522 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



The high donor concentration in the base region leads to low ohmic 

 base resistance (r^') and fixes the position of the base face of the deple- 

 tion layer. In the depletion layer, the concentration of impurities is so 

 low that the field region (space-charge layer) extends from the n-type 

 base to the p-type collector at low voltages. 



Depletion Layer 



The properties of the depletion layer which are important at high 

 frequencies are the capacitance across it (Ce) and the carrier transit 

 time through it {tc). These are determined primarily by the impurity 

 density, the thickness of the region, and the base-to-collector voltage. 

 Potential and field distributions in the depletion layer for both small and 



FIELD DISTRIBUTION 



POTENTIAL DISTRIBUTION 



DEPLETION REGION 



(a) Nd=Na 



(b) Nd<Na 



(c) Nd>Na 

 Fig. 3 — Field and potential distributions in depletion region of p-n-i-p transistor. 



