arating ketones from reaction mix-
tures: the derivative is crystallized
out and hydrolysed to give the ke-
tone.
A- Information about IUPAC nomenclature
semiclassical approximation An
approximation technique used to cal-
culate quantities in quantum me-
chanics. 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 theÜrst 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 Bril-
louin (1889–1969), who invented it
independently in 1926. The semi-
classical approximation is particu-
larly successful for calculations
involving the tunnel effect, such as
Üeld emission, and radioactive decay
producing alpha particles.
semiconductorA crystalline solid
with an electrical conductivity (typi-
cally 10^5 –10–7siemens per metre)
intermediate between that of a
conductor (up to 10^9 Sm–1) and an in-
sulator (as low as 10–15Sm–1). Semi-
conducting properties are a feature
of *metalloid elements, such as sili-
con 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. Con-
duction occurs in semiconductors as
the result of a net movement, under
the inÛuence of an electricÜeld, of
electrons in the conduction band and
empty states, called holes, in the va-
lence band. A hole behaves as if it
was an electron with a positive
charge. Electrons and holes are
known as the charge carriers in asemiconductor. The type of charge
carrier that predominates in a partic-
ular 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 va-
lence band as a result of thermal ex-
citation, each electron that makes
the jump leaving behind a hole in
the valence band. Therefore, in an in-
trinsic semiconductor the charge car-
riers are equally divided between
electrons and holes. In extrinsic semi-
conductorsthe type of conduction
that predominates depends on the
number and valence of the impurity
atoms present. Germanium and sili-
con 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 ger-
manium or silicon. Thus extrinsic
semiconductors doped with atoms of
valenceÜve give rise to crystals with
electrons as majority carriers, the so-
called n-type conductors. Similarly, if
the impurity atoms have a valence of
three, such as boron, aluminium, in-
dium, or gallium, one hole per atom
is created by an unsatisÜed bond. The
majority carriers are therefore holes,
i.e. p-type conductors.
semiconductor laserA type of
*laser in which semiconductors pro-
vide the excitation. The laser action
results from electrons in the conduc-
tion band (see energy bands) being
stimulated to recombine with holes
in the valence band. When this oc-
curs the electrons give up the energy
corresponding to the band gap. Ma-semiclassical approximation 478s