P-N-I-P AND N-P-I-N JUNXTION TRANSISTOR TRIODES 523 



typical applied voltages are shown in Fig. 3 for p-n-i-p structures in 

 which the depletion layer contains no net impurities (a), a small acceptor 

 dominance (b), and a small donor dominance (c). When collector voltage 

 is increased from zero, the space charge layer thickens until it extends 

 from base to collector. Further increase of voltage simply increases the 

 field strength in the region, without significant further increase in its 

 thickness. 



The capacitance initially changes inversely as the square root of 

 collector potential, but becomes constant when depletion region thick- 

 ness becomes constant. The time required for holes to drift from base to 

 collector decreases with increase of depletion region field until scattering- 

 limited carrier velocities are reached (about 5 X 10 cm/sec for holes, 

 at 10,000 volts/cm). 1" It should be noted that normal operation does not 

 occur until the depletion layer extends from base to collector (particu- 

 larly if the depletion region is slightly n-type so that effective base 

 thickness is large at low collector voltages, see Fig. 3(c)). The breakdown 

 voltage of the collector is very high,* since the field strength in the deple- 

 tion region is relatively uniform by comparison with that in older types 

 of units, the region is wide, and strong fields are required to produce 

 carrier multipHcation. 



Base Region 



Base region design seeks the conflicting objectives of short diffusion 

 transit time, requiring a thin region, and low ohmic base resistance, 

 requiring a thick region. In practice, the region is made as thin as feasible, 

 but of low resistivity material, and base contact geometry is chosen to 

 minimize the ohmic resistance. In the p-n-i-p, very low base resistivity 

 is practical, because the collector breakdown potential is fixed by the 

 thickness of the intrinsic depletion layer rather than by the baise re- 

 sistivity as in fused junction p-n-p's. 



The large donor density in the base region together with the very 

 high frequencies of operation make the emitter depletion layer ca- 

 pacitance (Cxe) both larger and more important than in previous tran- 

 sistors. In order to reduce this capacitance, the emitter junction area 

 is made small, thus leading to emitter current densities of 1 to 100 

 amperes/cm . In general, as the dc current density is incrcuised, the 

 minimum dc collector voltage must also be increased in order to preserve 



* An avalanche mechanism similar to a Townsend discharge in gases is now 

 believed responsible for reverse voltage breakdown in junction structures. See 

 Reference 3. 



