121B_1_bjt

vcb n n q ab dc n ab n dc 2 vbi vcb 2

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Unformatted text preview: ed. Base push out can occur --- Kirk's effect. In the base-collector depletion region, the E-field is large i.e., the Poisson's eqn is: e sat J 2 - = = N ( x) - n = N ( x) - 2 q sat x where N ( x) = = ND ( x) in the collector in the base EE121B Spring 2010 = - N A ( x) Jason Woo If J / q sat is comparable to ND(x) in the collector, then the collector no longer has a depletion region with the +ve charge. E-field lines are terminated in the e-. log N e- ND NA n ND ND New Base Region Jason Woo EE121B Spring 2010 B E N + D e p NA WB (pre Kirk's Effect) e ND + ND C WB (post Kirk's Effect) For good gain at high IC, collector must be doped high enough to avoid Kirk's effect In other words, we need to have: J < N Dcollector q sat As the device size reduces, I constant, and J N Dcollector has to increase. Note that as a result, CBC and becomes very important in determining the transient behavior of the BJT. Jason Woo EE121B Spring 2010 Bipolar transistor Breakdown Start with J C = M T J En We are interested in the case when M>1. Again M is caused by impact ionization in the base-collector depletion region. i.e.: M= J n (W B + X BC ) J n...
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