"gateway drug to semi-conductor physics and electronics." => So true. But I will wait a little bit more than 9 years before teaching semi-conductor physics to my little boy ;-)
Why? When I was that age, I would tear apart all of the electronics in my possession trying to figure out how they worked. I never could figure it out, until much, much later. Not because I couldn't have understood it, but because I didn't have anyone to help guide me with regards to electronics.
edit:
I would have killed for someone to have gotten me something like http://www.amazon.com/Snap-Circuits-SC-300-Electronics-Disco... which is listed as appropriate for age 8
Semi-conductors physics may be too much for a 9-year-old, but an introduction to transistors or doing a H bridge to reverse the rotation direction of a motor could be pretty impressive.