ELECTRICAL NOISE IN SEMICONDUCTORS 



951 



is t ho spectrum. Tho spectral dousity fpowcM- per unit baiulwidlli) \-:irics 

 in\-orsch' as the tVcciucMicy, according lo llic relation 



(IW 



f "(If 



wiiei'e the exponent lies between L and 1.") with an average about 1.2. 

 This type of spectrum will \)c referred to as a 1/f spectrum. Measure- 

 mcMits of the spectra of silicon point contact diodes have been reported 

 by P. 11. Miller^ for the freciuency range 20 cycles to 300 kilocy(des. 

 Spectra of point contact transistors measured by the author have been 

 reported elsewhere"' "* for the range 20 to 15,000 cycles. Typical spectra 

 foi- p-n junction type diodes and transistors are shown in Fig. 1. Almost 

 witiiout (\\c(>ption, oiu' measurements and those reported in the litera- 

 ture have shown the 1/f spectrum over most of the frequency range 

 covered. There is some evidence from the related fields of flicker noise 

 and carbon microphone noise that the 1/f spectrum may extend to fre- 

 quencies well below 0.1 cycle per second. Some departures from this 

 type of spectrum have been noted in the neighborhood of 100 kc, as 

 shown in the curves. 



1000 

 800 



ai 80 

 O 60 





1 2 4 6 8 10 20 40 60 100 200 400 



FREQUENCY IN KILOCYCLES PER SECOND 



Fig. 1— The spectrum of noise in ii-p-n transistors varies inversely with fre- 

 quency. 



