Diode
Circuits
So far we have only
considered passive circuit elements. Now we will consider our first reactive circuit
element, the diode. Hopefully things will start to get a little more
interesting.
Along with the diode there
is the transistor, which will be discussed in future lectures.
These circuit elements are
commonly made from a semiconductor basic material. Semiconductors along with
the passive circuit elements are often integrated onto a single electronics chip.
If a hundred or so transistors are on a chip it is referred to as an integrated
circuit or IC. Large-scale integrated circuits, LSI, contain thousands of
transistors.
And today even very-large-scale
integrated circuits, VLSI, exist, with hundreds of thousands of transistors.
Now lets talk physics.
1 . Energy
Levels
The physical principles of
semiconductor devices can be understood by considering quantum energy levels in
the material. I will just give a sketchy view here and define some terminology.
Valance
electrons are the electrons outside the closed shells of an atom. In
silicon and
germanium there are four
valence electrons, arsenic has five and gallium three. The large number of
valance electrons favour these elements as semiconductors.
To sustain an electric
current, a material must have charge carriers that are free to move. There is
some probability that an atom may eject a valence electron which is then free
to move in the material. The conductivity of a material is thus a function of
the number of free charge carriers per unit volume. Based on these probability
densities it is common to divide materials into three categories: conductors,
semiconductors and insulators.
In crystals atoms interact
and bind by sharing valence electrons. The wave function is
no longer associated with
a single atom but extends over the entire crystal. One effect of the
interaction between the atoms is that the otherwise degenerate energy levels
split into closely spaced levels. Since the number of atoms is large, it is
common to refer to this set of levels as a continuous energy
band.
In solid materials there
usually exist a valance band which is an energy region
where
the states are filled or
partially filled by valence electrons. The conduction band is
defined to be the lowest unfilled energy band. So our three materials can be
characterized by their band structure. An insulator has the valence and
conduction band well separated.
semiconductor has the
valence band close to the conduction band – separated by about a1 eV gap.
Conductors on the other hand have the conduction and valence bands overlapping.
The interesting property
of a semiconductor is that thermally excited electrons can move
from the valence band to
the conduction band and conduct current. Silicon and germanium have thermally
excited electrons at room temperature and hence their common use in diodesand
transistors.
When an electron has been
excited into the conduction band, the hole left behind in the
valence band is also free
to move through the crystal. A quantum mechanical treatment of this effect puts
the hole on an approximately equal footing with the electron. Temperaturecauses
the thermal generation of electron-hole pairs. One of the components of the
pair will add a little to the majority charge carriers. The
other component of the electron-hole pair will become the minority
charge carrier. Minority charge carriers limit ideal performance and
increase with increasing temperature.
A common method for
generating even more charge carriers in a semiconductor is by
doping. That
is, replacing a few atoms of the base material with atoms of a different
element.
These impurities will
contribute an excess electron or hole which is loosely bound and hence can be
excited into the conduction band by thermal energy. In N-type semiconductors
the majority of free charge carriers are negative, while in a P-type
semiconductor the majority are positive.
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