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For large capacitive loads the BJT delay is smaller than that of the MOSFET delay & so in BICMOS the output stage is formed of BJT transistor while internal stages are made of CMOS which is faster than the BJT for internal small in-between capacitive loadings.
While, BICMOS technology in analog design have additional advantages of using BJT in places where using ordinary CMOS transistor is hard.
Well, the bjt part of the biCMOS is the fast one in the sense that for the same gm the current needed is quite smaller than the one needed for the MOST. As a result the size of the BJT is smaller, its output resistance is bigger. If you build an amplifier with the same dc gain with BJT and MOS, because the bjt needed is smaller the bjt amplifier is capable of working to higher frequencies than that with the MOST. Also I think still bjt has higher Ft than the MOS transistor although they are getting closer with the new technologies.
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