//()/./•: i.x.ijx ii().\ i.\ c,j:i<m AMI \i 



y:^-) 



this is that tlie lirst liules to arrive are those which iiave thffused somewhat 

 ahead of the rest and move in material of low conductivit}-. Tlie later holes 

 travel in an en\-ir()nment of relatively hij;h conductivity and, consequently, 

 in a lower electric held. (Since the current is the same at all points between 

 emitter and collector, the held is in\-ersely proportional to the conductivity.) 

 The transit time for the later holes is, therefore, longer anrl the hole density 

 builds u|) more slowly for the latter part of the incoming i)ulse of holes. 

 The wave form obtained from the trailing edge of the emitter pulse, curve D, 

 is in striking contrast with the leading edge. The first gradual decay, up to 



A & B 



TIME,t (POINTS AT lO-MICROSECOND INTERVALS) — *■ 



EMITTER CURRENT SMALL, ABOUT 4% OF I^^, SO THAT ALL HOLES 

 MOVE IN THE SAME FIELD. 



LEADING EDGE OF PULSE FOR I^ = 2/3 I j^. 



I TRAILING EDGE C^ HOLE PULSE FOR If = 2/3 1 1,, SHOWING SHARPENING 



FROM X TO Y DUE TO TENDENCY OF LAGGING HOLES TO CATCH UP. 



Fig. 8 — Collector current characteristics for the circuit shown in Fig. 1. 



point A', is due to recombination of holes and electrons; at /s the emitter 

 current becomes zero; consequently, the electric field is reduced and the 

 holes arriving at A' have taken a longer transit time than the holes arriving 

 at t'i and a larger fraction of them have recombined with electrons. The 

 true trailing edge, running from A' to I', is appreciably sharper than the 

 leading edge. The reason for this is that holes lagging behind the main body 

 of holes are in a region of relatively low conductivity and high electric field 

 and tend to catch up with the main body. Thus the same effect which 

 lengthens wave C acts to shorten wave D. 



C. Herring has been able to obtain mathematical solutions for the appro- 

 priate equations bearing on the matters just discussed. His theory is pre- 

 sented elsewhere in this issue.'" 



