bjt vs fet bandwidth

Thread Starter

indianhits

Joined Jul 25, 2009
86
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!!!
 

mik3

Joined Feb 4, 2008
4,843
Yes BJTs are slower than MOS (less bandwidth) but they have greater transconductance than MOS (higher gain).
 

mik3

Joined Feb 4, 2008
4,843
can you please tell me the reason for its slowness
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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