Delete comment from: Ken Shirriff's blog
"bite": on the chips I've seen, NPN transistors are usually rectangular and PNP are usually round, because NPN transistors are vertical while PNP transistors are lateral with the base and collector surrounding the emitter. (I'm using "The Art of Analog Layout" page 80 as a reference.) But I don't know if that's true in the 556 you're looking at.
As for Q21, yes it looks like a diode. This is one of the components I puzzled over for a while, since a diode doesn't make sense in the circuit but there's not obvious collector. After looking closely, I think what's happening is there's no isolation "moat" between Q21 and Q22 so they share the same N region connected to Vcc. In other words, Q22's Vcc-to-collector connection also serves as Q21's Vcc-to-collector connection.
Mar 18, 2016, 1:39:02 PM
Posted to 555 timer teardown: inside the world's most popular IC