824 IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, zyxwvuts VOL. 27, NO. 5, SEPTEMBERIOCTOBER 1991 Pyroelectric Detectors and Their Applications Akram Hossain and Muhammad H. Rashid Abstract-This paper reviews the principles of operations of pyroelectric detectors. The equations for current responsivity, voltage responsivity, noise, noise equivalent power, and detec- tivity are derived from the fundamental thermodynamic model of a pyroelectric detector. The frequency-dependent current and voltage responsivities are explained. Factors influencing the choice of pyroelectric material for various applications are briefly summarized. The possibilities of various new applications and the developments of pyroelectric detectors are also briefly dis- cussed. A laboratory prototype demonstration system has been built using an ELTEC detector type 5192. For a high signal-to- noise ratio, an electric charge integration technique is applied. High-precision operational amplifiers are employed for high-qu- ality signal conditioning. The pyroelectric detector consists of two parallelly opposed dual lithium tantalate pyroelectric single crystals with a JFET source follower sealed in a standard TO-5 housing. I. INTRODUCTION HE GENERATION of an electric dipole moment in an T insulating material due to a homogeneous change in temperature is called pyroelectricity. An inhomogeneous change in temperature in a piezoelectric material produces stress that may create false pyroelectricity. Therefore, the true pyroelectricity can be found only in materials having a unique polar axis. A pyroelectric material generates electric dipole moments, and this in turn generates current. This current is proportional to the change in the temperature of the crystal rather than the temperature itself. Thus, pyroelectric detectors are not thermoelectric detectors such as thermocou- ples. For a pyroelectric effect to exist in a material, the reversibility of electric dipole (polarization) is not necessary. Thus, all ferroelectric materials are pyroelectric, but the converse is not necessarily true. Pyroelectrics are basically high-frequency thermal detectors because the maximum re- sponse can be obtained at a time that is shorter than the thermal relaxation time. A pyroelectric detector can be em- ployed to detect any radiation that results due to a change in the temperature. 11. THEORY OF PYROELECTRICITY There are various kinds of electric dipole moments that exist in dielectric materials. One of these kinds of dipole moments is created by the asymmetric distribution between Paper IUSD 8940, approved by the Industrial Control Committee of the IEEE Industry Applications Society for presentation at the 1989 Industry Applications Society Annual Meeting, San Diego, CA, October 1-5. Manuscript released for publication February 20, 1991, A. Hossain is with the Electrical Engineering Technology Department, Purdue University Calumet, Hammond, IN 45323-2094. M. H. Rashid is with the Engineering Department, Indiana University Purdue University, Fort Wayne, IN 46805-1499. IEEE Log Number 9100921. the unlike partners of molecules. These types of dipoles are called permanent electric dipole moments. The materials, which have these types of permanent dipole moments, are called pyroelectric materials. These permanent dipole mo- ments exist even in the absence of an external electric field. The behavior of these materials is fundamentally different from that of normal thermal detectors because they exhibit spontaneous polarization. When a pyroelectric crystal experi- ences a homogeneous change in temperature, these dipoles orient themselves in one direction and maintain a net polar- ization. The net polarization vector is the dipole moment per unit volume, and it is also proportional to the charge per unit volume. These charges appear at the surface of the crystal electrode. When a pyroelectric crystal remains at equilibrium (i.e., the crystal experiences no change in temperature), free charges at the crystal surface are neutralized by an internal depolarization field. Thus, if the change in crystal tempera- ture is larger the polarization vector will be larger. This means a larger amount of charge binds at the electrode. An electric polarization is the amount of charge freed at the electrode. The change in electric charge per unit time is the current. Thus, a pyroelectric material produces current as it experiences a change in temperature. If a change in polariza- tion zyxwvut dP, is produced by a change in temperature dT in time dt, then the pyroelectric current per unit area of the crystal can expressed as [l], [2] dP, dT I = - - - zyxw ’’ [ dT][ dt] where T temperature t time P, polarization per unit volume. The quantity dP, / d T is known as the pyroelectric coef- ficient and is denoted by p, dTldt is the rate of change of temperature. Thus, a pyroelectric material works as a charge generator due to homogeneous change in temperature. Al- though the physical concept of the pyroelectric effect is very simple, the physics and thermodynamics associated with the behavior are quite complicated. Figs. 1 [3] and 2 [3] are typical illustrations of pyroelectric spontaneous polarization versus temperature and pyroelectric coefficient versus tem- perature characteristics, respectively. Table I [ 11, [2], [9] shows the pyroelectric coefficients and other pertaining prop- erties of a few pyroelectric materials. It can be noticed that dP, / dT has the highest value at the Curie temperature. Since a temperature variation can easily be achieved by means of electromagnetic radiation, the response of a pyro- electric device due to a step function of electromagnetic 0093-9994/91/0900-0824$01.00 zyxwvut 0 1991 IEEE