PROPERTIES OF IONIC BOMBARDED SILICON 



115 



Table I — Effect of Bombardment Voltage on Eb and Ef 



corresponding forward voltages Ef have been scaled from the above 

 drawings and have been tabulated below. Since they vary only slightly 

 with surface temperature, only their mean values are regarded as signifi- 

 cant. Mean values of Eb are plotted in Fig. 6b. 



In order to isolate further the effects of bombardment we have sub- 

 tracted from the mean values of Eb value of Eb for untreated sihcoii. 

 Thus the curve marked Eb represents the improvement in backward 

 voltage that has accrued from bombardment alone. This is also tabu- 

 lated as the mean Eb in Table I. 



EFFECT OF TOTAL CHARGE 



Tests hav( been made to determine the effect of time of bombard- 

 ment on the rectifying properties of silicon surfaces. In these tests, 

 specimens taken from neighboring regions of the same melt were ex- 

 posed for progressively longer periods all at the same bombarding po- 

 tential of 30 kv and the same rate and density of application, 5 micro- 

 amperes per square centimeter. Representative current-voltage charac- 

 teristics are plotted in Figs. 7a and 7b for two neighboring regions. 

 The results are summarized in Fig. 7c. The latter show a rather rapid 

 improvement of back voltage Eb with total charge up to perhaps 50 

 microcoulombs per square centimeter.'^ Thereafter the improvement is 

 small. For purposes of comparison, there is plotted as a vertical line a 

 value of bombarding charge that would account theoretically for one 

 positive ion in each unit crystallographic cell on the surface layer. This 

 suggests that when all surface cells have been penetrated bj' a single 

 ion, no marked increase in back voltage can be effected. 



* Specifying results in terms of microcoulombs implies that bombarding ef- 

 fects are independent of the rate of application. This is known to be true only 

 between factor limits of | and 2 of the boml)arding current. 



