DIFFUSED p-7l JUNCTION SILICON RECTIFIERS 679 



enter or leave the package and thus no changes of the device due to the 

 enclosed gas should occur as long as the gas does not react with the sili- 

 con, solder or package. However, no seal is absolutely vacuum tight 

 and thus care should be used in choosing a package design so that mini- 

 mum effects should occur to the electrical properties during the use of 

 the device. The third requirement of the disposal of the internally de- 

 veloped heat suggested the use of copper due to its high thermal conduc- 

 tivity. However, a small package alone is capable of dissipating only a 

 small amount of heat without reaching a temperature that is too high 

 for the device. This necessitates the use of cooling fins in conjunction 

 with the device to make use of its electrical properties. This thermal 

 requirement demands a package to which thermal fins can be attached. 

 This is met by having the package contain a bolt terminal to which 

 thermal fins can be attached or by which the unit can be mounted to a 

 chassis for cooling. The fourth requirement consists of two parts; the 

 package must have two leads that are electrically separated from one 

 another and the leads must be sufficiently heavy to conduct the maxi- 

 mum currents. The first of these requirements is met by using glass-to- 

 metal seals in the package and the second is met by using copper leads 

 of sufficiently heavy cross-section. The resulting packages for the units 

 discussed in this paper are shown in Fig. 6. It should be remembered 

 that the packages are only intermediate development packages and that 

 further work will probably alter these both in size and in shape. How- 

 ever, all the requirements mentioned will be applicable to any package. 

 6.2 The units pictured in Fig. 6 have a range of dc current ratings 

 associated with them. The lower rating of each device corresponds to 

 the maximum rating of the next smaller device. Of course, the larger 

 units could be used for smaller current applications; however, such use 

 M'ould be like using a freight car to haul a pound of coal. The maximiuu 

 rating of each de^'ice has been arbitrarily chosen for it to operate with 

 a reasonable sized cooling fin at an ambient of 125°C and no forced air 

 or water cooling. It is known that the ratings could be increased by 

 either method of forced cooling. It has been found that a copper con- 

 vection cooling fin is able to dissipate 8 milliwatts per square inch per 

 degree centigrade. This cooling rate is obtained from the difference be- 

 tween the average temperature of the fin and the ambient temperature 

 over the effective exposed area of the fin. For example, a copper fin 

 S}4, inches scjuare when mounted so that both surfaces are effective for 

 cooling will })e able to dissipate ten watts and at the same time prevent 

 the temiK'rature of the fin from exceeding 50°C above the ambient tem- 

 perature. Another thermal drop is found between the junction and the 



