Here is a link to the question pool home which will give an idea of what to look into:
http://www.ncvec.org/page.php?id=356
Near the bottom of the page there is a link to diagrams:
http://www.ncvec.org/downloads/2020ExtraClassPoolDiagrams.pdf
And the question pool itself:
http://www.ncvec.org/downloads/2020ExtraClassPoolJan22.txt
It looks like section E6B is the diode section but look at the other places that mention diodes too as that will show you other things you may be interested in. For instance there is a diode question in E6A that would be good to understand.
This is only a start, the next step is to figure out the things that are unfamiliar, so look up the different types of diodes like Zener, Schottky, PIN, LED, sheesh there are a lot, this seems like a good start:
Check out the diagrams and see if you can identify all the diode symbols in Figure E6-2, then figure out what the application would be.
E6 is the circuit components section, good stuff to know for sure, but if it is a super rough section for you make sure to balance your study with the other sections where you are stronger and just give E6 your best shot.
Each section has an indication in the pool of how many questions will come from that section. E1A was my nemesis, memorizing the frequencies was just not working out... so I stopped worrying about it and figured out enough of them to get some right but even if I would have gotten everyone in that section wrong I would still be ok (and I have a handy chart to look them up for when I do need to know: http://www.arrl.org/graphical-frequency-allocations)
