arating ketones from reaction mixtures: the derivative is crystallized
out and hydrolysed to give the ketone.
A
• Information about IUPAC nomenclature
semiclassical approximation An
approximation technique used to calculate quantities in quantum mechanics. This technique is called the
semiclassical approximation because
the *wave function is written as an
asymptotic series with ascending
powers of the Planck constant, h,
with the Ürst term being purely
classical. It is also known as the
Wentzel–Kramers–Brillouin (WKB)
approximation, named after Gregor
Wentzel (1898–1978), Hendrik Anton
Kramers (1894–1952), and Léon Brillouin (1889–1969), who invented it
independently in 1926. The semiclassical approximation is particularly successful for calculations
involving the *tunnel effect, such as
Üeld emission, and radioactive decay
producing alpha particles.
semiconductor A crystalline solid
with an electrical conductivity (typically 10
5 –10
–7 siemens per metre)
intermediate between that of a
conductor (up to 10
9 S m
–1
) and an insulator (as low as 10
–15 S m
–1
). Semiconducting properties are a feature
of *metalloid elements, such as silicon and germanium. As the atoms in
a crystalline solid are close together,
the orbitals of their electrons overlap
and their individual energy levels are
spread out into energy bands. Conduction occurs in semiconductors as
the result of a net movement, under
the inÛuence of an electric Üeld, of
electrons in the conduction band and
empty states, called holes, in the valence band. A hole behaves as if it
was an electron with a positive
charge. Electrons and holes are
known as the charge carriers in a
semiconductor. The type of charge
carrier that predominates in a particular region or material is called the
majority carrier and that with the
lower concentration is the minority
carrier. An intrinsic semiconductor is
one in which the concentration of
charge carriers is a characteristic of
the material itself; electrons jump to
the conduction band from the valence band as a result of thermal excitation, each electron that makes
the jump leaving behind a hole in
the valence band. Therefore, in an intrinsic semiconductor the charge carriers are equally divided between
electrons and holes. In extrinsic semiconductors the type of conduction
that predominates depends on the
number and valence of the impurity
atoms present. Germanium and silicon atoms have a valence of four. If
impurity atoms with a valence of
Üve, such as arsenic, antimony, or
phosphorus, are added to the lattice,
there will be an extra electron per
atom available for conduction, i.e.
one that is not required to pair with
the four valence electrons of the germanium or silicon. Thus extrinsic
semiconductors doped with atoms of
valence Üve give rise to crystals with
electrons as majority carriers, the socalled n-type conductors. Similarly, if
the impurity atoms have a valence of
three, such as boron, aluminium, indium, or gallium, one hole per atom
is created by an unsatisÜed bond. The
majority carriers are therefore holes,
i.e. p-type conductors.
semiconductor laser A type of
*laser in which semiconductors provide the excitation. The laser action
results from electrons in the conduction band (see energy bands) being
stimulated to recombine with holes
in the valence band. When this occurs the electrons give up the energy
corresponding to the band gap. Masemiclassical approximation
478
s
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out and hydrolysed to give the ketone.
A
• Information about IUPAC nomenclature
semiclassical approximation An
approximation technique used to calculate quantities in quantum mechanics. This technique is called the
semiclassical approximation because
the *wave function is written as an
asymptotic series with ascending
powers of the Planck constant, h,
with the Ürst term being purely
classical. It is also known as the
Wentzel–Kramers–Brillouin (WKB)
approximation, named after Gregor
Wentzel (1898–1978), Hendrik Anton
Kramers (1894–1952), and Léon Brillouin (1889–1969), who invented it
independently in 1926. The semiclassical approximation is particularly successful for calculations
involving the *tunnel effect, such as
Üeld emission, and radioactive decay
producing alpha particles.
semiconductor A crystalline solid
with an electrical conductivity (typically 10
5 –10
–7 siemens per metre)
intermediate between that of a
conductor (up to 10
9 S m
–1
) and an insulator (as low as 10
–15 S m
–1
). Semiconducting properties are a feature
of *metalloid elements, such as silicon and germanium. As the atoms in
a crystalline solid are close together,
the orbitals of their electrons overlap
and their individual energy levels are
spread out into energy bands. Conduction occurs in semiconductors as
the result of a net movement, under
the inÛuence of an electric Üeld, of
electrons in the conduction band and
empty states, called holes, in the valence band. A hole behaves as if it
was an electron with a positive
charge. Electrons and holes are
known as the charge carriers in a
semiconductor. The type of charge
carrier that predominates in a particular region or material is called the
majority carrier and that with the
lower concentration is the minority
carrier. An intrinsic semiconductor is
one in which the concentration of
charge carriers is a characteristic of
the material itself; electrons jump to
the conduction band from the valence band as a result of thermal excitation, each electron that makes
the jump leaving behind a hole in
the valence band. Therefore, in an intrinsic semiconductor the charge carriers are equally divided between
electrons and holes. In extrinsic semiconductors the type of conduction
that predominates depends on the
number and valence of the impurity
atoms present. Germanium and silicon atoms have a valence of four. If
impurity atoms with a valence of
Üve, such as arsenic, antimony, or
phosphorus, are added to the lattice,
there will be an extra electron per
atom available for conduction, i.e.
one that is not required to pair with
the four valence electrons of the germanium or silicon. Thus extrinsic
semiconductors doped with atoms of
valence Üve give rise to crystals with
electrons as majority carriers, the socalled n-type conductors. Similarly, if
the impurity atoms have a valence of
three, such as boron, aluminium, indium, or gallium, one hole per atom
is created by an unsatisÜed bond. The
majority carriers are therefore holes,
i.e. p-type conductors.
semiconductor laser A type of
*laser in which semiconductors provide the excitation. The laser action
results from electrons in the conduction band (see energy bands) being
stimulated to recombine with holes
in the valence band. When this occurs the electrons give up the energy
corresponding to the band gap. Masemiclassical approximation
478
s
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