32 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1956 



Dn ^ diffusion constant of electrons 



Vi = voltage across collector junction 



Tic = density of electrons on the p-type side of the collector junction 



Te = lifetime of electrons in collector body 



Le = diffusion length of electrons in the collector body 



Since the inverse cutoff frequency is well below that associated with the 

 base region, we may regard the injected hole current as independent of 

 the frequency in this region. The injection efficiency is low so that 



7 ;^ ^ « 1 



J e 



Thus at a frequency where 



then 



cor, 



»1 



I 



-1/2 



An interesting feature of these transistors was the very high current 

 densities at which the emitter could be operated without appreciable loss 

 of injection efficiency. Fig. 5 shows the transmission of a 50 millimicro- 

 second pulse up to currents of 18 milliamperes which corresponds to a 

 current density of 1800 amperes/cm". The injection efficiency should 

 remain high as long as the electron density at the emitter edge of the 

 base region remains small compared to the acceptor density in the 

 emitter regrowth layer. When high injection levels are reached the in- 

 jected hole density at the emitter greatly exceeds the donor density in th(> 

 base region. In order to preserve charge neutrality then 



p ^ n 



where p = hole density 



n = electron density 



As the inject(Hl hole density is raised still further the electron density 

 will eventually become comparable to the acceptor density in the 

 emitter regrowth layer. Tlie density of acceptors in the emitter regrowth 



