How to grep the difference between two files into a file

Saying that I have two files: a.txt and b.txt. The content of a.txt:

mosfet23
cap098
inductors 98
metal96

The content of b.txt:

test 23 cap098
drc45 metal96
lvs cap098

Of course I can use vimdiff to check their difference My question is how to record devices which exists in a.txt but doesn't exist in b.txt into a file? also we cant use comm as b.txt is not sorted? How to achieve using tcl,grep or sed?