From the time semiconductor engineering came to existence, several numbering systems were adopted by different countries. However, the accepted numbering system is that announced by Proelectron Standardization Authority in Belgium. According to this system of numbering semiconductor devices:

1. Every semiconductor device is numbered by five alpha – numeric symbols, comprising either two letters and three numbers (e.g. BC149) or three letters and two numbers (e.g. BFX63). When two numbers are included in the symbol, the device is intended for industrial and professional equipment. When the symbol contains three numbers, the device is intended for entertainment or consumer equipment.
2. The first letter indicates the nature of semiconductor material.
   A = Germanium, B = Silicon, C = Gallium Arsenide and R = Compound material (e.g. Cadmium Sulphide). Thus AC125 is a Germanium transistor and BC149 is a Silicon transistor.
3. The second letter indicates the device and circuit function.

A = Diode
B = Variable capacitance diode
C = A. F. low powered transistor
D = A. F. power transistor
E = Tunnel diode
F = H. F. low power transistor
G = Multiple device
H = Magnetic sensitive diode
K = hall – effect device
L = H. F. low power transistor
M = Hall – effect modulator
P = Radiation sensitive device
Q = Radiation generating diode
R = Thyristor (SCR or Triac)
S = Low power switching transistor
T = Thyristor, power
U = Power switching transistor
X = Diode, multiplier
Y = Power device
Z = Zener diode