112 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1952 



14 and 40. While all four gases worked very well, helium was the easiest 

 to handle. In the course of the tests, identically prepared samples each 

 |-inch square, taken from a high-piiiity silicon melt, were bombarded 

 with ions formed in the particular gas under test. Particular conditions 

 known to be good for producing good rectifier units were adopted as 

 standard for these tests. They corresponded to a total bombarding charge 

 of 600 microcoulombs per sq. cm., a surface temperature of 395°C a 

 contact force of about 10 grams and a bombarding potential of 30 kv. 



That the effect of bombardment varies with different gases is seen at 

 a glance from the characteristic current-voltage curves shown in Fig. 4. 

 Figs. 4b to 4e in particular indicate that as compared with an untreated 

 sample. Fig. 4a, the effect of bombardment is in general that already 

 noted of separating the two significant points of inflection, B and C. A 

 rather substantial increase in the forward voltage appears in the case 

 of argon as compared with hydrogen, helium and nitrogen. In contrast 

 with a small increase in the forward voltage resulting from the bom- 

 bardment of helium, there is a very substantial increase in the backward 

 voltage. Though substantial for all four gases, the effect of bombardment 

 is largest for helium with progressively smaller effects noted respectively 

 for argon, hydrogen and nitrogen. A particular characteristic of helium 

 bombardment, as compared with argon, not readily appreciated from a 

 linear scale, is shown in Fig. 4f. It will be noted that at the one volt level, 

 the ratio of forward to reverse current for helium is about 130 whereas 

 for argon it is about 25. At other levels the difference is even greater. 

 At the moment helium is regarded as a preferred source of ions. 



The log-log current-voltage curves show as before that the lowest 

 voltage at which substantial forward currents flow occurs in helium, while 

 the highest forward voltages occur for argon. In a similar way the 

 voltages for substantial reverse currents are highest for helium and 

 lowest for argon. The sharp break in the reverse current characteristic, 

 evident in these cases, has been observed so generally that it is now 

 accepted as typical of bombarded surfaces. 



EFFECTS OF TEMPERATURE 



Investigations have been made of the properties of silicon surfaces 

 as affected by the temperature at which bombardment was carried out. 

 This has been done not only for surfaces used as rectifiers but surfaces 

 used as transistors and as photo-electric cells as well. In the case of recti- 

 fiers, a procedure was adopted similar to that used in the previous tests. 

 Measurements were made at five different temperatures ranging from 



