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
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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