P-X-I-P AXD X-P-I-\ JUNCTIOX TRANSISTOR TRIODES 



521 



ceptors) is the fundamental structural characteristic of the junction 

 transistor. Fig. 2(a) shows an impuiity density profile for a p-n-i-p 

 along an axial line running through emitter, base, collector space-charge 

 layer, and collector. Similar profiles for step junction (alloy) and graded 

 junction (grown crystal) p-n-p's are sho^^^l in Figs. 2(b) and (c). 



The emitter and collector regions of the p-n-i-p have very high im- 

 l)urity concentrations (low resistivities), while the impurity density in 

 the l)ase is moderately high and the depletion layer is almost free of 

 impurities. The high acceptor density in the emitter forces most of the 

 emitter current to flow as holes, giving an injection ratio (7) close to 

 unity. The high densitj^ in the collector gives a low^ collector body re- 

 sistance and fixes the position of one face of the collector depletion layer. 



(a) STEP-BASE p-n-L-p 



lO'S-10'9 



10'8-io'5 



EMITTER 



COLLECTOR 



(b) STEP (alloy) p-n-p 



,10'''-10'8 



COLLECTOR 



(c) GROWN (graded) p-n-p 



Fig. 2 — Impurity den.sit}' profiles. 



