What are the operating condition of your MOSFET?
When used as a switch, the MOSFET is most of the time in two states:
- Blocked: High \$V_{\text{ds}}\$ voltage, no current -> no dissipated power
- Conduction: Very low \$V_{\text{ds}}\$ voltage (\$I_{\text{d}} \times R_{\text{ds_on}}\$), high current (\$I_{\text{d}}\$) -> small dissipated power (\$R_{\text{ds_on}} \times I_{\text{d}}^2\$)
The MOSFET is in a third state, during a very small amount of time. And this third state is when it is conducting a little:
- Non negligible \$V_{\text{ds}}\$ voltage, non negligible current. \$I_{\text{d}} \times V_{\text{ds}}\$ may be high! -> possibly big dissipated power.
If you plan, by design, to put your MOSFET longer into this third state, you have to ensure that the increase of the temperature of its junction won't let it pass above the maximum allowed temperature for that junction. (found in the datasheet) Reducing the slew rate of a MOSFET has to be carefully studied.
I don't know what you are driving with it. If it is a LED and you want to have it becoming brighter and brighter, but slowly, you would better use a PWM on the gate of your MOSFET and still use it as a switch. If the PWM is very fast, it won't be noticeable to a human eye.
The same approach is also valid for driving a motor.
