MOSFET(Gate and source shorted)

[Mani-chan]

I want to know how will the voltage current characteristics of a MOSFET will be once we connect gate directly to the source?

 

[betwixt]

What kind of MOSFET, small signal RF types or power types?

If you are talking about power MOSFETs which are enhancement mode devices, with the gate connected to the source only leakage current will flow.

Brian.

 

[dimmich]

betwixt, please explain what the difference with the small signal transistors? Difference will be only in RF signal due to parasitic capacitances in the transistor?

 

[betwixt]

MOSFETs are a big family of devices.

The small ones are used in signal amplifiers, particularly at high frequencies VHF/UHF/Microwave. These are usually "depletion mode" MOSFETS, and only capable of carrying a few mA at low voltage. In a depletion mode device, current will flow through the drain-source junction unless the gate voltage is used to stop it. So connecting the gate to the source leave the device in full conduction - no G-S voltage to prevent current flowing.

If the MOSFET you mean are the power devices used in switching circuits or some high power audio amplifiers, they are usually "enhancement mode" devices. They do not conduct between drain and source (except for a tiny leakage current) unless a voltage is applied between gate and source. So shorting the gate and source together means here can be no voltage and the device will not conduct.

depletion mode = voltage needed to STOP them conducting.
enhancement mode = voltage needed to START them conducting.

Brian.

 

[Mani-chan]

Okay. Thank you for your help