Combustible gas diode sensor

Details Combustible gas diode sensor Combustible gas diode sensor

1998-02-20

2001-10-09

Larkin, Daniel S. (Department: 2856)

Measuring and testing

Gas analysis

Detector detail

C073S031050, C073S023310, C422S094000

Reexamination Certificate

active

06298710

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a combustible gas diode sensor including a SiC semiconductor substrate on top of which an AlN layer and a catalytic metal “gate” electrode are deposited. The resulting devices can be operated in either a D.C. forward conduction or an A.C. reverse bias mode.
BACKGROUND
MOS combustible gas sensors operate by catalytic oxidation of combustible gases at the “gate”. Substantial efforts have been expended in recent years towards the development of combustible gas sensors using semiconductor MOS technology.
Generally, the MOS gas sensor consists of a semiconductor substrate with an ohmic contact on one side and with the other side covered by a SiO
2
insulating layer with a metal gate on top. The metal gate is composed of a metal capable of catalyzing the oxidation of combustible gases. As a result of catalytic redox reactions on the gate surface, certain atomic and molecular species are generated which can diffuse through the porous gate to the metal gate/insulator interface where they can ionize. These ions can penetrate through the insulator thereby changing the potential distribution across the device. This changes the potential of the insulator/semiconductor interface and thus the depletion layer inside the semiconductor which in turn shifts the voltage dependent A.C. admittance characteristic of the device along the voltage axis.
In order to be sensitive to combustibles other then H
2
, the catalytic gate and, therefore, the device have to be operated at temperatures above 400° C., requiring the use of a wide band gap semiconductor, such as SiC instead of Si. However, at such high temperatures the SiO
2
layer becomes less insulating as the ionic charges within the layer become mobile. Under these conditions, the device acts as a true capacitor only when biased in depletion (e.g., for n-type SiC, the gate voltage is negative with regard to the back contact). In accumulation, it begins showing some D.C. conduction. However, as the conduction process in the SiO
2
is different from that in the SiC, there will be a finite voltage (the barrier potential) at which there is onset of this forward conduction. This barrier potential will depend on the charges injected into the SiO
2
insulator by the chemical processes at the gate. As changes in this barrier voltage are directly reflected as changes in the current/voltage D.C. characteristic in the forward direction, a sensor response can be obtained by measuring changes in this characteristic as a function of changes in the combustible concentration near the catalytic gate. However, the mobility of charges in SiO
2
is still relatively low. Therefore, to obtain a reasonable forward current with reasonable applied voltage, the thickness of this SiO
2
layer has to be very small, leading to breakdown instabilities.
SUMMARY OF THE INVENTION
The present invention provides a combustible gas sensor consisting of a diode structure which includes a silicon carbide semiconductor substrate having a metal back contact juxtaposed thereto. An AlN layer is deposited onto the SiC semiconductor substrate and a thin porous catalytically active metal electrode “gate” is deposited on the AlN layer.
This diode structure allows the detection of combustibles, such as hydrocarbons and carbon monoxide in a gas flow in a wide concentration range with the adjustment of certain parameters, such as the oxygen concentration. This device is compatible with semiconductor electronic technology and is substantially more robust than devices requiring membrane support for low thermal mass as the micro-calorimeter. The device further can be operated in either a forward conduction D.C. or a reverse bias A.C. response mode, the former not requiring radio frequency techniques.
In the D.C. mode embodiment, there is provided a method of detecting combustibles in a gas stream by placing a forward biased diode sensor in contact with the gas stream, wherein the diode sensor comprises a silicon carbide semiconductor substrate; a metal back contact juxtaposed to the silicon carbide semiconductor substrate; an AlN layer deposited onto the silicon carbide semiconductor substrate; and a catalytically active metal “gate” electrode deposited on the AlN layer. The method further includes the step of applying a constant forward current to the diode sensor and measuring changes in the forward voltage drop across the diode sensor as the response to changes of the combustible concentration in the gas stream. Alternatively, a constant forward bias may be applied across the sensor measuring changes in the forward current through the device as the response to changes of the combustible concentration in the gas stream.
In the A.C. mode, the present invention also provides a method of detecting combustibles in a gas stream, by placing the reverse biased diode sensor in contact with the gas stream, wherein the diode sensor comprises a silicon carbide semiconductor substrate; a metal back contact in mating engagement with the silicon carbide semiconductor substrate; an AlN layer deposited onto the SiC semiconductor substrate; and a catalytically active metal gate deposited on the AlN layer. The method next involves the step of measuring the capacitance of the diode sensor at frequencies between 10 and 3000 KHz. A constant reverse bias voltage is applied across the diode sensor. Lastly, the method requires the step of detecting a change in the capacitance in response to a change of the combustible concentration in the gas stream.


REFERENCES:
patent: 4450428 (1984-05-01), Ohta et al.
patent: 4928513 (1990-05-01), Sugihara et al.
patent: 4984446 (1991-01-01), Yagawara et al.
patent: 5417821 (1995-05-01), Pyke
patent: 5523589 (1996-06-01), Edmond et al.
patent: 5591321 (1997-01-01), Pyke
patent: 5698771 (1997-12-01), Shields et al.
patent: 5942674 (1999-08-01), Logothetis et al.
patent: 63-71647 (1988-04-01), None
patent: WO /09534 (1996-03-01), None
“Improved AIN/SiC Device Structures and Ohmic Contact Process”, IBM Technical Disclosure Bulletin, vol. 38, No. 09, Sep. 1995, pp. 415-419.*
Epitaxial Growth of A1N By Plasma-Assisted, Gas-Source Molecular Beam Epitaxy, by L.B. Rowland et al, J. Mater. Res., vol. 8, No. 9, Sep. 1993, pp. 2310-2314.
Epitaxial Growth Of Cubic A1N Films On (100) and (111) Silicon By Pulsed Laser Ablation, by Wen-Tai Lin et al, Appl. Phys. Lett., 66(16), Apr. 17, 1995, pp. 2066-2068.
Gas Sensitive Field Effect Devices For High Temperatures, by A. Baranzahi et al, Sensors & Actuators B 26-27, 1995, pp. 165-169.
Epitaxial Growth Of A1N Thin Films On Silicon (111) Substrates By Pulsed Laser Deposition, by R.D. Vespute et al, J. Appl. Phys. 77 (9), May 1, 1995, pp. 4724-4728.
High Quality A1N And GaN Epilayers Grown On (00.1) Sapphire, (100), and (111) Silicon Substrates, by P. Kung et al, Appl. Phys. Lett. 66 (22), May 29, 1995, pp. 2958-2960.
Fast Chemical Sensing With Metal-Insulator Silicon Carbide Structures, by Peter Tobias et al, IEEE Electron Device Letters, vol. 18, No. 6, Jun. 1977, pp. 287-289.

Affiliated with

Also associated with

Rating

Combustible gas diode sensor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Combustible gas diode sensor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Combustible gas diode sensor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2575199