PROPERTIES OF lOMC liOMBAKDED SILICON 109 



1)0 small. For coi'tain kinds of voltage limiters, Ep should l)e large. In 

 oilher ease the iuflectiou interval should be large. 



In an alternate graphical representation, see Fig. 2b, voltage-current 

 data are plotted to a log-log scale. This form of representation is of 

 ])articnlar value when large ranges of data are to be shown. It is also of 

 value in determining the resistance (Rp) at small voltages. Corresponding 

 points on the two curves shown in Fig. 2 are identifiable by the letters A 

 1^, C, antl D. Ciu'ves of both kinds are used interchangeably to show the 

 effects of the several variables of the experiment. 



EFFECT OF CONTACT PRESSURE 



In point contact rectifiers,^ pressure is of considerable importance. 

 Usually the best pressure is a compromise between good electrical charac- 

 teristics, usually obtainable only with light pressures, and good stability 

 usually obtainable with higher pressures. Experiments have been per- 

 formed with a range of contact pressures both on bombarded and unbom- 

 barded materials. In general, the results are highly variable, particularly 

 in the case of unbombarded material. From this wide range of data, 

 however, two characteristics have been selected that may be regarded 

 as typical for 10-gram and 60-gram pressure. They are shown in Fig. 3 

 for silicon taken from nearby portions of the same sample. Significant 

 points on these several curves may be compared with their idealized 

 counterparts shown in Fig. 2. Although the samples chosen show some- 

 what more than the usual intrinsic resistance typical of p-type silicon, 

 the etfects of contact pressure are nevertheless regarded as representative. 

 As indicated in Figs. 3a and 3b, the effect of increased contact pressure,^ 

 particularly in the case of unbombarded material, is of reducing the low 

 voltage resistance, Rp, see Fig. 2b. The more desirable higher resistance 

 is obtainable only with light contact, a condition unfavorable for high 

 mechanical stability. In the case of bombarded material, the effect of 

 contact pressure is less important. Thus it is possible in this case to 

 incorporate in the design higher contact pressures and obtain thereby 

 higher stabilities. For purposes of this paper a contact force of 10 grams 

 has been accepted as standard. 



In addition to showing the effect of contact pressure, Fig. 3 shows some 

 overall effects of bombardment. It will be noted, for example, that the 

 effect of bombardment, see Fig. 3b, has been that of shifting the plots 

 of Fig. 3a to the left by several orders of magnitude. Thus the resistance 

 (Rp) is increased by a factor of more than 10,000. It is to be noted also 



3 Scaff and Ohl, Bell System Tech. J., 26, Jan. 1947. 

 ^ Realy contact force. 



