188 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1953 



with the special cases 



{c^) = const -aX {^^ (27b) 



for constant mean free path and 



(cz) = const -ar (27c) 



for constant mean free time. Comparison of (26c) and (27c) might lead 

 one to surmise that we have here twice the same formula. This is indeed 

 the case, as will be shown in Section III A. 



Proceeding now to the diffusion problem, we observe from (20), (23) 

 and (24) that we must add the quantity 



nk 

 NT 



to the previous list of parameters when computing the diffusion current. 

 However, the current is always linear in this quantity, which means 

 that the diffusion coefficients contain the factor 



_1_ 

 NT 



and beyond this factor depend on the same variables as previously. This 

 gives in the high field case 



with the special cases 



D = const -a'^V (28b) 



and 



D = const -aV^ (28c) 



In the low field case, the diffusion process becomes independent of the 

 field and we get 



■kT\^ 



^ = --^-A^ iw) ' (^^^) 



with the special formulas 



D = const- X 0^] (29b) 



m) 



