Generally, there are thermal type infrared sensors (e.g., pyroelectric elements or thermopiles) that employ temperature changes generated by the absorption of infrared energy, and quantum type infrared sensors that employ changes in conductivity, or in electromotive force, that are generated by electrons excited by incident light energy. The thermal type, however, which can be operated at room temperature, has disadvantages in that it has no wavelength dependency and a low sensitivity and in that its response speed is low. On the other hand, the quantum type, although it must be cooled to a low temperature, has characteristics such as wavelength dependency and high sensitivity and a response speed that is high.
Typical examples for the use of infrared sensors are as human sensors that can detect human beings and that can automatically turn on or off home electrical appliances, such as lights, air conditioners or TVs, and as surveillance sensors used for security. Recently, very much attention has been drawn to infrared sensors because of a desire to save energy and to use them for home automation, security systems, etc.
An infrared sensor currently used as a human sensor is a pyroelectric infrared sensor that employs pyroelectric effects. As described in non-patent document 1, a pyroelectric infrared sensor is easily affected by electromagnetic noise and thermal fluctuation, because the impedance of a pyroelectric element is extremely high. Therefore, a shield such as a metal Can package is required. In addition, a large R or C is necessary for an I-V conversion circuit, so that a reduction in size is difficult.
On the other hand, for a quantum infrared sensor, HgCdTe (MCT) or an InSb material has been used as a typical material. When MCT or the InSb material is used, the sensor must be cooled using liquid nitrogen or liquid helium, or by using electronic cooling that employs the Peltier effect, etc. Generally, with a quantum infrared sensor that is cooled, a high sensitivity 100 times or more that of a pyroelectric sensor can be obtained. Furthermore, the device resistance is small, i.e., several tens to several hundreds Ω, and is not easily affected by electromagnetic noise and thermal fluctuation. It should be noted, however, that a strong metal package is used in order for it to be cooled to a low temperature.
Moreover, since an MCT sensor provides the highest sensitivity of all the quantum infrared sensors, the Hg vapor pressure used for this sensor is high. Therefore, the control and the reproduction of the composition for crystal growth are difficult, and a uniform film is not easily obtained. In addition, during the element production process, the mechanical strength is low, and Hg diffusion or leakage problems have arisen.
For the InSb material, a mixed crystal of InAsxSb1-x has been studied in consonance with a wavelength to be detected. For example, use of a method (see patent document 1) has been attempted whereby an InSb substrate is used, and one part of the InSb is replaced with As to obtain an epitaxial growth on the substrate.
Further, a monolithic structure (see patent document 2) has been proposed wherein an infrared sensor is grown on a base member in which a reader and a signal processing circuit are integrated. However, use of the technique whereby a compound semiconductor thin film, which is an infrared sensor, is grown on a signal processing circuit is extremely difficult, and a film having a quality that makes it available for use as a practical device is not easily obtained. In addition, a problem encountered is that heat generated by activating the signal processing circuit becomes thermal fluctuation noise, and provides an error signal for an infrared sensor that is monolithically formed on the signal processing circuit. Therefore, in order to suppress the affect of this thermal fluctuation, the entire sensor must be cooled using liquid nitrogen, etc. Such a cooling method is not appropriate for a human sensing application that is to be used for common home electrical appliances and lights.
Patent Document 1: Japanese Patent Application Laid-Open No. Sho 53-58791
Patent Document 2: Japanese Patent Application Laid-Open No. Hei 2-502326
Non-patent Document 1: Kunihiko Matsui, “Practical Guidance for Sensor Uses 141”, CQ Publisher, May 20, 2001, page 56
Non-patent Document 2: A. G. Thompson and J. C. Woolley, Can. J. Phys., 45, 255 (1967)