In fact, this question has been asked on the EE site, but it's not well-answered. I suppose it might be more on-topic here.
According to this answer:
Note that the holes injected into Emitter are supplied from Base
electrode (Base current), whereas the electrons injected into the Base
are supplied from Emitter electrode (Emitter current). The ratio
between these currents is what makes BJT a current amplifying device –
small current at Base terminal can cause a much higher current at
Emitter terminal. The conventional current amplification is defined as
Collector-to-Base currents ratio, but it is the ratio between the
above currents which makes any current amplification possible.
First off, Why collector current increases as base current increase? Is the former causes the later, or the later causes the former, or something else (voltage on electrodes, maybe) causes both?
And here is my question, Why collector current always increases more than the increment of base current? Say after something changes, a
extra holes are "injected into" emitter region, and b
extra electrons are injected into base region. Then why b
is greater to a
?
Best Answer
Answering questions:
But the C-B diode is not forward biased. This is an application where the BJT is used as a diode and no "classical" amplification is possible (transition region between saturation and amplifying region).
It does not matter too much - on the other hand: Look at the Ic=f(VCE) curves. Ic slowly rises with VCE because of the Early-effect.
In this case (VCE < VBE) the C-B diode is open and there is a small current Ic which has a direction opposite to the "normal" Ic direction. Example: For VCE=0 we have a current Ic which is negative (The Ic=f(VCE) curves do NOT cross the origin!).