Figure: A diode symbol with associated P and N regions.
The next step in understanding diodes is to understand the physical
principles behind the diode. A diode is composed of two regions
of semiconductor (eg. silicon, germanium). A semiconductor is neither a good conductor such
as copper, nor an insulator (dielectric) such as glass. A slab of
semiconductor can be thought of as a resistor whose resistance depends
on the amount of doping (impurities such as gallium or arsenic) that is
implanted in the silicon. One region of semiconductor within the
diode is called the P region. The P region has been doped with
an impurity which causes the presence of holes, or incomplete covalent
bonds in the semiconductor crystal. These holes in the P region attract electrons
and therefore are analogous to positive charges. Hence, the P region may
be thought of as being Positive (even though it is electrically neutral).
The N region has been doped with
an impurity which causes the presence of more electrons than are needed for
the covalent bonds in the crystal. The ``extra" electrons (the ones not
involved in covalent bonding) are available for
conduction. The N region may be thought of as the Negative region due to
the presence of the electrons available for conduction.
At the junction between the two regions, a potential field is built-up. This
field produces a barrier potential and
is responsible for the (approx.) 0.7 V turn-on voltage (for silicon diodes)
required in the forward direction to establish current flow.
When a voltage is applied in the reverse direction we say that
the diode is reverse biased. When the diode is reverse biased,
the potential barrier builds up
across the junction and opposes current flow through the diode. The
greater the applied reverse voltage, the larger the barrier.
When reverse biased, only a very small leakage current flows.
This leakage current is called the reverse saturation current
(
).