1284 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1956 



length but small compared to the length of the intrinsic region, then the 

 voltage drop at the ends of the intrinsic region will be proportional to 

 L while the current, and consequently the minimum field, will be propor- 

 tional to L. Thus r? will be proportional to 1/L and will decrease as V 

 increases and the region becomes more swept. Finally the two space 

 charge regions meet; then ?? rises again with V and approaches unity. 

 Hence, for a given -q and length of intrinsic region, there will be three 

 different values of V. For lower L the dip in the i] versus V curve will be 

 less, and there will be only one V for some values of 77. Since -q starts 

 from zero at the built-in voltage and approaches unity for infinite volt- 

 age, there must be either one or three values of V for every r?. Thus 

 when the V versus L curve (or in Prim's notation the U versus L curve) 

 branches, it branches at once into three curves. Prim's plot gives the 

 upper branch in cases where all three are present. 



