I was thinking about MOSFETs. it's a question to me that Why do they say +- for Vgs(gate to source) of e.g. an N-channel MOSFET?

 

Because the gate can be only say 20V above the source, or 20V below the source, anything more will destroy the transistor.

 

That's a voltage limit, not an operating condition.
For example, you can apply a negative gate-source voltage to an enhancement-mode N-MOSFET but the device would stay turned off, the same as if Vgs were zero volts.

 

If you look at the data sheet of any semiconductor, from a simple diode to the fanciest graphics processor, there will exist, at least, these two categories: (1) Absolute maximum ratings, and (2) Operating conditions. If you go outside the "absolute maximum ratings", then you'll need to buy some new parts.

That Vgs spec is the "Absolute maximum ratings" table. For MOSFETs, the gate (g), relative to the source (s), can tolerate 20V in either polarity, before being damaged. Usually, that is a conservative number, and it can handle more, before being damaged. But you don't want to try it. In fact, most designers, try to stay at least 25% away, from it, or no more than 15V, if 20V rated.

As stated before, an enhancement mode N-Channel MOSFET, will only turn ON, with some positive voltage, applied to the gate, relative to the source. Just about all power MOSFETs, are enhancement mode types. There are depletion mode MOSFETs. Those are ON with zero volts, at the gate. A negative voltage (if an N-Channel), needs to be applied, to turn it OFF. BUT ... in both enhancement mode and depletion mode (again all N-Channel), the more positive the gate, more current flows thru drain to source; the more negative, less current flows.

 

Thanks for the extended explanation Threeneurons, an by the way welcome to the forum