Unlike the American EIA JEDEC (and to their credit) the Japanese EIAJ took the time to categorize various transistors. For JEDEC, as an example, the 2N prefix does not allow any definition of category, so a 2N4401 is NPN, 2N4403 is PNP, 2N7002 is N-FET, and I had trouble finding anyone who had a 2N that is a P-FET. It seems that company-held prefixes are taking the place of JEDEC numbers (at least in some of the newer devices). Here is a breakdown of EIAJ prefixes:

• 2SA-PNP
• 2SB-PNP
• 2SC-NPN
• 2SD-NPN
• 2SJ: P-CH FET (J-FET/depletion mode or Enhancement mode)
• 2SK N-CH FET (J-FET/depletion mode or Enhancement mode)

Each 2S number is registered with a particular transistor manufacturer. Only one company would have the same number. For example, 2SA1037A only belongs to ROHM Co., LTD. (This is unlike the JEDEC 2N, where for example, 2N4401 can be made by many companies, Fairchild, On Semi, ROHM...etc.)

Generally, the 2SB and 2SD devices are lower frequency operation. In many cases they are higher power devices; with the switching characteristics not defined in detail quite as often as is more the case for 2SA and 2SC. I do not believe there is a hard and fast rule, which remains true. 2SA versus 2SB categories are a general case.

As can be observed, there is an ability to learn initial information about an EIAJ transistor just by looking at its prefix a rather ingenious numbering system.