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How transistors really work

S

sanjoealex

Jan 1, 1970
0
My doubt is that the current in CB junction of a transisitor in
saturation doesn't flow as in a forward biased diode even though the
collector base junction is forward biased in saturation. Why is this
so......???? I have searched many textbooks and sites and still i
couldn't find an answer. Can anyone help me????
 
R

Robert Baer

Jan 1, 1970
0
sanjoealex said:
My doubt is that the current in CB junction of a transisitor in
saturation doesn't flow as in a forward biased diode even though the
collector base junction is forward biased in saturation. Why is this
so......???? I have searched many textbooks and sites and still i
couldn't find an answer. Can anyone help me????

If *both* E-B junction and C-B junction are forward biased, then by
definition they both carry current, and the device is not acting like a
transistor - which means the term "saturation" does not apply.
Now if only the E-B junction is forward biased, and the collector is
at the same potential as the emitter, then technically the above is
correct, but the device *acts* as if it were still operational.
One can set up an experiment by making a small signal one transistor
amplifier; bias thebase and AC couple a signal in; use a load resistor
and a variable bias on the collector.
Lower the collector voltage while monitoring the collector voltage (AC
and DC).
 
Z

Zorknob

Jan 1, 1970
0
My doubt is that the current in CB junction of a transisitor in
saturation doesn't flow as in a forward biased diode even though the
collector base junction is forward biased in saturation. Why is this
so......???? I have searched many textbooks and sites and still i
couldn't find an answer. Can anyone help me????

The CB current is very small (i.e. doesn't act like a forward biased
diode) even in saturation because the base is relatively lightly
doped. The available majority carriers in the base are consumed by
the emitter/base current leaving no (or few) majority carriers
available to support collector/base current.

The majority carriers in the base are attracted to the emitter rather
than the collector because, even in saturation, the emitter has a
higher energy potential than the collector. I. e. for an NPN
transistor at saturation in a common emitter configuration the emitter
is at ground, but the collector is only at "virtual" ground, so the
holes in the base are more strongly attracted to the electrons in the
emitter than the collector.
 
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