Transistor overheating
Another simple way to look at this is just by resistance ratio and power ratio.

If you have a 3.6W load and only a TO-92 switch, what switch resistance and conditions are required to use this?

Consider Rja= 200'C/W lets consider 1/4W max for a 50'C junction temperature rise above ambient.

Loss/Load= 0.25/3.6W x100% = 7%

Load = 12V/0.3A= 40 ohms

Loss = 7% of 40 ohms = 2.8 Ohms


BLDC fans have a steady-state (SS) proportional current above stall speed so may be considered like a SS resistive load.
What switches are &lt;= 2.8 Ohms at &lt;= 300 mA ?

How do you choose ?

For NPN BJT's which are slightly better than PNP then you look on the datasheet for Vce/Ic at the current you intend to use.  We call this $$R_{CE} =\dfrac{V_{CE}}{I_{C}}$$
Rce only appears on datasheets of &quot;superbeta&quot; transistors because in saturation Vcb is starting to forward bias and shunt the current gain, hFE towards about 10% of it's max hFE  rating as practical limit.  But you can compute this for any BJT.
Why does hFE reduce when it saturates.


2N3904 TO-92 ON SEMI fig2 : Rce=  0.12V @ 100mA Max @ hFE=10 ( BAD choice , not rated for 300 mA, only 200 mA and you should have heat margin so derate 50%)

PN2222A TO-92 can be made to work with Rce &lt;=2 ohms  but a power transistor or low RdsOn power FET is clearly a better choice