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Reverse-bias
Connecting the P-type region to the negative terminal of the battery and the
N-type region to the positive terminal, produces the reverse-bias effect. The
connections are illustrated in the following diagram:
A silicon p-n junction in Reverse-bias.Because the P-type region is now
connected to the negative terminal of the power supply, the 'holes' in the
P-type region are pulled away from the junction, causing the width of the
nonconducting depletion zone to increase. Similarly, because the N-type region
is connected to the positive terminal, the electrons will also be pulled away
from the junction.
This effectively increases the potential barrier and greatly increases the
electrical resistance against the flow of charge carriers. For this reason there
will be no (or minimal) electric current across the junction.
At the middle of the junction of the p-n material, a depletion region is created
to stand-off the reverse voltage. The width of the depletion region grows larger
with higher voltage. The electric field grows as the reverse voltage increases.
When the electric field increases beyond a critical level, the junction breaks
down and current begins to flow by avalanche breakdown.
Summary
The forward-bias and reverse-bias properties of the p-n junction imply that it
can be used as a diode. A p-n junction diode allows electric charges to flow in
one direction, but not in the opposite direction. When the p-n junction is
forward-biased, electric charge flows freely due to reduced resistance of the
p-n junction. When the p-n junction is reverse-biased, however, the junction
barrier (and therefore resistance) becomes greater and charge flow is minimal.
Non-rectifying junctions
In the above diagrams, contact between the metal wires and the semiconductor
material also creates metal-semiconductor junctions called Schottky diodes. In a
simplified ideal situation a semiconductor diode would never function, since it
would be composed of several diodes connected back-to-front in series. But in
practice, surface impurities within the part of the semiconductor which touches
the metal terminals will greatly reduce the width of those depletion layers to
such an extent that the metal-semiconductor junctions do not act as diodes.
These "nonrectifying junctions" behave as ohmic contacts regardless of applied
voltage polarity.
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