Measuring and testing – Gas analysis – Detector detail
Reexamination Certificate
2000-05-04
2001-09-25
Williams, Hezron (Department: 2856)
Measuring and testing
Gas analysis
Detector detail
C422S098000
Reexamination Certificate
active
06293137
ABSTRACT:
FIELD OF THE INVENTION
The present invention is related to a highly sensitive Pd/InP hydrogen sensor. In particular, this new Pd/InP hydrogen sensor has advantages of a small size and a simple fabrication process.
BACKGROUND OF THE INVENTION
Due to the technology developments, modern industrial and medical applications use a large quantity of hydrogen as a raw material or other purposes. Hydrogen, however, is a flammable and explosive gas. When the concentration of leakage hydrogen reaches 4.65 vol % or more in air, a hazard of explosion emerges. Therefore, based on the considerations of industrial safety and environmental concern, hydrogen sensors are widely used in factories, laboratories and hospitals in order to accurately monitor the concentration of leakage hydrogen. However, in addition to a large volume and a high production cost, one disadvantage of conventional hydrogen sensors is that most belong to the category of passive elements. The other additional equipment or a conversion circuit is required to perform the analysis or amplification. Therefore, the conventional hydrogen sensors can not become intelligent sensors. As a result, the development of a new and effective hydrogen sensor that is intelligent and of the active type has become an important topic in modern industries.
In recent years, due to the advance of silicon semiconductor technology, much attention has been attracted on the use of a Pd metaloxide-semiconductor (MOS) structure as a semiconductor hydrogen sensor. The reason for using the Pd metal in the hydrogen sensor lies in that Pd has a good catalytic activity and can dissociate the hydrogen molecule adsorbed to the surface into hydrogen atoms. A portion of the hydrogen atoms diffuses through the Pd metal and is adsorbed to the interface between the metal and the oxide layer. These hydrogen atoms, after polarization, cause a change in the Schottky barrier height between the oxide layer and the silicon semiconductor and thus the electrical properties of the device. In the early days, I. Lundstrom proposed a Pd/SiO
2
/Si MOS field effect transistor structure with a Pd gate [Lundstrom, M. S. Shivaraman, and C. Svensson, J. Appl. Phys., 46, 3876 (1975)]. After the hydrogen being adsorbed to the Pd gate, the altered threshold voltage and terminal capacitance are used as the two bases for the detection of hydrogen. However, the use of a three-terminal device to realize the functions of a two-terminal device not only increases the cost, but also has elevated process difficulties. Furthermore, the quality of the oxide layer will also influence the hydrogen detection capability. In addition to the problem of reliability, the quality of an oxide layer becomes unstable due to the growth of the thin oxide layer is contaminated by the ions or the increase of defects. This results in the surface state pinning of Fermi-level of silicon semiconductor. Therefore, Schottky barrier height is less influenced by the polarized hydrogen atoms and subsequently the hydrogen sensitivity is lower. Many researches focus on how to improve such a problem. For example, A. Dutta et al. [A. Dutta, T. K. Chaudhuri, and S. Basu, Materials Science Engineering, B14, 31 (1992)] used zinc oxide (ZnO) and L. Yadava et al. [L. Yadava, R. Dwivedi, and S. K. Srivastava, Solid-St. Electron., 33, 1229 (1990)] used titanium dioxide (TiO
2
) to replace the oxide layer of silicon dioxide. On the other hand, the use of a two-terminal type Schottky barrier diode seems to be a more intuitive approach. Without the unstable factors of the oxide layer, the sensitivity of the device to hydrogen has a significant improvement. Therefore, for example, M. C. Steelee et al. [M. C. Steele and B. A. Maciver, Appl. Phys. Lett., 28, 687 (1976)] proposed a Pd/CdS structure, and K. Ito et al. [K. Ito, Surface Sci., 86, 345 (1982)] proposed a Pd/ZnO structure. The using II-VI compound semiconductor as the material is mainly due to the less effect of surface states of II-VI compound semiconductor as compared to the polarized hydrogen atoms.
SUMMARY OF THE INVENTION
A primary objective of the present invention is to provide a highly sensitive hydrogen sensor, which comprises:
a semiconductor substrate;
an n-type semiconductor film formed on said semiconductor substrate; and
an anode and a cathode formed on the same surface of said semiconductor film and isolated from each other, wherein a first metal as said anode forms a Schottky contact with said semiconductor film and a second metal as said cathode forms an Ohmic contact with said semiconductor film, wherein a thickness of said first metal and a material of which said first metal is made enable a Schottky barrier height of said Schottky contact to decrease when hydrogen contacts an exposed surface of said first metal.
In a hydrogen sensor according to the present invention, the material and the thickness of said first metal enable hydrogen to be dissociated into hydrogen atoms when the hydrogen comes into contact with the exposed surface of said first metal. Also, said hydrogen atoms diffuse through said first metal, so said Schottky barrier height decreases.
Preferably, said first metal is Pd or Pd alloy, more preferably Pd. Said Pd, preferably, has a thickness of 2000 Angstrom to 5 micron.
In a hydrogen sensor according to the present invention, preferably, said semiconductor substrate is made of a semi-insulating InP material.
In a hydrogen sensor according to the present invention, preferably, said semiconductor film is an n-type III-V Group compound, more preferably, an n-type InP (n-InP). An appropriate doping concentration of said n-InP is of 1×10
16
to 5×10
17
cm
−3
, and an appropriate thickness thereof is 1000 Angstrom to 5000 Angstrom.
In a hydrogen sensor according to the present invention, said second metal preferably is an AuGe alloy. Said AuGe alloy preferably has a thickness of 3000 Angstrom to 5 micron.
In a hydrogen sensor according to the present invention, preferably, said anode has a C shape or a C-like shape. Said cathode has a shape corresponding to the shape of said anode such that said cathode is encompassed by said anode. Alternatively, said cathode has a C shape or a C-like shape and said anode has a shape corresponding to the shape of said cathode such that said anode is encompassed by said cathode.
In a Pd/InP hydrogen sensor made according to one of the preferred embodiments of the present invention, a Pd film is used as a catalytic metal to dissociate a hydrogen molecule into hydrogen atoms. The interface between the InP film and the Pd film being capable of adsorbing a large amount of hydrogen atoms is used to obtain a significant linear variation of the diode electrical properties, thereby detecting a low concentration of the hydrogen content.
In order to further elaborate the objectives, characteristics and merits of the present invention, a preferred embodiment together with related figures are disclosed hereinafter.
REFERENCES:
Dutta et al., “Deposition and Characterization on Zinc Oxide Thin Films for Hydrogen Sensor Devices”, Materials Science and Engineering B14:31-35, 1992.
Yadava et al., “A Titanium Dioxide-Based MOS Hydrogen Sensor”, Solid State Electronics 33:1229-1234, 1990.
Chen Huey-lng
Liu Wen-Chau
Pan Hsi-Jen
Fish & Richardson P.C.
National Science Council
Politzer Jay L
Williams Hezron
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