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Question About Transistors.

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westkid85

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hi guys,
may i know that what is the major difference between MOSFET and BJT transistor?
what is the advantage of using anyone of them?
 
A MOSFET is a majority carrier davice, vs. a bjt, which is a minority carrier device.

FETs are unipolar, i.e., drain to source current conducts through one type of material (n or p). A bjt, is of course "BIpolar"", meaning that collector to emitter current conducts through two types of material, both n and p.

FETs present a very high impedance at the gate with non-linear capacitance. They are best suited for being driven with a low impedance constant voltage source. They need current as well as voltage, but a CVS provides both. A bjt presents a low impedance at the base and is best driven by a high impedance CCS (constant current source). A bjt requires both current and voltage, but the CCS provides both.

FETs switch faster between fully on and off (switch mode) than bjt since they are majority carrier devices. But bjt switches fastest when switched between active and cutoff regions. A bjt is slower when switched between saturation and cutoff.

The bjt is generally a higher (power) gain device. A FET stage presents higher input impedance, which can be advantageous, but the current gain voltage gain product for a bjt stage is usually higher than for a FET stage.

These are just some characteristics off the top of my head. Good peer-reviewed textbooks will elaborate and give mathematical insight. I'd recommend university libraries and approved texts for reference. Also, makers of FETs and bjts know them better than anyone. Visit the sites of On Semi, Fairchild, National Semiconductor, Texas Instruments, etc. Avoid self-proclaimed experts, as the web is filled with those.
 
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