I'ROl'ERTIES OF lOXIC 1U)M!{A KDKU SILICON 107 



diameter. This has the effect of removiiif>; some of the coiitaminatiiig 

 impurities derived from the crucihle as well as providing samples of 

 convenient size. This U-inch cylinder is then sliced transversely into 

 thin wafers which sul)se(iuently are polishetl on one side. Except as other- 

 wise noted the material coxeretl by this jjaper had an impurity content 

 of about 0.1 per cent. The exception will be found in the data of column 

 (a) of Fig. 8. 



BOMHARDMENT PROCEDURE 



The wafer, as prepared abo\e, is placed in the bombarding apparatus 

 with the rough face contacting the graphite support. The vacuum cham- 

 ber is sealed by placing the ion generator in position and the whole 

 assembly is evacuated. The sample is then heated to the proper tempera- 

 ture and the desired kind of gas is admitted, the pressure being estimated 

 from the ion current. When stable conditions prevail, the accelerating 

 voltage is applied to the target and the bombardment is carried out for 

 the proper length of time. A convenient current density is 5 micro- 

 amperes per sciuare centimeter of target area. The target area of our 

 present apparatus, including the silicon and a portion of the graphite 

 support, being 20 square centimeters, the ion current is generally around 

 100 microamperes. The dosage is sometimes specified in microcoulombs. 



After l)ombardment, the sample is removed from the apparatus and 

 the rough surface is covered with a tiiin layer of evaporated rhodium. 

 For most of the tests outlined below the 1^-inch diameter wafers were 

 cut into |-inch sciuares, a size convenient for testing. 



GRAPHICAL REPRESENTATION 



In considering the merits of non-linear materials such as silicon, per- 

 haps the simplest and most useful characteristic is the voltage-current 

 relation. If this is plotted to a linear scale, it results in a smooth curve 

 of the general form shown in Fig. 2a. Specific curves obtained in practice 

 may depart widely from that shown but in general, all may be regarded 

 as made up of two semiparabolas, one in the first fiuadrant and one in 

 the third, joined by a nearly horizontal straight line. For present pur- 

 poses, we shall further simplify this idealized characteristic by consider- 

 ing it as made up of three straight lines. The first, AB, is associated with 

 the reverse voltage current characteristic. The third, CD, is associated 

 with the forward voltage current characteristic. These two character- 

 istics are joined by the nearly horizontal line, BC. The slopes of these 

 three lines correspond to resistances. The section BC for example, corre- 



