PN Diode Equilibrium S' Tewksbury

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PN Diode - EquilibriumS. Tewksbury
These notes summarize the PN diode under
equilibrium conditions (zero current either
because the voltage across the diode is zero or
because the contacts are not connected to
anything. Apologies for the poor speech - not
well rehearsed.
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Equilibrium No current flowing - Fermi level
constant everywhere.
EE471 PN Diode - Equilibrium
P-type doping p NA -gt largen (ni)2 /p -gt
small
N-type doping n Nd -gt largep (ni)2 /n -gt
small
P-type semiconductor with large density of holes,
negligible electrons and N-type semiconductor
with large density of electrons, negligible
holes.
Mobile electron Mobile hole
Fixed donor ion Fixed aceptor ion
Since fixed ions do not move, the figure below
has eliminated their symbols so we can consider
only the mobile charge. Mimicking the diode
situation, we show the P-type (N-type) regions
connected to regions with negligible
hole(electron) densities.
Small hole density
Small electron density
Holes will diffuse to the right from region of
high hole density into region of low hole density.
Electrons will diffuse to the left from region of
high electron density into region of low ele tron
density.
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Nonzero E in Depletion Region As shown by the
small arrow (dashed) from a positive ion to a
negative ion, electric field lines extend from
each positive ion on the right to a negative ion
on the left. These lead to a net nonzero
electric field in the depletion layer. The
direction of the electric field is shown
correctly (with position x increasing to the
right, the electric field is negative).
Electrons move in a direction opposite to the
electric field, whereas holes move in the same
direction.
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Junctionx 0
Junctionx 0
Junctionx 0
xn
xn
xn
-xp
-xp
-xp
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THE EQUATIONS YOU SHOULD KNOW FOR EQUILIBRIUM (no
current) CONDITION
The built-in voltage Vbi. We will show where
this comes from when we discuss currents in
diodes.
The relationship between xp and xn, allowing you
to calculate one as per above equations and then
quickly calculate the other.
KT/q 0.026 volts at 300K.
The depletion layer widths xp and xn on the
P-side and N-side of the junction , and the total
depletion layer width w xp xn
The calculation of equilibrium carrier densities
in the charge neutral regions outside the
depletion layers. This is the same as our
familiar calculations but with new subscripts
used to distinguish what is where.