Hi,

the device voltage determines the normal regime of operation of the transistor, i.e. a 2.5V device is supposed to be operated under supply voltages of 2.5V±10%. It will certainly not break down when being subject to a somewhat larger gate or drain-source voltage for some time, but this is not recommended for reliable operation over a longer time scale. You can have a look at the reliability information usually included in the design kits for more information on ageing under stress.

Some people also use devices under supply voltages exceeding the nominal value and make sure that each individual device isn't subject to a GB, GS, GD or DS voltage exceeding the nominal value. This practice is considered arguable by others.

Paul