66G THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1956 



t = total time of diffusion 



X 



y = /jyr- = variable of integration 



A plot of C/Co = erfc y versus y is given in Fig. 3. Co is the surface solu- 

 bility density and depends upon the tempers are of the diffusion proc- 

 ess/ At some depth, Xj , the concentration C equals the original im- 

 purity concentration where the silicon will change conductivity type 

 resulting in a junction. In order to obtain desirable depths of the dif- 

 fused layers, A^+ and P+, it is necessary to diffuse at temperatures in 

 the range of 1000°G to 1300°C for periods of hours. With such periods 

 it is obvious that the diffusion process lends itself to easy control and 

 reproducibiUty. 



.3.0 CONDUCTIVITY MODULATION 



3.1 It is well known that the series resistance of a power rectifier is 

 the most important electrical parameter to control and should be made 

 as small as possible for several reasons. The series resistance consists 

 essentially of two parts; the body resistance of the semiconductor and 

 the contact resistance to the semiconductor. In the early stages of recti- 

 fier development both parts of the series resistance contributed about 

 equally to the total series resistance. However, methods were soon found 

 to reduce the contact resistance. It then became apparent that in order 

 to reduce the body resistance, the geometry would have to be changed 

 and the resistivity chosen carefullJ^ By going to larger, thinner wafers 

 it was possible to reduce this body resistance. However, the cost of 

 pure silicon made it important that conductivity modulation (described 

 below) be incorporated in these devices as a method for reducing the 

 body resistance. Our initial attempts were successful due to the fact 

 that higher lifetime of minority carriers could be maintained in the ex- 

 tremely thin wafers that were used as compared to the lifetime remain- ^ 

 ing after the diffusion process in thicker wafers. 



3.2 A complete mathematical description of the I-V characteristic 

 for the conductivity modulated rectifier is practically impossible due 

 to the fact that the equations are transcendental. However, it is easy 

 to understand the operation of the device physically. 



When the device is biased in the forward direction, electrons from 

 the heavily doped N-\- region are injected into the high resistivity ir 

 region. If the lifetime for these electrons in the tt region is long enough, 

 the electrons will diffuse across the w region and reach the P-f region 



* Fuller, C. S., and Ditzenberger, J. A., J. Appl. Phys., 25, p. 143!), li)54. 



