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 Vds voltage, no current -> no dissipated power
- Conduction: Very low Vds voltage (Id*Rds_on), high current (Id) -> small dissipated power (Rds_on^2 * Id)
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 Vds voltage, non negligible current. Id * Vds 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's a LED and you want to have if becoming brighter and brighter but slowly, your 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.