310 



THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



of a is smaller the higher is the specific resistance, a behavior just opposite 

 to that for thick films with film resistances of 100 ohms. Associated with this 

 "crossing over" of the curves is the circumstance that, to obtain the lowest 

 values of a over a range of film resistances, the boron content of the film 

 must increase with increase in film resistance. The envelope of the curve 

 family of Fig. 24 gives, as shown, the relationship between minimum a 

 and film resistance. 



1.0 

 0.8 



0.6 

 0.5 

 0.4 

 0.3 



0.2 



0.10 

 0.08 



0.06 

 0.05 

 0.04 



0.03 

 0.02 



0.010 

 0.008 



0.006 

 0.005 

 0.004 



0.003 

 0.002 



0.001 



-10 -20 -30-40 -60 -100 -200 -400-600 -1000 -3000 



TEMPERATURE COEFFICIENT OF RESISTANCE IN PARTS PER MILLION 



PER DEGREE CENTIGRADE 



Fig. 23 — Relationship between the resistivities of thick borocarbon films and their tem- 

 perature coefficients of resistance. 



This envelope is reproduced in Fig. 25 and is there compared with the 

 curve of Fig. 14 for carbon films. For films of low resistance, the temperature 

 coefficient, a, for borocarbon films is about one tenth of that for carbon 

 fihns. This difference decreases with increasing film resistance until, between 

 10* and 10* ohms for a square, it becomes negligible and the curves appear 

 to coincide, the addition of boron to carbon then appearing to offer little 

 advantage. However, with attainable helixing factors, small (as well as 

 large) borocarbon resistors of about 10 megohms resistance can be produced 



