bjt vs fet bandwidth

Discussion in 'General Electronics Chat' started by indianhits, Nov 4, 2009.

  1. indianhits

    Thread Starter Active Member

    Jul 25, 2009
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    hello my doubt is in bjt we get a small bandwidth with better gain but in fet we get a larger bandwidth when compared to bjt but gain is not better when compared to bjt

    can anyone please clarify my doubt

    thank you!!!
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    Yes BJTs are slower than MOS (less bandwidth) but they have greater transconductance than MOS (higher gain).
     
  3. indianhits

    Thread Starter Active Member

    Jul 25, 2009
    86
    0
    can you please tell me the reason for its slowness
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    A BJT is a minority carrier device, i.e its operation relies on minority carriers rather majority carries like the MOS. When the BJT is forward biased, minority carriers are accumulate in the p and n regions near the junction. In the p region there is accumulation of electrons and in the n region accumulation of holes. These accumulated charges are separated by the depletion region and that can be though as a capacitor, charges separated by something. This capacitance is called minority carrier capacitance. The greater the forward current in the base, the more charge is accumulated, the more thinner the depletion region and thus greater the capacitance. Therefore, when you turn off the transistor this stored charge (minority carrier) has to be removed as to tun off the transistor. This process is slower than the process required to turn off a MOS and thus a BJT is much slower device than a MOS basically due to its greater turn off time.
     
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