As Elliot Anderson suggested an NPN would be better. The NPN has a rougly 0.7V drop across it, which if the fan has a tach or PWM speed output, running the base of the fan at 0.7V might not be good for the PWM output or tach output. The NPN will also burn up power in it if the fan has a lot of current. 

(0.7V x fan current) = Watts dissipated in NPN 

Here are some basic ideas for circuits that could be used in a design like this

A better more lossless way uses two nmos transistors. M4 inverts the logic and pulls the gate of M3 to the fan Vcc.

 If you wanted to invert the logic, remove M4 and run M3's gate directly from \Therm and the 10k pullup


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