In my latex installation, \bigcup produces pdf using F73 fonts!!!, it is Type 3 and I don't know how to make it become embedded subset.

-- UPDATED --

The way to fix this problem is to use the option [cmex10] for the amsmath package. It is stated in the new version (1.7) of the IEEE transaction template by Michael Shell.

In short, just use

\usepackage[cmex10]{amsmath}