Unformatted text preview: - I gen is small * Of course, I gen is large when the PN junction is close to or at breakdown.
Jason Woo 24 EE121B Spring 2010 We can write I C =M I nC I C = M 1 I nC i.e. , if VBC is not too large (much less than BVBC) Together, we have 0 = T M For advanced BJTs, 1 - >> 1 - T
1 1 i.e., >> 1 - T 1-
Therefore, Typically 0 = T M Jason Woo 25 EE121B Spring 2010 That is, J n ( 0) 0 = = 1 - 0 1 - J p ( - x E ) + J rec J n ( 0) DEWB N AB J p ( - x E ) DBWE N DE if Jrec<< Jp(-xE), i.e., JE is not too small Therefore, to maximize , we need to make WE NDE>>WB NAB (i.e., heavily doped the emitter). Note that in the low current regime when Jrec Jp(-xE),
J n ( 0) 1- J rec = D B ni WB N AB W DBE rec e qVBE n -1 n kT can be rather small
Jason Woo 26 EE121B Spring 2010 At moderate current =
At high current, DBW E N DE constant DEWB N AB
- and I C n I n -1 n C - IC n Irec dominates IB VBC<0
- IC 1
Jason Woo IC if the base reaches high level injection condition.
27 EE121B Spring 2010 reality, drops with large IC due to base-pu...
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