TRANSISTORS IN SWITCHING CIRCUITS 1227 



Til Rof2;ioii IT, /•,„ has, of course, high vahies and in general r^ » /"/. . 

 Pending further investigation, r,„ will be assumed finite, but very small 

 in Region III. 



IDEALIZATION OF R^i 



In the output family of Fig. 13 it may be noted that 7^22 has two 

 values, a high value for h > —aU and a low value for Ic < —al, . 

 The high value corresponds to Regions I and II and the low value to 

 Region III. To a first order the two values are separately constant 

 which was not true of earlier transistors in which 7^22 underwent ex- 

 tensi\'e degradation in magnitude as Ic and /« increased. 



The lower limit to which 7^22 can fall in Region III is n , since 7^22 = 

 Tc + /■(, , implying that Tc is zero in Region III. This is approximately, 

 but not acciu'ately true. As a/, approaches —Ic in magnitude, the 

 \-oltage across the collector barrier becomes nearly zero so that r^ has 

 a low, but finite value. Under this condition, the hole current is very 

 high and heavy conductivity modulation of the collector barrier re- 

 sistance occurs. Thus the collector resistance in Region III is indeed 

 quite low and may be neglected for many circuit computations. 



In the functional relation R21 = /(/« , /c) it has been assumed that 7?22 

 is a function of Ic alone. Further, the approximation involves first order 

 terms only and hence the functional relation Vc = /(/e , /c) may be 

 written as: 



Vc = RnL + 7^22/0 (9) 



Here again, as in the input case, the currents and voltages are total 

 instantaneous or dc values as indicated by the capital letters. 



It is believed desirable, however, to give one more consideration to 

 the output relations. When 7^ = 0, the collector characteristic is ap- 

 proximately that of a diode in the reverse direction. A diode has low 

 reverse resistance until the voltage across the barrier exceeds a few 

 tenths of a volt and then has quite high resistance, approaching infinite 

 slope in the case of junction diodes. ' This effect is shown exaggerated 

 in the idealized output family of Fig. 15. The current and voltage at 

 the break in the /« = curve have been termed /co and Vgo respectively. 

 I tin and Fco are quite evident in junction devices; in point contact devices 

 they are not nearly so evident due to the lower value of R22 and the 

 higher voltages and currents normally employed. Where currents and 



* See Reference 2. 



' Holes and Electrons, W. Shockley, Van Nostrand, p. 91, 1950. 



