Portland, Ore. Ferroelectric transistorsultra-small gateless two-terminal junctions between semiconductor and oxideswitch by reversing the polarity of their junction. This is the region where semiconductors are tuned for a single wavelength in a laser diode. But because the region's actions are not well understood, no one has successfully stabilized the ferroelectric transistor, leaving it a glint in the eye of starry-eyed researchers.
"Until now we have not had a way to think about these problem in generalevery solution, like that enabling the laser, has been a one-off. There was a tool missing from our kit," said McKee.
McKee and fellow researchers are creating a model of the so-called Schottky barrier in hopes of finding that missing tool. They said the work could lead to smaller, faster computers.
Current theory regarding semiconductor junctions derives from the seminal work of Siemens engineer Walter Schottky, who first explained the existence of "holes" in semiconductors. Schottky later detailed how one such hole was caused by an ion being displaced to a crystal's surface.
Such holes have since become known as "Schottky defects." Likewise the "Schottky diode" is based on his "bulk" termination" analysis of the Coulomb buffer between a metal and a semiconductorthe "Schottky barrier problem."
"When the electrostatics of semiconductor materials were originally conceived by Schottky and others, they were trying to understand the overall problem and did so by making approximations in their formulation namely that the intrinsic properties of the bulk materials on either side of the junction determine the barrier height," said McKee. "But invariably there is an interaction between the two components at the interface resulting in a third component, that was
left out of the original formulation."
The Schottkey barrier problem arose when first forming a
metal-to-semiconductor junction, when current flows from the metal into the semiconductor in order to come to equilibrium. The electrons only flow about a nanometer into the semiconductor leaving a so-called "space charge" there that forms a physical barrier to electrons when the junction is reverse biased, thereby rectifying any current flow in
between the materials.
Physists call the diode-effect of the Schottky barrierelectricity is conducted in only one directiona result of the Coulomb buffer presented to charges trying to cross in the wrong direction.
Current can't reverse bias the buffer since the space-charge fills all the available conduction paths. To model this effect, McKee enlisted the help of assistant professor of physics at North Carolina State University, Marco Buongiorno Nardelli, who created a detailed model of the Schottky barrier.
"Barrier height is no longer a problem, but an opportunity. We believe our theory will change common beliefs in the field of semiconductor physics, and could open the way for smaller, faster and smarter computers," said Nardelli, who also holds a research post at Oak Ridge National Laboratories.
In their experiment, McKee tested Nardelli's model's predictions by putting oxide on silicon and observing that the more active semiconductor region donated electrons to the oxide side, in a manner similar to the way electrons flow into the semiconductor from the metal in a classical Schottky diode. For the semiconductor-oxide junction, electrons flowed from the semiconductor over to the oxide and reached equilibrium.
As a result, the semiconductor side becomes positively charged, and the oxide where the electrons flowed becomes negatively charged.
The opposing positive and negatively charged regions forms a dipole, in effect, that presents a barrier to reverse biased current flow.
Depending on its size, the researcher then tested ways of
adjusting the two sides of this junction, in order to change the way the semiconductor conducts electricity, such as switching the barrier on and off like a switch (the long-sought ferroelectric transistor).
"This concept is not new. If you adjust the conduction bands of the two sides of a semiconductor-to-semiconductor junction, that's how you make a laser work. You tune the ability of the electrons to exchange between the two materials," said McKee.