The characteristic curve for a tunnel diode shows an
area of negative resistance. When forward biased the
current in the diode rises at first, but later it can
be seen to fall with increasing voltage, before
finally rising again. The reason for this is that
there are a number of different components to forming
the overall curve. The main two are the normal diode
current across the junction, and the current arising
from the tunnelling effect. It is this last component
that is of interest in a tunnel diode.
Tunnelling is an effect that is caused by quantum
mechanical effects when electrons pass through a
potential barrier. It can be visualised in very basic
terms by them "tunnelling" through the barrier.
The tunnelling only occurs under certain conditions.
This means that it peaks when a certain voltage is
placed across the junction. This results in the
current increasing to a point beyond that which would
be expected for a standard pn junction. As the
voltage across the diode is increased the effect
reduces and the current through the device falls.
This results in a negative resistance region on the
curve of te diode that can be used to provide gain.
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