PIC driven transistor - smoothed signal edge - why?

[flotulopex]

Hello,

I'm driving a transitor with a PIC; it is meant to control an IR led for a remote with a 38kHz carrier freq.

Depending on what PNP or NPN transistor I use, the final signal edge is smoothed (green arrow).

Clearly, the signal is much better with a PNP transistor (BC307b graph).

Since both transistor suppose to be "complementary" and have identical specs, I don't understand why the edge (BC237 graph) is smoothed that way while using a BC237 NPN transistor.

Any clue? Is this due to the IR-Led's capacity?
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2672&stc=1&d=121353203 4">