A HIGH-FREQUENCY DIFFUSED BASE GERMANIUM TRANSISTOR 31 



in 



_i 



LU 



m 

 o 



LU 



a 



z 



10 



20 



30 40 50 60 80 100 200 300 400 



FREQUENCY IN MEGACYCLES PER SECOND 



600 800 1000 



Fig. 4 

 current. 



The variation of the alpha-cutoff frequency as a function of emitter 



levels exist in the range of emitter current shown in Fig. 4. The conclu- 

 sion to be drawn then is that electric fields produced by impurity 

 gradients in the base region are not the dominant factor in the transport 

 of minority carriers in these transistors. 



The emitter current for a low level of injection could not be deter- 

 mined by measuring /„ versus /« because the high input impedance at 

 very low levels was shorted by the input capacity of the header and 

 socket. Thus at very small emitter currents the measured cutoff fre- 

 quency was due to an emitter cutoff and was roughly proportional to 

 the emitter current. At /e ^ 1 ma this effect is small, but here at least 

 intermediate levels of injection already exist. 



A further attempt to measure the effect of any "built-in" fields by 

 turning the transistor around and measuring the inverse alpha proved 

 fruitless for two reasons. The unfavorable geometrical factor of a large 

 collector area an a small emitter area as well as a poor injection effi- 

 ciency gave an alpha of only 



a 



= 0.1 



Secondly, the injection efficiency turns out in this case to be proportional 

 to oT^^'^ giving a cutoff freciuency of less than 1 mc/sec. The sciuare-root 

 dependence of the injection efficiency on freciuency may be readily seen. 

 The electron current injected into the collector body may be expressed as 



Je = qDnN 



1 -)- iu^Te 



1/2 



where q = electronic charge 



