960 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



no significant differences in noise behavior. It should be noted, however, 

 that small variations might be hidden in the large scatter in the data 

 from undetermined causes. 



IV. NOISE AND MAGNETIC FIELDS 



An important role for the minority carrier in the noise mechanism 

 was first clearly indicated in experiments on the effect of a magnetic 

 field on noise in germanium filaments. It has been found experimentally 

 that the noise in a single crystal filament may change by a substantial 

 factor when the filament is subjected to a steady transverse magnetic 

 field. The following discussion will show that this behavior is in har- 

 mony with the hypothesis of noisy injection of minority carriers, as set 

 forth in a preceding section.* 



The physical picture on which this treatment is based involves the 

 random injection of holes into an n-type filament by hole sources which 

 may be either in the interior or on the surface of the filament. f It is 

 assumed that the spectrum of the noise arises from the fluctuating na- 

 ture of the noise source. The effect which any source has will depend 

 upon the lifetime of the holes which it emits. If these holes remain in 

 the filament for a long time, they will produce more noise than if they 

 remain in the filament for a short time. We shall be concerned with 

 the effect of magnetic fields upon these lengths of time and shall not 

 deal in this paper with the fluctuations of the noise sources themselves. 

 If a transverse magnetic field is applied to an n-type germanium fila- 

 ment, a Hall effect voltage is set up and the holes will be deflected to- 

 wards one surface of the filament. Since recombination takes place prin- 

 cipally at the surfaces, this may cause a substantial change in the lifetime 

 of the holes. In order to determine the effect of the magnetic field on 

 the noise we proceed along the following lines. 



(a) We assume that the observed noise is due to fluctuations in the 

 conductivity of the filament produced by fluctuations in the hole con- 

 centration. Since these fluctuations are small, we may take the change 

 in conducitivity to be proportional to the change in average hole den- 



* The following semi-quantitative theory of the dependence of noise on mag- 

 netic field is taken with some modification from unpublished work of W. Shock - 

 ley and H. Suhl, on the basis of which the calculations leading to the curves of 

 Figs. 10 and 11 were carried out. It is hoped that this work may be published in 

 the near future. 



t To simplify the terminology, the discussion is based on n-type material with 

 holes as minority carrier. An exactly similar argument could be made for p-type 

 material with electrons as the minority carrier. There is some experimental evi- 

 dence of the similarity of behavior of n- and p-type germanium, though most of 

 the experimental work has been done with n-type. 



