Humidity Sensors: A Review of Materials and Mechanisms

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Copyright © 2005 American Scientific Publishers
All rights reserved
Printed in the United States of America

SENSOR LETTERS
Vol. 3, 274–295, 2005

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Humidity Sensors: A Review of
Materials and Mechanisms

Zhi Chen∗ and Chi Lu

Department of Electrical and Computer Engineering and Center for Nanoscale Science and Engineering,
University of Kentucky, Lexington, Kentucky 40506, USA

(Received: 22 July 2005. Accepted: 27 July 2005)

We have reviewed humidity sensors based on various materials for both relative and absolute
humidity, including ceramic, semiconducting, and polymer materials. In the majority of publications,
there are few papers dealing with absolute humidity sensors, which have extensive applications in
industry. We reviewed extensively absolute humidity sensors in this article, which is unique com-
paring with other reviews of humidity sensors. The electrical properties of humidity sensors such
as sensitivity, response time, and stability have been described in details for various materials and
a considerable part of the review is focused on the sensing mechanisms. In addition, preparation
and characterization of sensing materials are also described. For absolute humidity sensors, mirror-
based dew-point sensors and solid-state Al2O3 moisture sensors have been described. As the major
problem in Al2O3 moisture sensors, long-term instability, has been solved, (cid:1)-Al2O3 moisture sensors
may have promising future in industry.

Keywords: Humidity Sensor, Mechanisms, Humidity-Sensing, Relative Humidity, Absolute

Humidity, Dew Point, Frost Point.

CONTENTS

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 274
2. Classification of Humidity Sensors . . . . . . . . . . . . . . . . . . 275
. . . . . . . . . . . . . . . . . . . . . . . 275
3. Relative Humidity Sensors
3.1. Ceramic Sensing Materials . . . . . . . . . . . . . . . . . . . . 275
3.2. Semiconducting Sensing Materials . . . . . . . . . . . . . . . 280
3.3. Polymer-Based Humidity Sensors . . . . . . . . . . . . . . . . 281
. . . . . . . . . . . . . 286
4.1. Mirror-Based Dew/Frost Point Sensors (Hygrometers) . . . 286
4.2. Aluminum Oxide Moisture Sensors . . . . . . . . . . . . . . . 289
5. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292
References and Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

4. Absolute Humidity Sensors (Hygrometers)

1. INTRODUCTION

Humidity sensors have gained increasing applications
in industrial processing and environmental control.1 For
manufacturing highly sophisticated integrated circuits in
semiconductor industry, humidity or moisture levels are
constantly monitored in wafer processing. There are
many domestic applications, such as intelligent control of
the living environment in buildings, cooking control for

∗Corresponding author; E-mail: zhichen@engr.uky.edu

microwave ovens, and intelligent control of laundry etc.
In automobile industry, humidity sensors are used in rear-
window defoggers and motor assembly lines. In medical
field, humidity sensors are used in respiratory equip-
incubators, pharmaceutical processing,
ment, sterilizers,
and biological products. In agriculture, humidity sensors
are used for green-house air-conditioning, plantation pro-
tection (dew prevention), soil moisture monitoring, and
cereal storage. In general industry, humidity sensors are
used for humidity control in chemical gas purification, dry-
ers, ovens, film desiccation, paper and textile production,
and food processing.

In this paper, we aim to present extensive review of
research and development of humidity sensors for a wide
variety of applications. Because applications in each field
require different operating conditions, various types of
humidity sensors based on a variety of sensing materi-
als will be described. This paper is organized as follows.
It begins with brief review of classification of humidity
sensors based on types of sensing materials and detec-
tion ranges (Section 2). Then the relative humidity sensors
based on ceramic, semiconductor, and polymer materi-
als will be discussed in Section 3. Absolute humidity
sensors, which were not extensive studied but are found

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Sensor Lett. 2005, Vol. 3, No. 4

1546-198X/2005/3/274/022

doi:10.1166/sl.2005.045

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Humidity Sensors: A Review of Materials and Mechanisms

wide-spread applications in many industrial fields, will be
reviewed in Section 4.

–80 –70

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–40

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–20

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Dew/Frost Point (˚C)

2. CLASSIFICATION OF
HUMIDITY SENSORS

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10

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1000

10000

PPMv at 1 atm

Humidity measurement determines the amount of water
vapor present in a gas that can be a mixture, such as
air, or a pure gas, such as nitrogen or argon. Based on
measurement techniques, the most commonly used units
for humidity measurement are Relative Humidity (RH),
Dew/Frost point (D/F PT) and Parts Per Million (PPM).2
Relative Humidity (RH) is the ratio of the partial pressure
of water vapor present in a gas to the saturation vapor
pressure of the gas at a given temperature. RH is a func-
tion of temperature, and thus it is a relative measurement.
The RH measurement is expressed as a percentage. Dew
point is the temperature (above 0 (cid:3)C) at which the water
vapor in a gas condenses to liquid water. Frost point is the
temperature (below 0 (cid:3)C) at which the vapor condenses to
ice. D/F PT is a function of the pressure of the gas but
is independent of temperature and is therefore defined as
absolute humidity measurement. Parts Per Million (PPM)
represents water vapor content by volume fraction (PPMv)
or, if multiplied by the ratio of the molecular weight of
water to that of air, as PPMw. PPM is also an absolute
measurement. Although this measurement unit is more dif-
ficult to conceive, it has extensive applications in industry
especially for trace moisture measurement.

Figure 1 shows the correlation among Relative Humid-
ity (RH), Parts Per Million by volume (PPMv), and the
Dew/Frost Point (D/F PT). RH measurement covers higher
humidity range, PPMv covers lower humidity range, and

Relative Humidity (%) at 20 ˚C
25

3.5

10

1

50

100

Fig. 1. Correlation among humidity units: Relative Humidity (RH),
Dew/Frost point (D/F PT), and Parts Per Million by volume fraction
(PPMv).

D/F PT covers all the humidity range. Therefore, for daily
life, Relative Humidity is constantly used for ease under-
standing. For trace moisture measurement, it would better
to use PPMv or D/F PT, because it tells us the absolute
amount of water vapor in a gas or air. According to the
measurement units, humidity sensors are divided into two
types: Relative humidity (RH) sensors and absolute humid-
ity (moisture) sensors. Most humidity sensors are rela-
tive humidity sensors, which can be further classified into
ceramic, semiconductor, and polymer humidity sensors.
Two types of absolute humidity sensors or hygrometers
are available, including solid moisture sensor and mirror-
chilled hygrometer.

3. RELATIVE HUMIDITY SENSORS

3.1. Ceramic Sensing Materials

Humidity sensors based on water-phase protonic ceramic
materials are used widely in industry and research labo-
ratories. The adsorbed water condensed on the surface of

Zhi Chen received his B.S. degree in 1984 and M.S. degree in 1987 in electrical engineering
from University of Electronic Science and Technology, Chengdu, China. He obtained a
Ph.D. degree in electrical engineering from University of Illinois at Urbana-Champaign in
1999. He is currently an associate professor with Department of Electrical Engineering
and the associate director of Center for Nanoscale Science and Engineering, University
of Kentucky. He is a senior member of IEEE and won the National Science Foundation
CAREER Award in 2001. His research interests include micro/nano fabrication, nanoscale
devices and materials including growth of highly ordered carbon nanotubes for electronic
device applications, CMOS transistor reliability and deuterium processing, gate dielectrics
for MOS transistors, and microsensors.

Chi Lu received his B.S. degree in Environmental Engineering from Hebei University of
Science and Technology, China in 1996 and M.S. degree in Materials Science from Beijing
University of Chemical Technology, China in 1999. He is currently a Ph.D. student at
Department of Electrical Engineering, University of Kentucky. His research interests include
gas sensors and nanodevices.

Sensor Letters 3, 274–295, 2005

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Humidity Sensors: A Review of Materials and Mechanisms

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Fig. 2. Brief illustration of the Grotthuss mechanism.

the materials and protons will be conducted in the formed
aquatic layers. For ionic sensing materials, if the humidity
increases,
the conductivity decreases and the dielectric
constant increases.3(cid:1) 4 In bulk water, proton is the dominant
carrier responsible for the electrical conductivity. The con-
duction is due to the Grotthuss mechanism, through which
protons tunnel from one water molecule to the next via
hydrogen bonding that universally exists in liquid-phase
water (Fig. 2).

This mechanism was reported about 200 years ago.5
The mechanism of protonic conduction inside the adsorbed
water layers on the surface of the sensing materials was
discovered in study of TiO2 and (cid:4)-Fe2O3.6(cid:1) 7 As shown in
Figure 3, at the first stage of adsorption, a water molecule
is chemically adsorbed on an activated site (a) to form
an adsorption complex (b), which subsequently transfers
to surface hydroxyl groups (c). Then, another water mole-
cule comes to be adsorbed through hydrogen bonding on
the two neighboring hydroxyl groups as shown in (d).
The top water molecule condensed cannot move freely
due to the restriction from the two hydrogen bonding
(Fig. 3(d)). Thus this layer or the first physically-adsorbed
layer is immobile and there are not hydrogen bonds formed
between the water molecules in this layer. Therefore, no
proton could be conducted in this stage.

As water continues to condense on the surface of the
ceramic, an extra layer on top of the first physically-
adsorbed layer forms (Fig. 4). This layer is less ordered
than the first physically-adsorbed. For example,
there
may be only one hydrogen bond locally. If more layers

Fig. 3. Four stages of the adsorption. Reprinted with permission from
[6], T. Moromoto et al., J. Phys. Chem. 73, 243 (1969). © 1969, Ameri-
can Chemical Society.

276

Fig. 4. Multi-layer structure of condensed water. Reprinted with per-
mission from [7], E. McCafferty et al., Faraday Discussions 52, 239
(1971). © 1971, Royal Society of Chemistry.

condense, the ordering from the initial surface may grad-
ually disappear and protons may have more and more
freedom to move inside the condensed water through the
Grotthuss mechanism. In other words, from the second
physisorbed layer, water molecules become mobile and
finally almost identical to the bulk liquid water, and the
Grotthuss mechanism becomes dominant. This mechanism
indicates that sensors based purely on water-phase protonic
conduction would not be quite sensitive to low humidity,
at which the water vapor could rarely form continuous
mobile layers on the sensor surface.

The two immobile layers,

the chemisorbed and the
first physisorbed ones, while cannot contribute to proton-
conducting activity, could provide electron tunnelling
between donor water sites.8(cid:1) 9 The tunnelling effect, along
with the energy induced by the surface anions, facilitates
electrons to hop along the surface that is covered by the
immobile layers and therefore contributes to the conduc-
tivity. This mechanism is quite helpful for detecting low
humidity levels, at which there is not effective protonic
conduction. Nonetheless, the tunnelling effect is definitely
not the semiconducting mechanism that will be discussed
later.

In the following subsections, we will describe four basic
types of oxide-based sensing materials, including Al2O3,
TiO2, SiO2, and spinel compounds. The basic preparation
methods, humidity-sensing properties, and their advan-
tages and disadvantages will be discussed in detail.

3.1.1. Al2O3

Al2O3 is one of the most favorable ceramic sensing mate-
rials due to its independence of temperature at nearly
all range of relative humidity from 25 (cid:3)C to 80 (cid:3)C.10
The small pore radius makes Al2O3 sensitive to very
low water vapor pressure. Due to the electron tunnelling
effect inside the condensed immobile water layers, porous
Al2O3 is a competitive candidate for sensing low humid-
ity levels.8 In addition to capacitive and resistive sensors,
more complicated sensing devices based on Al2O3, e.g.,
MISFETs (metal-insulator-semiconductor field-effect tran-
sistors), were fabricated, and some of them have very good
linear response.11

There are several phases for Al2O3 whereas only two of
them are common and used in humidity sensing: (cid:3)-Al2O3

Sensor Letters 3, 274–295, 2005

Chen and Lu

Humidity Sensors: A Review of Materials and Mechanisms

(amorphous) and (cid:4)-Al2O3
(corundum). The former is
more sensitive than the latter due to its high porosity,
while the latter is most thermodynamically stable phase.
Although many Al2O3-based humidity sensing applica-
tions use the (cid:3)-phase or amorphous phase Al2O3, the films
are susceptible to change to (cid:3)-Al2O3
· H2O (boehmite),12
resulting in the gradual decrease of surface area and
porosity.13 Therefore the deposition or growth of humidity-
sensitive (porous) (cid:4)-Al2O3
is also important for sen-
sors required for long-term, non-regenerate applications.
Because (cid:3)-Al2O3 is always mixed with huge amount of
amorphous Al2O3, the crystal content is quite small and
amorphous Al2O3 formed by anodization or vacuum depo-
sition contains (cid:3)-phase to some degree, whereas the for-
mer is crystalline and the latter has no significant peaks in
X-ray diffraction except for one broad peak.

Many of the present Al2O3 humidity sensors are fabri-
cated through anodization. Because of its low-cost and easy
process, anodic Al2O3 has great priority over other ceram-
ics. The anodization technique can be divided into two cate-
gories, low voltage (<100 V) anodization and anodic spark deposition (usually >100 V). The low voltage anodiza-
tion produces (cid:3)-phase or amorphous Al2O3 and the anodic
spark deposition results in porous (cid:4)-Al2O3. These two
methods will be discussed first and other methods for fab-
rication of humidity-sensing Al2O3 will be discussed later.
The first humidity-sensitive Al2O3 layer formed through
anodization on Al metal surface was reported in 1953.4
The anodization was carried out in 3% H2CrO3 at 50 V.
The capacitance increased linearly while resistance dec-
reased exponentially to the relative humidity, both of the
capacitive and resistive sensitivities are considerably aff-
ected by the temperature. The anodization parameters con-
siderably affected the moisture sensitivity of the resulted
porous Al2O3 films. As reported in Refs. [14, 15], the
capacitance/ resistance versus humidity characteristic of
the sensor fabricated at low current density shows a weak
response at low humidity, whereas for anodization at high
current density or re-anodization a much steeper response
at low humidity is obtained. This phenomenon has been
attributed to trapping of anions of electrolytes at high
current density or into the pores by re-anodization. The
high charge density results in easy physisorption of water
molecules that form a liquid-like network within the pores
(as discussed in the previous section).

The primary problem of anodized amorphous Al2O3 as
discussed before is that when exposed for a long duration
in high humidity, significant degradation in the sensitiv-
ity and drift in the capacitance characteristics would be
expected. This was attributed to the widening of the pores
due to diffusion of the adsorbed water.16 The best solu-
tion would be to grow self-ordered porous films and elim-
inating the variability among the pores and irregularities
the microstructure of the film. Thermal annealing at about
400 (cid:3)C has been reported to have limited improvement of
the stability of anodized Al2O3 sensors.17

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200 nm

Fig. 5. Honeycomb structure of anodic aluminum oxide (AAO).

Although amorphous AAO (anodic aluminum oxide)
was found to be humidity-sensitive in 1950s, it was not
until 1978 did the researchers discover that it could form
regular microstructure.18 Low voltage anodization at cert-
ain conditions (always characterized by long-term anodi-
zation period at a constant voltage) in acidic electrolyte
solution forms Al2O3 layer consisting of hexagonal close-
packed cylindrical pores perpendicular to the metal surface
(Fig. 5). The diameters and depths of the pores can be
controlled by tuning the anodization conditions. Therefore,
the detection limit could be set very low by shrinking the
pore size (as mentioned before, the minimum detectable
humidity decreases as the pore radius decreases). In addi-
tion to its ease process, this honeycomb structure has great
potential applications in electronic, optical, and microme-
chanical devices.19(cid:1) 20

For humidity-sensitive field-effect

transistors (HUMI
FET),21 porous Al2O3 film is usually sandwiched between
a top Au electrode and an under-gate Al electrode. It
is also reported that a HUMIFET with a structure of
SiO2/Si3N4/Ta/Ta2O5/Al2O3 exhibits much higher sensitiv-
ity (less than 1 ppmv) and faster response time (less than
1 second) than conventionally anodized Al2O3.11

As a high voltage anodization procedure, anodic spark
22(cid:1) 23 films (∼10 (cid:2)m)
deposition can create porous (cid:4)-Al2O3
that would almost not degrade in humid environments.24(cid:1) 25
The electrolytes of anodic spark deposition are not water
solutions but high-temperature salt melts (usually alkali
salts).23 Due to the tremendous energy dissipated at very
large instantaneous current density (∼104 A/cm2),
the
already deposited Al2O3 barrier film breaks down and elec-
tric sparks occur. The extremely high temperature resulted
from the electric sparks melt the Al2O3 film locally, result-
ing in a porous structure (Fig. 6).24 Re-anodization of the
porous (cid:4)-Al2O3 in certain acid solutions at a low voltage
was found to be effective in increase of the film resistance

277

Humidity Sensors: A Review of Materials and Mechanisms

Chen and Lu

Desorption

Adsorption

100

)

%

i

(

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l

80

60

40

20

0

0

5

10

14

Time (sec)

Fig. 8. Time responses of the (cid:4)-Al2O3 sensor to relative humidity from
12 to 65% and from 95% to 65%. Reprinted with permission from [25],
Z. Chen et al., in Proc. 27th Annual Conf. IEEE Industry Appl. Soc.,
Houston, TX (1992), Vol. 2, p. 1668. © 1992, IEEE.

Other methods, such as electron beam evaporation, reac-
tive evaporation, sputtering, spray pyrolysis, etc., were also
utilized to deposit Al2O3 thin films. Unfortunately, simi-
lar to the films formed at low-voltage anodization, Al2O3
films prepared by vacuum methods at lower substrate tem-
peratures are usually (cid:3)-phase or amorphous, which suf-
fers from degradation as mentioned before. An effective
method to obtain porous (cid:4)-Al2O3 (stable) for humidity
sensing is reactive evaporation at elevated substrate tem-
peratures (800–1300 (cid:3)C),27 in which the metal aluminum
is evaporated and oxidized before the oxide particles are
deposited on the substrate. Reactively evaporated Al2O3
films have been reported to be sensitive to moisture levels
as low as 1 ppmv.28

Amorphous Al2O3 films deposited by spray pyrolysis
at 250–350 (cid:3)C from aluminum acetylacetonate dissolved
in dimethyl
formamide were found to be humidity-
sensitive.29 However, degradation was not mentioned in
the report. Humidity sensors based on bulk-sintered Al2O3
films are also reported. However, they are only sensitive to
water vapor levels higher than 50–100 ppmv due to the less
porosity.30(cid:1) 31 The Al2O3 sensors for absolute humidity mea-
surement will be described in more detail in Section 4.2.

10 µm

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Fig. 6. Porous Al2O3 by anodic spark deposition. Reprinted with per-
mission from [24], Z. Chen et al., J. Am. Ceram. Soc. 74, 1325 (1991).
© 1991, American Ceramic Society.

so that no short-circuit occurs.24(cid:1) 25 Figure 7 shows the
electrical characteristics of the (cid:4)-Al2O3 sensors versus the
relative humidity (RH) at various temperatures.25 Their
response time and long-term stability are also shown in
Figures 8 and 9.25 The (cid:4)-Al2O3 sensors showed very high
sensitivity and very fast response at RH range (<5 s). To test its long-term stability, it was exposed in the air for one year and its reading was still the same as its initial one (Fig. 9). Cathodically grown aluminum hydroxide, or hydrated Al2O3, can also be used as a humidity sensing materials.26 Using electroanalyzing saturated Al2(SO4)3 as solution and a hydrogen-adsorbing metal (palladium) as the cathode, aluminum hydroxide film can be deposited on the pal- ladium. Although this film has good response at high humidity, it is not sensitive to low humidity. Electroanal- ysis is not the only method to fabricate Al2O3 thin films. 10000 5000 ) f p ( e c n a t i c a p a C 1000 600 400 2000 40 ˚C 11 ˚C 25 ˚C 40 ˚C 25 ˚C 11 ˚C 107 106 105 ) Ω ( e c n a i t s s e R 100 ) H R % i ( g n d a e R r o s n e S 80 60 40 20 0 0 20 40 60 80 Relative Humidity (%) 104 100 0 50 300 Time (sec) 350 Fig. 7. Capacitance (—) and resistance (- - - -) response of the (cid:4)-Al2O3 sensor to relative humidity at 11 (cid:3)C, 25 (cid:3)C, and 40 (cid:3)C. Reprinted with permission from [25], Z. Chen et al., in Proc. 27th Annual Conf. IEEE Industry Appl. Soc., Houston, TX (1992), Vol. 2, p. 1668. © 1992, IEEE. Fig. 9. Long-term stability testing results of the (cid:4)-Al2O3 sensor in the RH range. Reprinted with permission from [25], Z. Chen et al., in Proc. 27th Annual Conf. IEEE Industry Appl. Soc., Houston, TX (1992), Vol. 2, p. 1668. © 1992, IEEE. 278 Sensor Letters 3, 274–295, 2005