964 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



^ = fIt- = l72tEH X 10"' (3) 



/vi/g 



where t = thickness of the filament (cm) 



H = magnetic field (oersteds) 



E = applied electric field (volts per cm) 



Eh = effective transverse component due to Hall effect (volts 

 per cm) 



q = unit electronic charge 



kT = Boltzman's constant X absolute temperature. 



The constant may be derived by noting that kT/q is 1/40 volt at room 

 temperature, and that the effective transverse field. Eh , may be ex- 

 pressed as follows. (See Reference 14, Section 8.8.) 



Eh = eE = (On + dp)E 



= (m„ + Hp)HE X 10"' 



= 4.3 X 10~' HE 



where 9 = Hall angle 



ju„ = Hall mobility for electrons (2800 cmV volt-sec) 



fXp = Hall mobility for holes (1500 cm /volt-sec). 



The other dimensionless parameter is proportional to the rate of surface 

 recombination, and is defined as the ratio of the surface recombination 

 velocity to the diffusion velocity from the center: 



t/' = st/2D = st/8Q 



where s = recombination velocity characteristic of the surface (cm/ 

 sec) 



D = diffusion constant (cm /sec). 



The numerical constant is given for holes at room temperature. The 

 noise changes are expressed in decibles, that is, ten times the common 

 logarithm of the ratio of noise powers with and without the magnetic 

 field. 



A second case is that in which generation and recombination are on 

 the surfaces, but the two surfaces have unequal absorption properties. 

 It might be expected that rather large increases in noise would result 

 when the magnetic field was poled to pull holes away from the surface 

 with high absorption properties, and this turns out to be the case when 

 the calculations are carried out. The results are shown in Fig. 11 for a 



