Before learning the internal semiconductor structure of a transistor, I understood that the value of alpha(Common base current gain) being close to one makes Common Emitter current amplification factor (beta) larger and increases amplification capability.

Now I am studying the internals of transistor currents from Neamen's book on Semiconductor physics. It shows that the emitter current has two major components, one is electrons diffusing from Emitter to Base and the second one is holes diffusing from Base to Emitter(NPN transistor). It says, because the latter contributor does not really contribute to the Collector current, it is not a part of basic transistor action. Hence we would like it to be as small as possible. Making this component small leads to alpha to be close to the value of 1. Hence it is desirable to make the value of alpha close to 1.

Later on from the circuit calculation point of view, it seems clear and makes sense that larger alpha can be helpful in more amplification but it doesn't make sense to me from semiconductor point of view as to what would we really gain by making alpha close to one, equivalent to saying, what would we really gain by making holes diffusion from base to Emitter very small?

  • \$\begingroup\$ \$\alpha = \frac{I_C}{I_E} = \frac{\beta I_B}{(\beta +1)I_B} = \frac{\beta}{\beta +1}\$ Thus the ideal value is 1 and \$I_E = I_C\$. All the electrons emitted by the emitter will be collected by the collector. electronics.stackexchange.com/questions/410972/… \$\endgroup\$
    – G36
    Commented May 1, 2020 at 11:19
  • \$\begingroup\$ I am afraid that your problems in understanding result from the fact that you believe the so called "current gain" would be important for voltage amplification in common emitter as well as common base configurations, right? In fact - alpha and beta factors have no major influence on voltage amplification. It is the transconductance which matters. \$\endgroup\$
    – LvW
    Commented May 1, 2020 at 14:20
  • \$\begingroup\$ @LvW I was just emphasizing the fact that essense of beta being large Or small becomes clearer in circuit analysis. I think I couldn't well explain what I meant by the 'essense'. I didn't really mean voltage amplification but current amplification. \$\endgroup\$
    – Bhuvnesh
    Commented May 2, 2020 at 8:32

1 Answer 1


bipolar transistors operate by injecting carriers from the external base lead (a gold? bond wire) to the large-base contact region that shrinks down and down to become the very narrow BASE region that is sandwiched between collector and emitter.

The EMITTER injects carriers of opposite polarity, these carriers attracted to the BASE carriers and trying to intercept and annihilate them.

Fortunately most of the EMITTED carriers MISS and can gathered up in the COLLECTOR.

The hit/miss ratio is crucial to useful bipolar behavior.


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