BJT (thing)

The Bipolar junction transistor (BJT) is one of the most used transistors today. It replaced the triode in the 1960's. The first BJT had two indium electrodes fused to a germanium substrate. The substrate was at the time called base so the connected electrode also got this name. The collector got its name since it 'collects' charge carriers, while the emitter sends out the carriers. A charge carrier is an electron (negative carrier) or the 'hole' left when an electron leaves the atom (since the atom now have more protons than electrons it gets a positive charge). This is also why the BJT is called bipolar (the conducting area has charge carriers of two polarities)

Buildup of the original
germanium transistor  
      _
     | | 
   e_| |_e
 .-(_| |_)-.
 |   | |   | 
 |  |   |  | 
 |  |___|  |     
 |    |    | 
 |    |    |
 C    B    E


C  Collector
B  Base(germanium substrate)
E  Emitter
e  indium electrodes

Modern BJTs are mostly made from silicon , and consists either of NPN or PNP junctions. The N material is doped with a material with an excess of negative charge carriers while the P material has more positive carriers.

Schematic buildup of a NPN transistor
      .-----------------.      
  C --|  N  |  P  |  N  |-- E
      `-----------------' 
               |
               B

The BJT is a current amplifier; A current flow from the base to the emitter results in a larger flow from the collector to the emitter. This secondary flow is caused by recombination of charge carriers in the 'base' area.
The current amplification might range from tens of times(in high effect transistors) to several hundred times in signal transistors. Darlington transistors easily have several thousand times amplification. The raw current amplification is called hFE or ß(beta).

The germanium transistor has a base-emitter voltage drop of about 0.2v, while the silicon version has a drop of approximately 0.6v. But the silicon transistor has a better temperature and high frequency stability and silicon is much cheaper than germanium. For microwave frequency transistors gallium arsenide is sometimes used. Most transistors today uses an epitaxial layer buildup.

Electrical symbol:
(European version is similar to the USA symbol
but it has a circle around it)
USA version
   NPN

         E
       _ |
     | \|
     |/
 B --|
     |\
     | \
         C
   PNP
         C
     | /
     |/
 B --|
     |\ _
     | |/
        | E