Active solid-state devices (e.g. – transistors – solid-state diode – Responsive to non-electrical signal
Reexamination Certificate
2001-08-09
2003-09-30
Jackson, Jerome (Department: 2815)
Active solid-state devices (e.g., transistors, solid-state diode
Responsive to non-electrical signal
C257S077000, C257S769000, C257S565000
Reexamination Certificate
active
06627964
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to gas sensors for detecting concentrations of H
2
, NH
3
, H
2
S, hydrocarbons and the like included in sample gases and, particularly, to a gas sensor using a pn junction diode as a detecting device. The sensor of the present invention is preferably adapted for detecting leakage of hydrogen from a fuel cell and the like, controlling or monitoring an ammonium selective reduction catalyst for use in a power plant, a co-generation system and the like, or monitoring a three way catalyst or an NOx adsorption catalyst and the like of an automotive vehicle.
2. Description of the Related Art
Gas sensors in which oxide semiconductors serve as detecting elements have been used for measuring concentrations of sample gases having hydrogen atoms, hydrocarbons and the like. One measurement principle is to detect an increase in conductivity caused by electron transport from a molecule having a hydrogen atom and the like adsorbed in an oxide semiconductor as a change in voltage. As an example, Japanese Patent Laid-Open No. 87291/1979 discloses a gas sensor in which an oxide semiconductor thin layer is formed on a surface of a pn junction diode via a dielectric thin layer.
However, the gas sensor employing an oxide semiconductor as a detecting element is provided with a heater for activating the oxide semiconductor. When a highly inflammable gas such as hydrogen and the like is used, there is a possibility of fire upon contact with the heater due to gas leakage. For this reason, as in the case of a fuel cell and the like, it is not preferable to use a gas sensor employing an oxide semiconductor as a detecting element in a system containing a highly inflammable a gas such as hydrogen and the like. Further, in the above case, there is another problem of large electricity consumption because of the need for powering the heater.
A gas sensor which uses a diode fabricated by finely processing a semiconductor has been studied as a detecting element which does not require a heater. A gas sensor based on a Shottky diode, a MIS (Metal-Insulator-Semiconductor) diode, a FET (Field Effect Transistor) diode or the like which utilizes a change in the rectification characteristics of a semiconductor due to absorption of sample gas components has been proposed. The measurement principle involves measuring a change in potential of the diode in a dipole layer caused by an adsorbed sample gas in an electrode layer having a catalytic effect.
International Patent Publication No. 505911/1998 discloses a gas sensor using an MIS-type diode having improved long-term stability and response by providing an intermediate layer such as TaSi
x
and the like in an electrode layer having catalytic activity to a sample gas. Japanese Patent Laid-Open No. 222027/1994 discloses a Shottky diode type gas sensor using diamond as a semiconductor.
Recently, various types of gas sensors in which SiC (silicon carbide) is used as a semiconductor have been investigated. SiC is more advantageous than Si in its voltage breakdown characteristics, response time and the like. Further, since Si has a band gap of about 1.1 eV, Si can not be used under circumstances in which the temperature is over 150° C. In contrast, since SiC has a large band gap of about 2.2 eV to about 3.3 eV (3C—SiC, 6H—SiC and 4H—SiC have band gaps of 2.23 eV, 3.03 eV and 3.26 eV, respectively where C represents cubic; H represents hexagonal and wherein the numerals, for example, 3, 4, 6 and the like, before C and H represent recurring periods of a crystal structure), SiC is advantageous in that it can be used even at a high temperature of about 600° C. For this reason, SiC is expected to be applied to a gas sensor which can be operated even at a high temperature.
A Shottky diode-type gas sensor has been studied at NASA Lewis Research Center. Further, a capacitor type or a shottky diode-type sensor has been studied at Linkong University, Sweden (Movement of Advanced Microsensor Process Technologies, IEEJ Technical Report, No. 727, pp. 18-22, 1999).
On the other hand, it is known that a surface of a SiC single crystal wafer has a surface-roughness on the order of nanometers (Proceedings of 60th Annual Meeting of JSAP, No. 1, 3p-R-1, p. 335, 1999). Further, it is known that a hollow through-hole called a “micropipe defect” is liable to be generated (Electronic Materials, pp. 57-61, November, 1998).
In order to fabricate a gas sensor using a SiC single crystal, strict process control is necessary so as to avoid the influence of surface roughness or a surface defect such as a micropipe and the like on the SiC single crystal.
Further, there is a concern of a change in diode characteristics of the Shottky diode-type gas sensor with time in long-term service under a high-temperature condition due to deterioration of the state of an interface between an electrode and a Shottky contact of a semiconductor. This change with time in long-term service is also a concern in the case employing a Si single crystal.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a gas sensor having a simple structure, exhibiting a small change of diode characteristics with time in long-term service in which either a Si single crystal or SiC single crystal is employed, and which is capable of detecting the concentration of a gas containing a molecule having a hydrogen atom, for example, H
2
, NH
3
, H
2
S, a hydrocarbon and the like.
Accordingly, the present invention provides a pn junction diode-type gas sensor comprising:
a first semiconductor layer which is either a p-type or n-type semiconductor layer;
a second semiconductor layer of a type different from said first semiconductor layer;
a semiconductor substrate on which said first semiconductor layer and said second semiconductor layer are disposed;
a first ohmic electrode in electrical contact with said first semiconductor layer;
a second ohmic electrode in electrical contact with said second semiconductor layer; and
a catalytic layer containing a metallic catalytic component disposed on one of said first ohmic electrode and said second ohmic electrode, said catalytic component dissociating hydrogen atom from a molecule having hydrogen atom in contact with the catalytic layer.
The pn junction diode-type gas sensor of the present invention comprises a catalytic layer containing a metallic catalytic component which dissociates hydrogen atom from a molecule having hydrogen atom. The catalytic layer is disposed on an ohmic electrode which works as a detection electrode. By using such a pn junction diode as a detecting element, it becomes possible to detect the concentration of a gas containing a molecule having a hydrogen atom. Ordinarily, the Shottky diode-type gas sensor could not obtain rectifying characteristics unless a thin, uniform electrical insulating film having a thickness of about 1 nm to about 10 nm was formed between the semiconductor and the electrode. When a defect is present in the electrical insulating film, a dielectric breakdown over time in long-term service will occur. However, it is very difficult to industrially prepare a thin, uniform film having a thickness of about 1 nm to 10 nm whereupon the production cost of the Shottky diode-type gas sensor becomes high. Particularly, this is conspicuous when SiC which is liable to have a defect such as a micropipe and the like therein is used as a semiconductor. In contrast, the pn junction diode-type gas sensor of the present invention does not always require an electrical insulating film of SiO
2
or the like between the semiconductor and the electrode unlike the Shottky diode-type gas sensor. Under these circumstances, the pn junction diode-type gas sensor has the advantage of lower production cost than a Shottky diode-type gas sensor even when a compound semiconductor substrate such as SiC which is liable to contain a defect such as a micropipe is employed therein. Further, since dielectric breakdown of the electrical insulating film under a high temperature cond
Nakashima Kenshiro
Okuyama Yasuo
Oshima Takafumi
Yokoi Hitoshi
Jackson Jerome
NGK Spark Plug Co. Ltd.
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