When no voltage is applied a ttransistor’s base, electrons in
the emitter are prevented from passing to the collector side because of
the pn junction.
If a negative voltage is applied
to the base, things get even worse as the pn junction between the base and emitter becomes reverse‑biased resulting in the formation
of a depletion region that prevents current flow.
If a positive
voltage (>0.6V) is applied
to the base of an npn transistor, the pn junction between the base and
emitter becomes forward-biased. During forward bias, escaping electrons are drawn to the positive base. Some electrons exit through the base, but because the p-type base is so thin, the onslaught of electrons that leave the emitter get close enough to the collector side that they begin jumping into the collector. Increasing the base voltage increases the emitter-to‑ collector electron flow. Recall, positive
current flow is in the direction opposite to the electron
flow ocurrent
flows from collector to emitter.
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