Sec. 5-1] RADIATION TRANSDUCERS 263 



free to move through the crystal lattice and form a current. It re- 

 mains in this state for the duration r, the statistical average lifetime 

 of the carrier in the conduction band. The total increase of the num- 

 ber of free carriers, caused by F excitations/sec, is 



N = tF 



The lifetime r of the carriers is determined by their rate of recombi- 

 nation ; the lifetime of commonly used photoconductors ranges from 

 10- 5 to 10- 2 sec. 



The photocurrent in a photoconductor can be expressed by 



eFr 



P m 



-* r 



where T r is the transit time of the carriers between the electrodes. If 

 an ohmic contact exists between the electrodes and the photocon- 

 ductor (i.e., in the absence of a barrier layer), new carriers can move 

 into the photoconductor as the primary carriers move through the 

 photoconductor and out of the space between the electrodes. Since 

 these new carriers take the place of the former, the carriers can be 

 considered to be still alive. The gain of the photoconductor, i.e., the 

 number of charges passing through the photoconductor per photon 

 excitation, is 



T 



J T 



The gain can range from less than unity to several orders of magni- 

 tude higher than unity. Gains of more than 10 4 have been observed. 1 

 transfee function. In general, the resistivity decreases and the 

 photocurrent increases with increasing incident light flux. The trans- 

 fer function, i.e., the characteristic of photocurrent versus incident 

 light intensity, can follow a wide variety of patterns; it can increase 

 linearly with the light intensity (t>, or it can follow a O n law in which 

 the exponent n can be smaller than unity (sublinearity, n between 

 0.5 and 1). It can also be larger than unity (superlinearity). The 

 situation is further complicated by the fact that the magnitude of 

 n not only depends upon the photoconductor but also upon the level 

 of incident light intensity and upon the temperature. Nonlinearity 

 of the transfer function can appear if the photoconductive trans- 

 ducer is connected in series with a load resistor, so that the partial 

 voltage applied to the photoconductor changes with the level of 

 illumination. A feedback circuit which improves the linearity of the 

 transfer function in such cases is described by Rittner. 2 



1 R. W. Smith, Phys. Rev., 97, 1525 (1955). 



2 E. S. Rittner, Rev. Sci. Instr., 18, 36 (1947). 



