Discontinued
Semiconductors
SLC
are suppliers of obsolete electronic components, parts, peripheral-devices.
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SLC is your
international independent source for obsolete,
hermetic, hard to find, DMS, discontinued semiconductors,transistors
and vacuum tubes. SLC offers hermetic components and vacuum
tubes that are obsolete or difficult to find. SLC guarantees and
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Types
of semiconductors
Pure
or intrinsic semiconductors.
A
silicon crystal is different from an insulator because at any temperature
above absolute zero temperature, there is a finite probability that
an electron in the lattice will be knocked loose from its position,
leaving behind an electron deficiency called a "hole". If
a voltage is applied, then both the electron and the hole can contribute
to a small current flow.
An
intrinsic semiconductor is one which is pure enough that impurities
do not appreciably affect its electrical behavior. In this case, all
carriers are created by thermally or optically exciting electrons
from the full valence band into the empty conduction band. Thus equal
numbers of electrons and holes are present in an intrinsic semiconductor.
Electrons and holes flow in opposite directions in an electric field,
though they contribute to current in the same direction since they
are oppositely charged. Hole current and electron current are not
necessarily equal in an intrinsic semiconductor, however, because
electrons and holes have different effective masses (crystalline analogues
to free inertial masses).
The concentration of carriers is strongly dependent on the temperature.
At low temperatures, the valence band is completely full, making the
material an insulator (see electrical conduction for more information).
Increasing the temperature leads to an increase in the number of carriers
and a corresponding increase in conductivity. This principle is used
in thermistors. This behavior contrasts sharply with that of most
metals, which tend to become less conductive at higher temperatures
due to increased phonon scattering.
Extrinsic
semiconductors.
An
extrinsic semiconductor is one that has been doped with impurities
to modify the number and type of free charge carriers.
N-type
doping
The
purpose of n-type doping is to produce an abundance of carrier electrons
in the material. To help understand how n-type doping is accomplished,
consider the case of silicon (Si). Si atoms have four valence electrons,
each of which is covalently bonded with one of four adjacent Si atoms.
If an atom with five valence electrons, such as those from group VA
of the periodic table (eg. phosphorus (P), arsenic (As), or antimony
(Sb)), is incorporated into the crystal lattice in place of a Si atom,
then that atom will have four covalent bonds and one unbonded electron.
This extra electron is only weakly bound to the atom and can easily
be excited into the conduction band. At normal temperatures, virtually
all such electrons are excited into the conduction band. Since excitation
of these electrons does not result in the formation of a hole, the
number of electrons in such a material far exceeds the number of holes.
In this case the electrons are the majority carriers and the holes
are the minority carriers.
P-type
doping
The
purpose of p-type doping is to create an abundance of holes. In the
case of silicon a trivalent atom, such as boron, is substituted into
the crystal lattice. The result is that an electron is missing from
one of the four possible covalent bonds. Thus the atom can accept
an electron from the valence band to complete the fourth bond, resulting
in the formation of a hole. Such dopants are called acceptors. When
a sufficiently large number of acceptors are added, the holes greatly
outnumber the excited electrons. Thus, the holes are the majority
carriers, while electrons are the minority carriers in p-type materials.
