NEW PHOTORESISTORS

Recently, reports have appeared in the literature on new photoresistors sensitive to the near-infrared part of the spectrum. Photoresistors made of lead sulfide, as well as those made of thallium sulfide, have been described. Lead-sulfide photoresistors are news in instrumental technology, while the sulfur-thallium ones, apparently thanks to improvements in fabrication technology, surpass in their qualities the “talofides” (Kesa) used earlier.

In the note by Li and Parker¹ it is reported that lead-sulfide photoresistors were made during the war by Goodwin, Cashman, and others (in Germany), and that work on these photoresistors is now being carried out in England. Li and Parker, however, do not report the method of their fabrication. In Oxley’s short note² the following indications are given on this matter: “In one of the known processes,” says Oxley, “molar solutions of lead acetate, thiourea, and NaOH are taken in the volume ratio 4:3:22. All solutions are heated to 45°C. When they are mixed, the thiourea is added last. The required surface is formed by three precipitations, 30 minutes each.” Sometimes heating to 125°C in air or in vacuum was used, which, according to Oxley, is employed to adjust the resistance and increase sensitivity.

The data concerning the parameters of these photoresistors given by various authors (Li and Parker¹, Oxley and Cashman³) are close to one another. The dimensions of the lead-sulfide photoresistors are from 3 to 30 mm²; the internal resistances range from 0.1 to several MΩ. The sensitivity maximum (according to Oxley) lies at 2.7 µ, the threshold at 3.6 µ. At 0.4 µ the sensitivity falls to 20% of the maximum. Thus, the photoresistors described can be used in the visible and near-infrared regions. Their great advantage is that they are able to withstand intense irradiation without noticeable changes in characteristics. Linearity of response is maintained up to intensities of 40 photo-candles. The signal response remains practically constant for frequencies up to 5000 Hz. According to Oxley, at the temperature of dry ice, lead-sulfide photoresistors operate even more stably and

their sensitivity under these conditions is 100 times greater than at room temperature. Cashman also reports data on new photoresistors made of lead selenide—they too are sensitive in the visible and near-infrared region (up to \(1.45\,\mu\)), linear up to 0.02 foot-candles, although saturation is not reached even at 10,000 foot-candles.

For modulation frequencies not exceeding several thousand Hz, the signal-to-noise ratio remains almost constant and considerably exceeds that obtainable with Cs—O—Ag vacuum and gas-filled photocells or photomultipliers under the same test conditions.

Lee and Parker emphasize the applicability of lead-sulfide photoresistors for measuring rapidly varying processes, since the response time of these photoresistors does not exceed 0.1 millisecond. These photocells were used by Lee and Parker to determine rapid changes in surface temperatures in certain technical processes.

A. A. Ilyina

References

1. E. Lee a. R. C. Parker, Nature 158, 518 (1946).
2. C. L. Oxley, J.O.S.A. 36, No. 6, 357 (1946).
3. R. J. Cashman, J.O.S.A. 36, No. 6, 356 (1946).