Lead Sulfide Photosensitive Resistor

Abstract

FOR missile instrumentation and sensing applications, there are unique advantages in the simplicity of the Kodak Ektron Detector, a photosensitive resistor. The Kodak Ektron Detector utilizes the peculiar electrical properties of lead sulfide. .Normally formed on a 0.30-in.-thick glass blank, it consists of a rectangular deposit of lead sulfide and gold electrodes deposited a t the edges of the sensitive area. Complex, exact arrays and mosaics of such resistors can be produced. The basic unit is overcoated with plastic for protection. Kodak Ektron Detectors are rugged and can be very small. Although they respond to x-ray radiation down to a few Angstroms in wavelength, their useful photosensitivity is usually considered to range from 0.25 micron (2500 Angstroms) to 3.5 microns (35,000 Angstroms). In the visible region, the detector responds to a 2500 K tungsten light source about the same as a red-sensitive gas-filled phototube of comparable sensitive area under comparable conditions. Though the detector competes with phototubes and photomultipliers when exposed to tungsten light, it reaches maximum sensitivity at a wavelength of about 2 microns in the infrared region. Operating at room temperature at 2 microns, the Kodak Ektron Detector gives hundreds of times the response of a good laboratory bolometer. At —40 C, its sensitivity at 2 microns increases about 25 times. The cell's response to chopped or pulsed light sources is usable from steady illumination to as high as 10,000 radiation pulses per sec. In addition to their exceptional spectra response, Kodak Ektron Detectors are characterized by high signal-to-noise ratio, time constants in the range of 400 to 1000 microsec under normal conditions, dark resistance of 0.2 to 0.8 megohm for any square cell, and a negative coefficient of resistance. In a uniform radiation field, the signal-to-noise ratio at a given voltage varies directly with the square root of the cell area. Since the most interesting properties of these photoresistors are associated with infrared response, investigations have been chiefly in the wavelength region from 1 to 3 microns. Use of geometry is made to obtain a desired dark-resistance value. Choice of cell configurations ranges over an extended scale of resistance values. Because of the permissible variety in cell geometry, high-impedance detectors need not be long, narrow units, but may be folded'' into compact rectangular patterns. Fig. 1 depicts three detectors of the same over-all size but with impedances of (a) 0.002 megohm, (b) 0.2 megohm, and (c) 20 megohms, respecFig. 1