838 



THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



tr 4.5 

 O 



UJ 4.0 

 O 



O 3.0 



^ 2.5 



2.0 



5 0.5 



30 40 50 60 70 80 90 100 

 MICROAMPERES FLOWING INTO THE BASE 



110 120 130 



Fig. 8 — Junction transistor static characteristics. 



current gain of this transistor is equal to 40. The measured a for this 

 transistor was 0.976. Substituting in the current gain formula, equation 

 (3) above, the calculated current gain is 40.6 which agrees with Fig. 8 

 within the accuracy of the measurements. 



2.34. how Voltage Characteristics 



Again referring to the curves in Fig. 8, it will be seen that transistors 

 operate at low collector to emitter potentials. The 1.5-volt curve is not 

 the minimum potential at which this transistor ^\dll operate. Some 

 transistors have good current amplification at potentials as low as two- 

 tenths of a volt. When the base current is reversed, the characteristics 

 in Fig. 8 can be extended to smaller collector current values. One might 

 assume that the collector current can be reduced to zero by causing 

 enough current to flow out of the base. This is not true. There is a mini- 

 mum collector current, called the saturation current, and increasing 

 current flow out of the base will not decrease the collector current below 

 this value. This saturation current is assigned the symbol Ico ■ This Ico 

 current is usually a few microamperes but it increases at the rate of 7 

 or 8 per cent per degree Centigrade increase in temperature of the col- 

 lector junction. Transistors also have a critical junction temperature 



