PHOTOELECTRIC DETECTORS 



Area n-p junctions can also be formed in lead sulphide by ensuring a 

 sulphur deficient surface on a /?-type single crystal. These photodiodes have 

 spectral characteristics similar to those for the photoconductive type and 

 have the great advantage of a time constant of about 1 //sec: the sensitivity 

 is correspondingly lower. These cells are of the photovoltaic type and operate 

 v/ithout an external power source. 



Cadmium-selenide cell 



A useful photoconductive cell with a high internal amplification has been 

 developed by E. Schwartz of Messrs Hilger and Watts, Ltd. It consists of a 



^,000 



6,000 8/)00 



Wavelength 



lopoo 



A 



Figure 28.47 Spectral sensitivity of t/ie Schwartz cadmium-selenide cell 



{Hilger and Watts, Ltd.) 



polycrystalline layer of treated cadmium-selenide on an insulating base, the 

 whole being placed in a small protective mount. The most sensitive cells 

 have a photosensitive surface of width only 0-2 mm and a length of 1-5 or 

 10 mm, and it is usually necessary to focus the light on to this narrow area. 

 Double cells in a single mount are also produced for comparison work or for 

 use as movement detectors. 



The spectral sensitivity of the Schwartz cell is shown in Figure 28.47. The 

 sensitivity extends below 400 m//, through the ultraviolet, down to X-ray 

 wavelengths. There is a sharp peak at 710 m/^, with a fairly rapid cut-off 

 thereafter. 



As these cells are of the photoconductive type they require a source of 

 power to activate them; however, only low voltages are required. The 

 characteristics of a cell with a sensitive area of 1 by 0-2 mm are shown in 

 Figure 28.48 for a variety of illumination intensities. It can be seen that for 

 a high illumination of, say, 1,000 lux (— 200 ^Im on the cell) about 1 mA 

 will flow with a voltage of 4. This is equivalent to a sensitivity of 5 A/lm, 

 with a dark current of 0-6 //A. At the low illumination level of 1 lux (=0-2 

 [Am. on the cell) about 50 /^A will flow with an applied voltage of 30. This 

 gives a sensitivity of 250 A/lm with a dark current of 5 [lA. These cells have 

 therefore a very high internal amplification and they can be used for energiz- 

 ing relays, etc., directly. If the incident energy cannot be focused as a small 



377 



