528 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



of Xm is a design balance between Cc and fa , with any desire for low 

 voltage operation weighting the scales toward smaller Xm . 



As collector voltage and therefore field strength is reduced below that 

 required for normal operation, transit time is increased because of the 

 reduced drift velocity. In addition, the holes in transit interact with the 

 ac field of the layer, thus increasing the output conductance Qcc ■ Further, 

 a larger density of holes in the layer is required to carry the same cur- 

 rent, disturbing the field distribution. If the voltage is reduced greatly, 

 space-charge limited emission may occur, ' producing much longer 

 effective transit times. 



If output voltage is reduced sufficiently, the collector field will not 

 extend all the way from base to collector. If the layer is someAvhat n-type, 

 the field region collapses toward the p-region of collector. If it is some- 

 what p-type, the field collapses toward the n-region of the base. The 

 latter arrangement has the advantage that /„ is less drasticall}^ reduced. 

 Further, in normal operation, the negatively charged acceptor atoms of a 

 slightly p-type layer will neutralize the charge of the holes in transit, 

 thus making the field more nearly constant from collector to base. The 

 effects of low voltage on the collector field distribution are indicated ap- 

 proximately by the dashed lines of Fig. 4.* 



Collector Region Design 



Acceptor concentration in the collector should be large for several 

 reasons. This gives a low collector body resistance, which virtually 

 eliminates internal series loading of the collector, and it aids operation 

 by fixing the position of the collector edge of the depletion layer. The 

 advantages obtained may be seen by considering a unit in which the 

 collector body is made somewhat p-type and a collector contact is at- 

 tached at some distance from the depletion layer. If 10 ohm-cm p-ma- 

 terial is used for the collector body and a collector contact fastened 2.5 

 mils from the collector resistance of 250-500 ohms will result. In addition, 

 because of the weak drift field at the collector edge of the depletion 

 layer, the hole transit time is about twice that for a true p-n-i-p. 



Alpha Cutoff Frequency 



A current transmission cutoff frequency /„ for the p-n-i-p is given 

 approximately byf 



* The field distributions occurring in an intrinsic depletion layer at low field 

 strengths have been discussed in Reference 11. 



t It is assumed that alpha is given by a = a o(l + jf/fa)- Equation (9) repre- 

 sents the phase of this expression quite well, but the amplitude rather poorly. 



