PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 305 



may give rise to conduction by positive holes, which in the extreme case 

 yields a positive Hall constant, rather than the negative Hall constant, 

 indicating conduction by electrons, which carbon normally possesses. 



6.4 Characteristics 



Figure 20 shows some types of pyrolytic carbon resistors produced, while 

 Table II summarizes the essential characteristics of some of the more widely 

 used varieties. Pyrolytic carbon resistors are compared in Table III with 

 representative carbon composition and wire-wound resistors. 



These tables show that for many uses pyrolytic carbon resistors are 

 superior to other available varieties. Thus, for high frequency applications, 

 particularly when high values of resistance or large power dissipations are 

 required, they are ahnost unique.^ Similarly, regardless of frequency or of 

 resistance, they exhibit greater stability in all respects than do carbon 

 composition types. The stabilities and the tolerances to which they can be 

 held are such that they could well serve as replacements for wire-wound 

 types in many applications if it were not for the numerically large values 

 of their temperature coefficients of resistance. 



It has, however, been found possible to decrease the temperature co- 

 efficients of resistance of resistors in all other respects equivalent to the 

 pyrolytic carbon type to values smaller than are, on the average, available 

 in wire-wound varieties. These are produced by modification of the pyrolytic 

 carbon film by the addition of boron and are known as "borocarbon resis- 

 tors "^^ The comparatively small temperature coefficients of these boro- 

 carbon resistors are, of course, of considerable interest. Besides being in- 

 creasingly requisite for applications in which appreciable amounts of high 

 frequency power must be dissipated, they greatly simplify production of 

 closely matched units for computer network and other applications, are of 

 particular advantage for electronic equipment which is subject to extremes 

 of temperature, and can be employed as replacements for wire- wound types 

 in many applications. 



7. BOROCARBON RESISTORS 



Investigations of the properties of carbon shortly after the turn of the 

 century indicated that they could be greatly modified by the addition of 

 boron.^2 So far as can be ascertained, however, the implications of this 

 early work for the pyrolytic carbon resistor went unnoticed until, during 

 the recent war, an investigation of the pyrolytic codeposition of carbon and 

 boron was undertaken in these Laboratories. Results of this preliminary 

 study indicated a good probabiUty 'that composite pyrolytic fihns of boron 

 and carbon would have appreciably smaller temperature coefficients of resist- 

 ance than films of carbon, as has been confirmed by subsequent development. 



