Conduction current in a bipolar transistor

[Jim Clark]

This would be "diffusion" current. Carriers injected into the base region from
the emitter will diffuse to the depletion region of the base-collector reverse biased 
pn junction, where they are then swept across.

Diffusion occurs merely because of the tendency for an imbalance in the concentration
of carriers to even out due to random thermal motions. The net motion (and hence
current) is in the direction of decreasing concentration (and thus from emitter to
collector).

Jim
  ----- Original Message ----- 
  From: Heiko van der Linden 
  To: synth-diy at node12b53.a2000.nl 
  Sent: Thursday, October 19, 2000 3:39 PM
  Subject: Conduction current in a bipolar transistor


  Hi All,
   
  This may sound like a stupid question but I would really like to know the answer.
   
  I was talking with a friend about bipolar transistors and how they work. This went ok for a while but
  after some talking we got into a problem: what kind of current flows through the collector during normal
  operation ?
  I understand that the majority carriers come from the emitter into the base. And then they feel the electric
  field coming from the collector and are swept over the reverse biased base collector junction.
  This sounds all very intuitive (the charge carriers are attracted by the oppositely charged collector)
  but the base collector junction is reverse biased. So the current that flows is not a "normal" current.
  What I would like to know is: what's the conduction mechanism of this current ? Is it an avalanche type 
  of conduction or something else ? And what's this type of conduction called ?
  I couldn't find a website that explained this aspect of transistors. Some of them say it's a transistor
  current etc. So that's not much use either when you want to understand what is really happening in
  those little black and silver cans....
   
  Thanks,
  Heiko
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