Journal of Electroceramics 2:4, 229±242, 1998 # 1998 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands. Pyroelectric Sensors M.H. LEE, R. GUO & A.S. BHALLA Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Submitted April 2, 1998; Revised April 2, 1998; Accepted July 28, 1998 Abstract. Status of the pyroelectric sensors is reviewed in brief. In this content the existing new materials, their pyroelectric properties, and ®gure of merit for various pyroelectric devices are compared. The assemblage components of a single pyroelectric sensor are reviewed and the discussions are extended to incorporate the large area and the multi-element devices suitable for wide range of applications. Keywords: pyroelectric sensors, pyroelectricity, infrared sensors, ferroelectric materials and devices 1. Introduction Pyroelectric sensors, in principle, work as a thermal transducer. The pyroelectric sensing element con- verts the non-quanti®ed thermal ¯ux into the output measurable quantity like charge, voltage or current. Therefore in designing and fabricating the sensor, the knowledge of the basic characteristics of the materials is desirable as those provide the under- standing of several unforeseeable noise sources which occur from microphonic effects, clamping, mechanical strain, mounting of sensing elements, and packaging. This also provides the guidance in bonding of the sensing element and other thermal ef®ciency related problems. Thus there are two major considerations when fabricating pyroelectric sensors namely, (i) materials and (ii) design parameters. In this article we discuss the sensor topic in the context of (i) characteristics of pyroelectric materials relevant to sensors and (ii) basic sensor design. In the ®rst part, the basics of the pyroelectric effect, evaluation of pyroelectric properties and material ®gures of merit for various pyroelectric devices are brie¯y discussed. In the second part, comments are made on the status of different components of a pyroelectric sensor. The comments are extended to cover the various intrinsic and extrinsic noise factors that limit device performance and the design approaches to fabricate large and multi-element pyroelectric devices. 1.1. Pyroelectric Effect The pyroelectric coef®cient p of a material under constant stress and electric ®eld is de®ned by the expression p  @ P @T   E;s 1 where P is the polarization, T the temperature, E the electric ®eld, and s the elastic stress [1±3]. The pyroelectric effect can be observed only in those materials whose point group symmetry is consistent with the vectorial property of the polariza- tion. Thus crystalline materials whose structures belong to the ten polar point groups namely, 1, m, 2, mm2, 3, 3m, 4, 4mm, 6, 6mm and ceramics, polymeric and composite materials whose structures belong to the textural or basic Curie point groups (1,1m) are pyroelectric materials. Thus the pyroelectrics can be classi®ed into two main categories, (i) non-ferro- electric pyroelectrics, those whose polarization can not be switched by an application of external electric ®eld (including some semiconductors and biological materials) and (ii) ferroelectric pyroelectrics those