An n-type semiconductor on top of a p-type semiconductor creates a vertical electric field (E, green arrow), while diffusion creates a depletion layer near the junction (orange), where the electric field is strongest. Heating one end of the device creates a heat gradient at right angles to the electric field (del T, red arrow). Electrons and holes moving in these fields are forced into loops of current, and a magnetic field is generated “sideways” (B, blue arrow), at right angles to both electric and thermal fields. |
Lawrence Berkeley National Laboratory scientists and their colleagues have discovered a new relation
among electric and magnetic fields and differences in temperature, which
may lead to more efficient thermoelectric devices that convert heat
into electricity or electricity into heat.
“In
the search for new sources of energy, thermopower?the ability to
convert temperature differences directly into electricity without
wasteful intervening steps?is tremendously promising,” says Junqiao Wu
of Berkeley Lab’s Materials Sciences Division (MSD), who led the
research team. Wu is also a professor of materials science and
engineering at the University of California at Berkeley. “But the new
effect we’ve discovered has been overlooked by the thermopower
community, and can greatly affect the efficiency of thermopower and
other devices.”
Wu
and his colleagues found that temperature gradients in semiconductors,
when one side of the device is hotter than the opposite side, can
produce electronic vortices?whirlpools of electric current?and can, at
the same time, create magnetic fields at right angles to both the plane
of the swirling electric currents and the direction of the heat
gradient. The researchers report their results in Physical Review B.
Wu
says, “There are four well-known effects that relate thermal, electric,
and magnetic fields”?for example, the familiar Hall effect, which
describes the voltage difference across an electric conductor in a
perpendicular magnetic field?“but in all these effects the magnetic
field is an input, not an outcome. We asked, ‘Why not use the electric
field and the heat gradient as inputs and try to generate a magnetic
field?’”
To
test the possibilities, the researchers modeled a practical device made
of two layers of silicon: a thin, negatively doped layer (N-type) with
an excess of electrons and a thicker, positively doped layer (P-type)
with an excess of holes, which are electron absences that behave as
positively charged particles.
At
the junction where the oppositely doped silicon layers meet, a third
kind of layer called a P-N junction forms, not physical but electronic:
electrons from the N-type layer diffuse across the physical boundary
into the P-type layer while holes move in the opposite direction,
forming a depletion layer where charges are “dried out”.
Given
the high density of mobile electrons at the surface of the N-type layer
and the high density of mobile holes at the surface of the P-type
layer, but few mobile charges in the depletion layer, the electric field
is strongest near the junction. This deep layer has profound effects,
when a heat gradient is applied to the joined silicon layers.
Wake up and smell the champagne
“There
are three ways charges can move?three kinds of currents,” says Wu. “One
is the diffusion current, in which particles move from denser areas to
less dense areas. This has nothing to do with charge. Think of a bottle
of champagne. I pop the cork, and a little while later you can smell the
champagne, because the molecules diffuse from their dense concentration
in the bottle into the air.”
The
second kind of current is called drift current. “If there’s a draft in
the room moving toward you, you may smell the champagne a little
earlier, or if it’s moving away from you, a little later,” Wu explains.
“In an electronic device, a drift current is caused by the voltage bias,
the electric field.”
Says
Wu, “So in an electronic device we have diffusion current away from the
dense charge areas, and drift current due to the electric field, and
now we add a third, the thermoelectric current, which is another form of
drift current in which charge carriers move from the hotter end of the
device to the cooler end.”
The
results would be uninteresting if all the currents were pointing in the
same direction, or in opposite directions, but they’re not. The
electric field sets up a drift current from the negatively charged top
layer toward the positively charged bottom layer of the device – moving
against the diffusion currents of the charge carriers. Meanwhile the
heat gradient sets up a drift current at right angles to the electric
field.
“In
these conflicting perpendicular forces, electrons and holes cannot
maintain straight motion but are sucked into vortices,” Wu says.
Instead
of a single vertical vortex in the device, vortices form in each layer
and are separated by the depletion layer. In the N-type layer, the
widely separated electrons near the depletion layer move with the
temperature gradient, from hot to cold, but move in the opposite
direction near the surface, where the electrons are bunched closer
together. The vortex formed by holes in the N-type layer is nearly a
mirror image of the electron vortex.
The
unusual result is that merely by applying heat to one end of a simple
silicon device, the researchers can generate a magnetic field
perpendicular to the twin vortices?a magnetic field that emerges at
right angles to the plane of the two silicon layers.
“The
immediate application is not that we can make a magnetic field, which
is relatively weak, but the realization that the efficiency of many
semiconductor devices, including commercial products, could be made more
efficient if we do it right,” Wu says. “For example, designing them to
make sure that their electric fields, and inhomogeneities in composition
or doping, are aligned with their heat gradients would avoid these
energy-wasting current vortices.”
Wu’s
fascination with the new effect he and his teammates discovered doesn’t
stop there, however. “My interest isn’t just in making more efficient
electronics but in making good things out of this. The first step is to
confirm with experiment what we’ve discovered through modeling. After
that, a whole new program of research opens up.”
Wu
explains that the remarkable electronic and magnetic effects caused by
temperature differences in the current model may well be duplicated by
other kinds of inhomogeneous excitation – for example, by the way light
falls on a solar cell. “Different intensities or different wavelengths
falling in different areas of a photovoltaic device will produce the
same kinds of electronic vortices and could affect solar cell
efficiency. Understanding this effect may be a good path to better
efficiency in electronics, thermal power, and solar energy as well.”
Study abstract: Electrothermally driven current vortices in inhomogeneous bipolar semiconductors.
