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Acceptors |
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Doping defects that introduce an energy state in the band gap
close to the valence band. |
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At medium temperatures - generally meaning room temperature -
electrons from the valence band will completely fill these states leading to a
concentration of holes in the valence band that is about equal to the
concentration of the acceptor states. |
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Conductivity s |
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A specific propery of any material, defined as the relation
between electrical field E and
current density j |
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Generally, the specific conductivity s is a tensor of second order and may depend on other
parameters including the firld strength E. |
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As long as the current-field relationship is linear (demanding
s ¹
s(E), the
material exhibits ohmic behavior. |
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s can always be expressed in
terms of the concentration n of carriers reponsible for
conduction, their charge q, and their mobiltiy µ,
via |
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Diffusion
length |
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The diffusion length L always refers to minority
carriers. It is the average distance a minority carrier moves away from its
point of origin, given by |
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With D = diffusion coefficient and t = time for the movement (= life time for a
minority carrier). |
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The diffusion length is a prime
material parameter that comes up in many formula, it can be rather
large (up to mm) in indirect semiconductors and it is very sensitive to
certain lattice defects. |
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Donors |
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Doping defects that introduce an energy state in the band gap
close to the conduction band that is occupied by an electron at low
temperatures. |
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At medium temperatures these electrons will move to the
copnduction band leading to a concentration of electrons in the conduction band
that is about equal to the concentration of the donor states. |
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Doping |
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Controlled introduction of lattice defects that introduce
energy states for electrons in the band gap |
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Often done with substitutional atoms that are to the left or
right of the element they replace in the periodic table |
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But all defects - e.g. dislocation sand grain boundaries - may
have energy levels in the band gap, too, and thus may introduce doping. |
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Electrons
in semiconductors |
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Practically always refers to
free electrons in the conduction band. |
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Holes in
semiconductors |
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States in the valence band not occupied by electrons. Behave
for all practical purposes like positively charged electrons. |
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Intrinsic
semiconductors |
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Undoped "perfect" semiconductors with properties
exclusively governed by the crystal. The Fermi energy is in the middle of the
band gap; electron and hole concentrations are equal ( =
ni). |
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Life time |
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Usually the average time t a
minority carrier, after it was generated by thermal fluctuations or other
energy expenditures, "lives" before it disappears again by
recombination. Refers to thermal equilibrium in this case. |
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In more complicated circumstances - e.g. in space charge
regions or in non-equilibrium conditions- life times must be considered more
carefully; a distinction between generation and recombination life time, e.g.,
might be necessary. |
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Majority
carriers |
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The kind of carrier - electrons or holes - that are present in
the larger concentration. |
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Majority carriers are electrons in the case of doping with
donors, and holes in the case of doping with acceptors. |
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Mass action
law |
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Traditional, albeit somewhat misleading name for the relation
between the concentration of electrons, ne and holes,
nh in doped semiconductors and the intrinsic
concentration, ni:. |
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Directly obtainable from considering charge neutrality |
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Minority
carriers |
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The kind of carrier - electrons or holes - that is present in
the smaller concentration. |
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Majority carriers are holes in the case of doping with donors,
and electrons in the case of doping with acceptors. |
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Perfect
semiconductors |
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Ideally, a perfect crystal contains only those crystal defects
necessary for device function - i.e. doping atoms and perfect interfaces. What
makes an interface perfect is hard to define - in case of doubt the absence of
interface states in the band gap. |
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Si crystals are closer to being perfect then anything
else (in the unanimated world, that is) . Their dislocation density is zero
(which has not been possible to achieve for practically all other (large)
crystals), the level of unwanted point defects is in the (1 - 10) ppm
region for O and C, respectively, and in the low ppb if
not ppt region for everything else. |
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There are, however, unavoidable remnants of the intrinsic
point defects (vacancies and self interstitals) that were present in thermal
equilibrium at high temperatures. They may occur in any forms of microclusters,
not yet fully understood. |
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pn-junction |
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A transition from p-type to n-type within one
piece of material. Electrically, a pn-junction is a diode. |
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The "ideal" pn-junction is a paradigm of
semiconductor science. It is usually highly idealized and considered to be
- An abrupt junction, i.e. the doping
changes from p-type to n-type abruptly,
- with constant doping levels on both
sides of the junction, being
- one-dimensional, and being
- "large", i.e. the p
and n areas are much longer than the diffusion length of the carriers.
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Real pn-junctions, especially in integrated circuits,
do not even come close to these assumptions. Nevertheless, the results of the
ideal pn-junction contain almost everything needed to tackle the real
junctions and thus should be well understood. |
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n-type
semiconductor; n-doped, n-doping |
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Semiconductors with the majority carriers being electrons.
Doping thus was done with donors. |
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p-type
semiconductor; p-doped, p-doping |
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Semiconductors with the majority carrierers being holes.
Doping thus was done with acceptors. |
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Do not mix up p-doped with
P-doped (Phosphorous doped) in Silicon! P is a Donor in
Si; P-doped Si is a n-tpype semiconductor |
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© H. Föll
2.2.1 Intrinsic Properties in Equilibrium 
To index