694 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1953 



From (78) and (79) 



$(5) ^ ^UABY'' exp [-2-"'L{AB)]. (80) 



Now using the expression just obtained for ^(B) in terms of (AB), 

 we can eliminate ^{B) from (43) and solve for C/" as a function of (AB). 

 After some manipulations we obtain 



f « i2ABy- - 2iL^ - I in (2^) . (81) 



For 2ABA or 2ABA~' of the order of 10~^ (40c, d) insure that the 

 right side of (81) will not change in order of magnitude if the two terms 

 multiplied by IT^ are dropped. Therefore 2ABA and 2ABA~^ will be of 

 order 10~^ or less if 



T 10 ^ ' To "^ • ^^^^ 



Thus for a given intrinsic layer thickness there is a ceiling on the im- 

 pressed voltages for which AB is negligibly small. 



