There are various series of transistor codes used:

• Codes beginning with B or A, for example BC108, BC478
  The first letter B stands for silicon while letter A stands for germanium (rarely used now). The second letter indicates the type; for example C means low power audio frequency; D means high power audio frequency; F means low power high frequency. Sometimes a letter is added at the end (e.g. BC108C) to identify a special type of the main type.
  
  
• Codes beginning with TIP and `2N` (example: - TIP42A and 2N5026)
  The word TIP in transistors refers to the manufacturer: Texas Instruments Power transistor. 2N denotes transistor for example 2N5026. Odd numbers indicate transistor is NPN while even numbers indicate transistor is PNP. The letter at the end identifies versions with different voltage ratings.