The difference between 2n3904 and 2n3906

Why the 2N3904 transistor will be popular:May just be traditional or familiar and people tend to have some 3904s and 3906s on hand, even if they have almost nothing else.It is the competitive part of the actual application built.

The difference between 2N2222 and 2N3904 NPN transistors:They are similar in specification. as a general purpose small signal NPN. The maximum Vce voltage was probably a little bit different but around 30–40 volts or more, plenty for transistorized and logic circuits, with a Hfe (beta) somewhere around 50–100..

The main difference is physical - the 2N2222 is in a metal hermetic TO-18 can and the 2N3904 was in a plastic encapsulated molded TO-92 case. The metal can was more suited for some heat sinking and thermal dissipation and the plastic not so much but was cheaper.

For small signal use with little power dissipation the 3904 would be cheaper by a fair amount due the the package. Though they were electrically close, they were not friendly to interchange or substitute for one another because the 3904 had the leads straight across and the 2222 had the leads in a right triangle and the PCB outlines would be round vs D-shaped leading to confusion.