The digit is one less than the number of connections, i.e. 2 for most. Zener diodes The serial number runs from 10-9999. The third letter, if used, is usually a W,X,Y or Z indicates non-commercial use. The serial number runs from 100-9999. BC204A, BAW45, BF299, BFY62.
Manufactures use their own type indictors as prefixes grouped as follows.
NTE downloaded software requires a registration form to be completed but the software is free. Click HERE Other interesting web sites you will want to see. Based on Pacific Semiconductor's 40,000 transistors database. Mister Transistor The Transistor Collector. Revolutionary Transistor Design Turns the Silicon World on End.