274 INSTRUMENTATION IN SCIENTIFIC RESEARCH [Chap. 5 



the order of 1 mm 2 . The operation of the photodiocle can be under- 

 stood from the characteristic of Fig. (5-1)33. Without illumination 

 the barrier layer acts as a rectifier. The forward current (first quad- 

 rant) rises sharply with the applied voltage -\-E in the forward 

 direction. The influence of the illumination is small, since the for- 

 ward dark current is high already. The dark current in the reverse 

 direction — E is small, but the current in the reverse direction 

 increases strongly with increased level of illumination. The photo- 

 diode is operated in this region of voltages and currents, i.e., in the 

 third quadrant. 



Figures for sensitivity of photodiodes quoted in the literature 

 vary from less than 10 to about 50 m A/lumen. The spectral 

 response extends from the visible to the infrared region, about 2.2 /j,, 

 with a maximum relative sensitivity in the vicinity of 1.5 /u. The 

 output noise level is around 3 to 10 juV and exceeds that of photo- 

 voltaic cells; the noise for 1 cps band width decreases with increased 

 frequency. Increase of temperature causes an increase of the photo- 

 current as well as of the dark current. The use of an a-c supply 

 voltage for the operation of barrier-layer cells has been described 

 by Sargrove. 1 



The combination of two photodiodes, back to back, forms a photo- 

 transistor. It can be connected in any one of the customary circuits 

 for npn transistors, the output current being controlled by light 

 falling into the central p region. Both the photocurrent and the 

 dark current appear amplified at the transistor output. 



5-14. Thermal-radiation Transducers 



Thermal-radiation transducers are primarily applicable to the 

 measurement of infrared radiation, where the energy of the single 

 quantum is too small to cause photoelectric effects. All thermal- 

 radiation transducers consist essentially of a thin metal strip which 

 absorbs radiation, so that its temperature is increased. The tem- 

 perature is measured either (a) with a thermoelement, (b) through 

 the accompanying variation of resistance with temperature (bolom- 

 eters), or (c) by increase of thermionic emission from a filament. 



a. Radiation Thermoelements. The radiation thermoelement 

 shown schematically in Fig. (5-1)38 consists of a thin disk F on 

 which a thermoelement T is fastened. Radiation striking the disk 

 is absorbed and causes an increase of the receiver temperature. The 

 system comes to equilibrium when the power dissipated from the 



1 J. A. Sargrove, J. Brit. Inst. Radio Engrs., 7, 86 (May-June, 1947). 



