You could start off on the most basic diode, the one done by Edison. Basically a vacuum tube.
Then you could move on to a triode, the vacuum tube with a wire mesh in between to control the electrons.
Then you could do the p-n junction diode. Then the transistor by Shockley et. al.
Move on to the MOSFET or FET etc..
Thats from the electronics and electrical engineering perspective. You could add another one from the computational and logic part like Babbages differential engine. And then move on to binary numbers and the AND OR NOR NOT gates...
That would pretty much fill up 20 pages. In the final pages you could outline how TODAY's computers work and put in stuff u could rip off from wikipedia with very basic or laymen knowledge like Quantum Computing, supercomputers blah blah..
And furthemore, in the middle, you could also add in stuff like software development and how business markets emerged for the computer. Like the birth of IBM and DOS with Microsoft. The open source movement with Linus Torvalds...
Or programming language development like Alan Kay and smalltalk..
the list just goes on and on.. the best thing to do is to start on one wikipedia article and keep clicking all the way in the given links on that page. By around 4 hours of reading, you would be wondering how to make all that information fit into 20 pages...
hope this silly rant helps..EDIT: ok, sorry, I just realized it was the "physics of computing" ... sorry if I babbled on unnecessary stuff..