1244 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



merit measurement shown as Fc(3, —5.5) in Fig. 21. This measurement 

 is the voltage from collector to base under the condition that I^ > 



— {Ic/a). In this instance /j has been chosen to be 3 mA and Ic to be 



— 5.5 mA. The collector current value is chosen on the basis of the 

 smallest tolerable value of alpha expected so as to place the point of 

 measurement near the 7^22 knee, but in Region III or overload. 



The T^c(3, —5.5) measurement is a good measurement for defining 

 the general behavior. T"c(3, —5.5) taken with the r^o measurement 

 constitute a very good defining set for checking the transistor as in 

 re-measuring. For design purposes, the Fc(3, —5.5) measurement is not 

 sufficient. It provides an approximate value for Tc , but does not define 

 r'/ and r„ . A second dc measurement, the collector to emitter voltage 

 drop, V,c , has been employed experimentally also. An improved char- 

 acterization will undoubtedly involve separate measurements of r^ , 

 III , /// 

 Tm and Tc . 



REGION-TO-REGION TRANSITION PROPERTIES 



The transition between Regions I and II is accompanied by abrupt 

 changes in r, and r^ . 



The theory assumes that both of these parameters change at an 

 infinite rate at a fixed emitter current, taken as h = 0. Unfortunately 

 neither of these assumptions is strictly true, r, undergoes a gradual 

 change from high to low values which is only approximated by the 

 three assigned values. In particular the behavior near /, = is of con- 

 cern when dealing with small triggers. 



The forward transfer impedance changes at a finite rate also. Further, 

 the emitter current at which the maximum rate of change occurs will 

 vary from unit to unit. Present practice also has been to measure a 

 rather than r„i . The rational for doing so is not too good since r„ is 

 quite likely the better parameter to characterize. Alpha has a strong phys- 

 ical appeal, fits well into the circuit problems and is easy to measure. 



Since a = (r^ + r,„)/(r6 + Vc) it is necessary to assume that n and 

 fc are constant near /, = 0, an only fair approximation. Having made 

 the approximation, the typical a behavior shown in Fig. 22 may be 

 taken as a measure of /•,„ . Three values are measured, the first of which, 

 ai , in Region II, is redundant to the Region II small-signal measure- 

 ments. The two limits, 0:2 and 0:3 , serve to place lower and upper limits 

 on the absolute values of a at the Regions I-II transition. These limits 

 in turn place a lower value on the rate of change in a within the I, ± A 

 range shown. 



It may be noted that a in Region I is finite. There is a lower limit 



