P-n junction sensor

#In some situations, it is desirable to monitor emissions such as the exhaust from smoke stacks, engines, industrial processes, etc. to determine the presence and concentration of particular gases and/or aerosols. Sensors employed for such purposes are often subjected to harsh conditions, for example, high temperature and/or corrosive environments. However, current commercially available gas sensors typically have designs which suffer from temperature limitations and are susceptible to corrosion. Consequently, these commercially available sensors are usually not suitable for use in such harsh environments. In addition, commercially available sensors often have further drawbacks, for example, sluggish response time, large sensor size, inconsistent quality and performance, performance degradation over time, high power requirements and high cost.

#The present invention provides a sensor for detecting the presence or absence of selected substances in an atmosphere such as gases, chemical species, aerosols, etc. Some embodiments of the present invention sensor can operate in high temperature and corrosive environments, can be small in size with low power requirements, can have a fast response time, can be low cost, and can be manufactured with consistent quality and provide consistent performance.

#One embodiment of the present invention is a gas sensor which includes a p-n junction for subjecting under a reverse electrical bias. A gas sensitive conductive layer is formed across the p-n junction for providing an alternative conductive path across the p-n junction. The conductivity of the conductive layer in the presence of a selected gas is different than in the absence of the selected gas, wherein the conductivity of the conductive layer is indicative of the presence or absence of the selected gas.

#The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

#FIG. 1 is a schematic drawing of an embodiment of the present invention sensor.

#FIG. 2 shows the p-n junction depletion zone and a crack/defect in the gas sensitive layer in the embodiment of FIG. 1.

#Referring to FIG. 1, sensor 10 is capable of sensing the presence and/or concentration of selected substances within an atmosphere or environment, such as gases, chemical species, aerosols, etc. In many applications, sensor 10 is employed as a gas sensor. Sensor 10 includes a semiconductor substrate 11 having an n-type semiconductor region or layer 12 abutting a p-type semiconductor region or layer 14 with a p-n junction 16 therebetween. Ohmic contacts 20 and 22 are formed on the n-type region 12 and the p-type region 14, respectively. A voltage source 24 is electrically connected to the n-type region 12 and p-type region 14 via lines 26 and 28, which are connected to respective contacts 20 and 22. Voltage source 24 provides a reverse electrical bias across the p-n junction 16. A substance sensitive conductive layer 18 is formed over the substrate 11 and across an exposed region of the p-n junction 16 to provide an alternative conductive path past or across the p-n junction 16. The conductive layer 18 has a conductivity that varies depending upon the concentration of a particular substance, such as a gas or gases, chemical species, aerosols, etc., coming into contact with conductive layer 18, and thereby modulates the flow of current. A measurement device such as a current meter 15 measures the flow of current between contacts 20 and 22, thereby measuring the flow of current through conductive layer 18. The concentration and/or presence of the selected substance can be determined by the level of measured current.

#In operation, when voltage source 24 subjects the p-n junction 16 to a reverse electrical bias, very little p-n junction current 32 (FIG. 3) passes from the n-type region 12 to the p-type region 14 through the p-n junction 16. The p-n junction 16 has a very short or narrow space charge width or depletion zone 19 (FIGS. 2 and 3), typically 0.5 μm or less. Referring to FIG. 3, although very little p-n junction current 32 flows through the p-n junction 16, a parallel or bypass leakage shorting current 34 flows from the n-type region 12 to the p-type region 14 through conductive layer 18. The bypass current 34 only has to flow around the depletion zone 19 through conductive layer 18 to flow past or across the p-n junction 16. The conductivity of the conductive layer 18 varies depending upon the concentration of a selected substance coming into contact with conductive layer 18. The conductivity is affected by the absorption, desorption, or reaction of the substance at the exposed conductive layer 18. As a result, the material from which conductive layer 18 is formed, is chosen to respond to a specific substance or family of substances to be monitored. By measuring the current I passing between contacts 20/22 with current meter 15, the current 34 through conductive layer 18 is measured. Measurements of different currents 34 through conductive layer 18 are correlated to different concentrations as well as the presence and absence of the selected substance or substances. Typically, current levels are predetermined for particular concentrations. If needed, the p-n junction current 32 is accounted for. Also, each sensor 10 can be calibrated to account for variations occurring in the manufacturing process.

#The very narrow depletion zone 19 (FIG. 3) of the p-n junction 16 around which the leaking shorting current 34 flows, allows the conductive layer 18 to be unaffected by most cracks or defects 17 (FIG. 2) in conductive layer 18. As seen in FIG. 3, the leaking shorting current 34 within conductive layer 18 can flow around the p-n junction 16 in close relationship thereto such that any cracks or defects 17 that are away from the p-n junction 16 have little effect on the flow of the leaking shorting current 34. Consequently, cracks and defects 17 can form in the conductive layer 18 during manufacturing and/or during use, and not affect the operation of sensor 10. Typically, only the cracks or defects 17 that are directly over the p-n junction 16 may possibly affect the performance of the sensor 10. Usually, such cracks or defects 17 would have to extend transverse to the flow of the leaking shorting current 34 to affect the flow of current 34. Since most cracks or defects 17 are typically short and would not extend the whole lateral distance across conductive layer 18 while directly over p-n junction, the leaking shorting current 34 is usually able to flow around the crack/defect 17. Cracks or defects 17 extending in the same direction as the leaking shorting current 34 might not affect the flow of current 34. If the cracks or defects 17 are formed during the manufacture of sensor 10, the sensor can be calibrated to compensate for any effects caused by the cracks or defects 17.