288 BELL SYSTEM TECHNICAL JOURNAL 



and that which can be boiled. Some of the melted metal lands on the nega- 

 tive electrode and, with repetition of the phenomenon, results in a mound 

 of metal transferred from the anode to the cathode. This transfer, which is 

 about 4 X lO^^"* cc of metal per erg, is the erosion which occurs on the 

 make of electrical contacts. 



The arc voltage is of the order of 15. If the initial circuit j)otential is 

 more than about 50 volts, there may be more than one arc discharge, suc- 

 cessive discharges being in opposite directions and resulting in the transfer 

 of metal in opposite directions — always to the electrode which is negative. 



The occurrence of an arc is dependent upon the condition of the electrode 

 surfaces and upon the circuit inductance. For "inactive" surfaces an arc 

 does not occur for inductances greater than about 3 microhenries. Platinum 

 surfaces can be "activated" by various organic vapors, and in the active 

 condition they give arcs even when the circuit inductance is greater than 

 this limiting value by a factor of 10^. 



The Conductivity of Silicon and Germanium as A ffected by Chemically In- 

 troduced Impurities. G. L. Pearson.^ Paper presented at A. I. E. E., Swamps- 

 cott, Mass., June 20-24, 1949. Included in compilation on semiconductors. 

 Elec. Engg., V. 68, pp. 1047-1056, Dec. 1949. 



Absteact — Silicon and germanium are semiconductors whose electrical 

 properties are highly dependent upon the amount of impurities present. 

 For example, the intrinsic conductivity of pure silicon at room temperature 

 is 4 X 10~® (ohm cm)~^ and the addition of one boron atom for each million 

 silicon atoms increases this to 0.8 (ohm cm)~\ a factor of 2 X 10\ 



Although such impurity concentrations are too weak to be detected by 

 standard chemical analysis, the use of radioactive tracers and the Hall 

 effect has made it possible to make quantitative measurements at impurity 

 concentrations as small as one part in 5 X 10*. 



Silicon and germanium are elements of the fourth group of the periodic 

 table with the same crystal structure as diamonds and they have respec- 

 tively 5.2 X lO" and 4.5 X lO-"' atoms per cubic centimeter. The addition 

 of impurity elements of the third group such as boron or aluminum gives 

 defect or p-type conductivity. Elements from the tifth group such as j^ihos- 

 ])horous, antimony or arsenic give excess or n-type conductivity. 



The conductivity at room temperature, where it has been shown that 

 each impurity atom contributes one conduction cliarge, is given by equa- 

 tion (1) where N is tlie number of solute atoms per cubic tentinicter. 



(7 = A + H.\. (1) 



' H.T.I.. 



