in Materials Science 55 (2010) 1–59
produced superior results to either a native oxide or UV ozone-induced oxide in the gate region. The
ungated HEMTs with Sc
2
O
3
in the gate region exhibited a linear change in current between pH 3 and
10 of 37
l
A/pH. The HEMT pH sensors show stable operation with a resolution of <0.1 pH over the
entire pH range. 100Å Sc
2
O
3
was deposited as a gate dielectric through a contact window of SiN
x
layer.
Before oxide deposition, the wafer was exposed to ozone for 25 min. It was then heated in situ at
300 °C for cleaning for 10 min inside the growth chamber. The Sc
2
O
3
was deposited by RF plasma-acti-
vated MBE at 100 °C using elemental Sc evaporated from a standard effusion at 1130 °C and O
2
derived
from an Oxford RF plasma source. For comparison, we also fabricated devices with just the native
oxide present in the gate region and also with the UV ozone-induced oxide. Fig. 17 shows a scanning
electron microscopy (SEM) image (top) and a cross-sectional schematic (bottom) of the completed de-

25