Science Buddies: "Ask an Expert"

How do diodes function properly?
I get that there is a semiconductor junction, and that makes sense; however attaching metal wires themselves should create another junction. Cats whisker diodes should have the same problem, and why do they need to have a point contact?

Since solar panels can be used to produce light, and silicon ones are a semiconductor junction, are they intrinsically the same as LEDs, differing only in geometry? And are photo diodes in some photo sensors the same as light emitting diodes minus the difference in shape?
And what is different between a LED and a laser diode?
And why do some laser diodes have three pins?

Hello. Please keep all questions for your project to one post so that our experts can best reply based on what has already been discussed. Good luck, and thanks for using Science Buddies!