1242 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



virtually all of the device parameters should be constant from unit to 

 unit and with ambient conditions. 



It can be shown that for small-signals a device may be uniquely 

 characterized by five measurements. In terms of the parameters used 

 here these might be ^n , Rn , R22 , R21 and the dc bias point or equally, 

 ''e , n , re , Vm and the bias point. Since the problem was linearized in the 

 approximation, it follows that 15 such measurements, five in each 

 region, are necessary for proper switching device characterization. The 

 indicated extensive testing required may be reduced somewhat by 

 suitable approximations. It is clear that the switching device designer 

 and producer must reconcile themselves to making more tests for ac- 

 curate characterization than when small-signal devices are concerned. 



What will be given here is a description of typical developmental 

 switching transistors in terms of the parameters which have evolved as a 

 result of practical approximations. The method will be to discuss device 

 properties and measurements region by region; then to discuss the 

 properties at the transition points. Temperature, frequency and life 

 behavior will be taken up separately. 



REGION I PROPERTIES 



In Region I, the emitter current is negative. Hence the emitter 

 resistance r^ is large and is essentially that of a diode in the reverse 

 direction. At present r^ is measured by a simple dc test of the current 

 which flows at a nominal —10 volts. Both r^ and n will be discussed 

 further under the Region I-Region II transition properties. 



The Region I collector resistance is one of the most important param- 

 eters in switching. This is because of its determining nature in the 

 turning point voltages in Figs. 17, 18, and 19. Actually, what is of 

 concern is not the small-signal slope shown as r^* in Fig. 21, but rather 

 the dc current and voltage relationship shown as Tco . For example in 

 Fig. 17, it may be seen that V^p is given by the voltage drop determined 

 by the product of Rb and the dc collector current. 



Fig. 21 is an idealization of the R22 characteristic and has been de- 

 signed to bring out the diode nature of the collector by emphasizing 

 the saturation current and voltage, Ico and Vco . In junction devices the 

 break in the /^ = characteristic at Ico is quite evident whereas in 

 present point contact devices the transition is smooth due to the much 

 lower values of re . The device significance is the same, however; Ico 

 varies rapidly with temperature whereas Tc varies at a considerably 

 lower rate. 



* The actual parameter is of course ^22 , but since Ri2 = re -\- ri, and n <^ re 

 R22 is taken as re . 



