r-X-I-1' AND X-P-I-\ JUN'CTION TRANSISTOR TKIODES 



525 



These advantages are, however, balanced in part ])y an increase in the 

 (Miiitter depletion rojiion capacitance associated with the low resistivity 

 base material. 



DESICX TIIKORY 



General 



The piiiicipal objectives in the initial i)-n-i-p desif>;ii liave been high 

 alpha cntoff l're(inency, low collector capacitance, and low ohmic base 

 resistance. The eqnivalent circnit employed is shown in Fig. 5. The 

 output and feedback admittances which are important in earlier jnnc- 



Ypo — 



qle 

 kT 



inh 2 



Fig. 5 — Equivalent circuit of the p-u-i-p transistor. 



tion triodes are omitted, since the space charge layer widening factor 



(Hi" or Hec , ■ — — F7-) is very small. ' ^Fhe transfer admittance is shown 

 qw dVc 



as a current generator (aie) with cutoff fre(|uency (| a' | '^3 db down) 



of /a because this gives explicit recognition to base region diffusion transit 



time Tb and allows it to be combined with space charge layer transit 



time Tc . 



Km) tier Region Design 



Emitter region acceptor concentration should be very lai'ge (10 — 10 

 atoms/cc) in order to keep the injection ratio y close to unity at both 

 low and high frequencies.^ At low frequencies, y is determined by emitter 

 resistivity and carrier life path or diffusion length, base resistivity and 



width, as 



1 



7 = To = 1 + (TbW 



O'eLne 



