Some FET parameters question

[saturn]

idss fet

Hello,
What is the meaning of Idss (Zero-Gate Voltage Drain Current) of a FET. Is it the smaller the value, the better the FET?

Also what is Vgs, the smaller the better?

Thanks & best regards.

 

[IanP]

fet parameters

Idss ia a current when Vgs=0V.
Value is just different for different FETs and do not descibe the transistors quality ..
With Vgs you probably mean Vgsoff which is a Vgs voltage at which Id=0 (or near to). Again, different value for diffrent transistor with no preferences ..

 

[saturn]

fet idss

Thanks! Ian.

Still I didn't get much from what you said. For example Idss (as you mentioned when Vgs=0V). In this case, the FET is off, right? Then an ideal FET should have Idss=0?

 

[IanP]

idss in fet

At Vgs=0 FETs conduct certain current: for example the popular BF245C has Idss=12mA, but BF245A has only Idss=2mA.
As I mentioned befor FET is off at Vgsoff what can be for N-FET somewhere below -5V and varies fro differen FETs.

 

[Kral]

fet parameterts

saturn,

A depletion mode junction FET is ON with Vgs = 0. Under this condition, the drain current is relatively independent of the Drain-Source voltage.
This is the Idss.
Idss is not a measure of quality. It is a measure of the maximum current that you can expect the FET to support.

Vgs, or Vgsoff is the magnitude of Gate-to-Source voltage that is required to turn the FET OFF. It is given for a maximum Drain current, e.g., 200uA, and a specified Drain-to-Source voltage, e.g., 10V. For an N-channel depletion mode junction FET, Vgsoff is negative; for a P-channel FET, it is positive. Vgs is not a measure of quality.

For a given family of FETs, the higher the Vgsoff is, the higher will be the Idss.

For a given family of FETs, the higher the Idss value, the lower will be the rdson (drain-to source resistance). The rdson parameter is usually only specified for FETS that are intended for switching applications.

Regards,
Kral