ELECTRICAL NOISE IN SEMICONDUCTORS 



957 



to'* 



102 





1 



2 5 ,2 5 -.2 5 A Z 5 ^2 5 n 



10 10^ I0-* 10^ 10^ 10^ 



FREQUENCY IN CYCLES 



Fig. 5 — Typical spectra of noise in single crystal filaments carrying a dc cur- 

 rent. 



exceptions to this trend. Also, there are large variations in the magni- 

 tude of the noise. An average unit shows a noise voltage about three 

 times Johnson noise at a bias of 10 volts per centimeter. 



The noise behavior at reduced temperatures has been investigated. 

 Results on three different bridges are shown in Fig. 7. The open circuit 

 noise voltage is shown as a function of temperature for constant bias 

 voltage. Although the curves show rather large irregularities, there 

 seems to be no general trend for noise to decrease with decreasing tem- 

 perature over the range covered, from — 200°C to room temperature. 



The surface treatment applied to a bridge may affect the noise very 

 substantially. A sandblasted surface usually gives the lowest noise. 

 Etching the surface may raise the noise voltage by a factor of ten or 

 more, though the iV' resistance changes only a few per cent. The tech- 

 nique of washing and drying the surface may have an important effect 



