mosfets better than bjt and jfet?

why fet is less noise than bjt

why ppl mostly use mosfets over bjt or jfets....wat r the special advantages of mosfets..i would also like to knw abt the frequency response of mosfet.

thnx frds.

why fet small in size than bjt?

MOSFET is better than BJT and FET because it has large i/p impedance and less noisy.

bjt mosfet jfet noise

It all depends upon the criteria. JFET is the lowest noise, BJT is the next lowest. MOS takes up less room on a wafer surface area and therefore is lowest cost.

bjt compared to mosfet

Well.. Its not always better..

JFETS are not used extensively as MOSFETs usually outperform them with the advantage of no gate current.

In Integrated Circuit:
1. In Digital, we use MOSFETs because they take a very small size and can make the required logic functions with a very few number of transistors compared to the BJTs. They also don't absorb base current as BJTs and also they dissipate less power.

2. In Analog, we have two cases:
i. We use MOSFETs as in digital usually because most circuits now are not purely analog, but mixed signal circuits including both digital and analog parts, so we use it because its already in the process, although we sometimes need the speed and gain and less noise of the BJT which is quite attractive.
ii. If you can get to a BiCMOS process then most of the HIGH SPEED analog parts are made using BJTs to make use of the advantages mentioned above together with MOSFETs for the digital parts.


In Discrete circuits:
BJTs are used extensively for their high gain and speed and less noise, as the working environment is usually noisy with high capacitance or load. Here the BJTs dominate and outperform the MOSFETs also becuase they are not ESD sensetive devices unlike MOSFETs. Also, in discrete circuits mostly the package sizes are large enough to make any decrease in the device size insignificant, although power requirements might still be a bottleneck for the BJTs.

advantage fet over bjt

CMOS comes out of non-active operating region more quickly (BJT slow out of saturation).

However, the actual speed depends on what device you are designing and what is their application. Anolog designs using BJT still is faster because it has more speed, transconductance per amount of bias current.

Also, CMOS has faster swithing speed when used in digital design due to smaller and smaller transistor size.

On top of that, the main reason most people use CMOS is because process cost advantage. It is compatible with digital CMOS (mixed signal).

are bjt less susceptible to esd -patent

In digital domain, MOS is always better than BJT since there is base current in BJT and thus increasing the power dissipation.

In analog domain, BJT could be better than MOS in the following aspects:
- high gain and higher current per unit area
- smaller Vcesat than Vdsat (higher output swing)
- better Vbe control compared to Vt (only depends on doping concentration)
- lower noise
- higher breakdown voltage


** The cost of using BJT is complex process and thus higher cost. **

jfets with the lowest noise

I totally agree with Elmolla ...

bjt has higher gain than mosfet

In this world everything is based on money ,gain and easy.
All these features r there with MOSFET..
1.Its easy to operate.
2.its easy to fabricate.
3. it has infinite i/p resistance ..so no loading probs.
4.more economical...
This is the case with digita.
Now coming to analog ,there we use both BJT,MOSFET based on the parameters we want lik gain,noise,linearity etc.
These days combining the advantages of BJT ,new technology BiCMOS is coming..

doping noise bjt

suvendu said:

MOSFET is better than BJT and FET because it has large i/p impedance and less noisy.

Click to expand...

MOSFETS are terribly noisy below ~1MHz.

Added after 3 minutes:

flatulent said:

JFET is the lowest noise, BJT is the next lowest.

Click to expand...

It depends on Ic and Rs, see Fundamentals of Low-Noise Analog Circuit Design, W. MARSHALL LEACH, JR.