Pro Electron

• Unlike the tube naming convention, if there are two transistors in a single envelope, the type letter was never repeated - so a dual NPN RF transistor might get a type "BFM505" rather than something like "BFF505" for instance.
• Although some of the most popular devices conform to a pattern of serial numbers that identified package type and polarity, many do not.
• The letters assigned for the second character of transistor and diode type numbers differ in several ways, e.g.
  • "B" tends to be used for dual varicap diodes
  • "L" in the context of transistors designates RF power (transmitting) transistors; for valves it meant a high-power pentode tube (the usual choice for power RF)
  • "Z" is used for semiconductor Zener diodes instead of (full-wave) rectifier valves (tubes).

• • A Germanium (or any semiconductor with junctions in a material with a band gap of 0.6 to 1.0eV)
  • B Silicon (or band gap of 1.0 to 1.3eV)
  • C III-V semiconductors with a band gap of 1.3eV or more, like gallium arsenide in LEDs
  • D may be...
    • Semiconductors with a band gap less than 0.6eV, such as indium antimonide in infrared detectors (rarely used), or
    • (Mullard–Philips) 1.4V (or less) filament tubes
  • E (Mullard–Philips) tubes with a 6.3V heater
  • F Digital integrated circuits
  • P (Mullard–Philips) tubes for a 300mA series heater supply
  • R Devices without junctions, e.g. cadmium sulfide in a photoresistor
  • S Solitary digital integrated circuits
  • T Linear integrated circuits
  • U may be...
    • (Mullard–Philips) tubes for a 100mA series heater supply, or
    • Mixed digital/analogue integrated circuits

(see above)

Following these two letters is a 3- or 4-digit serial number (or another letter then digits), assigned by Pro Electron. It is not always merely a sequence number; there is sometimes information conveyed in the number:

• In early devices only, the serial number often indicated the case/package type (e.g. AF114-7 for TO-5 case, while AF124-7 were TO-72 versions of the same transistors); modern surface-mount devices often begin with "8",
• early silicon transistors followed the convention of using a middle digit of 0-5 for NPN and 6-9 for PNP.
• the last digit often indicated a particular specification or application grouping, e.g. the AF117 and AF127 were similar IF amplifier devices in different cases; the BC109, BC149, BC169 and BC549 are similar low-noise transistors).
• some modern devices use letters, such as "B" to indicate HBT bipolar transistors.[1]

Suffixes may be used, letters or perhaps blocks of digits delimited by "/" or "-" from the serial number, often without fixed meanings but some of the more common conventions are:

• for small-signal transistors "A" to "C" often means low to high h_FE, such as in: BC549C[2]),
• numeric suffixes may be used as an alternative way to show h_FE (e.g. BC327-25), or voltage rating (e.g. BUK854-800A[3]).
• for voltage reference diodes letters show the tolerance ("A","B","C","D","E" indicate 1%/2%/5%/10*/20%) and may be followed by the V_z value, e.g. 6V8 for 6.8 Volts or 18V for 18 volts.
• "R" can mean "reverse polarity".

Examples of suffixes and manufacturers' extensions to the basic sequence number include:

Note: A BC546 might only be marked "C546" by some manufacturers, thus possibly creating confusion with JIS abbreviated markings, because a transistor marked "C546" might also be a 2SC546.

Short summary of the most common semiconductor diode and transistor designations:

      BC549C
     / |--- \___ variant (A,B,C for transistors implies low, medium or high gain)
    /  |   \____ serial number (at least 3 digits or letter and 2 digits)
   /  device type:
A=Ge     A=Signal diode
B=Si     C=LF low-power transistor
         D=LF Power transistor
         F=RF transistor (or FET) 
         P=Photosensitive transistor etc.
         T=Triac or thyristor
         Y=Rectifier diode
         Z=Zener diode

Poland, Hungary, Romania, and Cuba mostly used Pro Electron designations for discrete semiconductors just like Western Europe. Kombinat Mikroelektronik Erfurt (KME) in East Germany and Tesla (Czechoslovak company) used designations derived from the Pro Electron scheme. In particular, the first letter specifying the material differed while the second letter followed the table above (with the few exceptions for KME noted below).[4]

Examples: GD241C - Germanium power transistor from KME; MB111 - optoisolator from KME; KD503 - Silicon power transistor from Tesla; LQ100 - LED from Tesla.