P-N-I-P AND X-P-I-N JUNCTION TKANSISTOU TIUODES 527 



large dc emitter current /« so that the minority carrier emitter admit- 

 tance ijee is at least of the order of magnitude of jwCxe • Total emitter 

 admittance is 



|^(l+jW)'«coth 



Vee + JCoCtc = 



(1 + J(^t )w' 



2-|l/2 



1/2 



coth 





+ M\e (4) 



Depletion Layer Design 



As mentioned previously, the most important characteristics of the 

 depletion layer in the p-n-i-p are the transit time for holes, Tc , and the 

 capacitance, Crc • The minimum voltage for normal operation, Fmin , and 

 maximum or breakdown voltage, Fmax , are also significant. 



The minimum voltage for "normal" operation is reached when the 

 electric field between the n-type base and p-type collector is strong 

 enough so that the holes drift at their limiting velocity of 5 X 10^ 

 cm/sec* The collector to base voltage required for normal operation is 

 the product of the minimum field strength for the limiting velocity and 

 the thickness of the depletion layer and is given by 



F:ni„ = 10,000 X,n (5) 



in which Xm is depletion layer thickness in cm. The maxunum field ob- 

 tainable before reverse voltage breakdown is not known exactly, but is in 

 practice near 100,000 volts/ cm, so that 



F:„ax ^ 100,000 x„ . (6) 



Depletion layer capacitance is nearly independent of collector voltage 

 in normal operation and is inversely proportional to layer thickness. 



Cro = "^ (7) 



Xm 



Transit time for holes increases directly with layer thickness, however, 

 being 



Since increase of Tc decreases the alpha cutoff frequency /„ , the choice 



* At lower field strengths, the transit time for holes is longer, giving a lower 

 alpha cutoff frequency. The "normal" is the best, rather than the only possible, 

 operating condition. 



