Ah !!
A good old’men recommandation
Yep! Any BC108 or BC547 will do and 1N4001 for the reverse bias protection. 1N4148 doesn’t have the power rating to sick the high ENF induced current.
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By chance, I also came across a variant with a MOSFET that should be able to drive also 12V fans.
Apparently, the software PWM control of a fan is as annoying as the backlight. I found another article explaining the troubles (more noise, of course!). The article is rather verbose and also explains in detail how everything works (practical for beginners & forever-beginners! :)) and the difference between using a BJT and MOSFET transistor (and why the latter might be preferred).
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Thks, interesting lecture, they are there a couple of good posts 
Hm. Looks like my cheapy creepy little fan is not really PWM friendly. It does not turn unless I set 100% duty cycle. If I set the frequency up to 1MHz, I can go some way down from 100%, but it makes an annoying high frequency sound/noise.
Or may it be a problem of the wiring? I used a BS170 mosfet. When connected directly to constant 3.3V instead of 5V, it turns well and more slowly.