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BELL SYSTEM TECHNICAL JOURNAL 



current leaving the emitter electrode in the form of electrons and any current 

 to the left oix = 0. 



It should also be clear that the entire analysis of this paper, though it has 

 for definiteness been formulated for the case where holes are injected into an 

 excess semiconductor, applies just as well to any case where electrons can 

 be injected into a defect semiconductor. For the latter case it is merely 

 necessary to interchange the subscripts e and h in the formulas. Though the 

 types of experiments discussed in this paper have to date only been reported 

 for «-type germanium, the occurrence of similar phenomena in /'-type speci- 

 mens is indicated by the successful use of such specimens in transistors.^ 



An interesting and possibly quite useful phenomenon should occur when, 

 after establishment of a steady state, the current je is suddenly turned off. 

 There will result a transient disturbance propagated in the direction of in- 



Fig. 11 — Schematic variation of hole density m with distance x, illustrating formation 

 of a sliock wave by quickly decreasing j^ to zero, for the case where j = je + ja is kept 

 constant. 



(a) Immediately after reduction of 7e to zero. 



(b) Later time. 



creasing x, which is very much like a shock wave in a gas. This, the most 

 interesting feature of the phenomenon, will occur regardless of whether ja 

 remains constant when je is cut off; however, the simplest example for il- 

 lustrative purposes is the case where ja is increased by the amount je at the 

 instant when the latter is cut off, so thatj remains constant. For this case, 

 illustrated in Fig. 11, the values of Uh ahead of the advancing front will 

 remain the same at each point as in the previous steady state. Just behind 

 the front, Uh must drop abruptly to zero. If j/J is large, where J is given 

 by (25), the drop will be extremely sharp. For the change in the form of the 

 front with time is compounded out of diffusion and propagation with variable 

 velocity along descending curves, as shown schematically in Fig. 4. Since 

 the latter propagation involves a more rapid motion to the right, the smaller 

 Hh , it tends to steepen the front, and this steepening must continue until 



' W. G. Pfann and J. H. Scaff, paper presented at the Cambridge Meeting of the Ameri- 

 can Physical Society, June 16-18, 1949. 



