Trying for a Terahertz Transistor
Researchers at the University of Rochester believe they know how to make a transistor that, at room temperature, could operate at a blazing three terahertz.
The device, dubbed a ballistic deflection transistor, won’t be in products anytime soon. But it does embody some interesting design theories.
The semiconductor industry is eagerly looking for alternatives to the traditional transistor because engineers predict the device will reach its speed and size limits within the next two decades. “A lot of people are curious and trying to figure out what the next significant thing will be,” says Stan Williams, director of quantum science research at Hewlett-Packard in Palo Alto.
Currently, the fastest transistors–found in telecommunication technology–are clocked at a couple hundred gigahertz. Some transistors can run at about 500 gigahertz, but only when they are cooled to low temperatures. The ballistic deflection transistor could break those speed records at room temperature by channeling electrons in a different way.
This early prototype could have potential, says Williams. But he’s skeptical that this design will replace present-day transistors. Researchers at HP worked on a similar idea many years ago, Williams said, but decided not to pursue it because, at the time, the transistors required extremely low temperatures to work. However, if the researchers’ prototype works as well as they predict at room temperature, he says, “then they’ve got something.”
In contemporary transistors, electrons slog through layers of semiconductor material and collect on a capacitor: a charged capacitor represents a binary 1, or “on” state, whereas a discharged capacitor represents a 0, or “off” state.
In the researcher’s prototype, however, electrons would flow in a straight line along a two-dimensional path. An electric field would manipulate their direction as they bounce off of a tiny triangular structure in the path: deflecting to the right corresponds to a 1, and deflecting to the left corresponds to a 0.
The Rochester group isn’t the first to propose using an electric field to steer electrons in a transistor; for years, researchers have been experimenting with a Y-shaped structure to deflect electrons to the right or left. But the Rochester design differs slightly. In other designs, the electric field has to do all the steering, moving the electrons right or left. In the Rochester team’s model, electrons ricochet off the triangle. The electric field, therefore, wouldn’t be required to do as much work. This could make the ballistic deflection transistor more efficient.
The challenge for the Rochester team was finding the right materials for their innovative design, and silicon wasn’t an option. At room temperature, electrons in silicon will bounce off other particles, on average, every 10 nanometers. This distance–scientists call it the mean free path–was too small to be usable. The electron channels and triangle would need to be impossibly small for the whole thing to work.
Instead, the researchers chose other semiconductor materials called indium-gallium-arsenide and indium-phosphide. These materials, explains Martin Margala, professor of electrical and computer engineering at the University of Rochester, have a mean free path of about 220 nanometers. The bigger mean free path means the transistor’s features can be larger.
Etching out features at the required dimensions was still a challenge, however. Part of the researchers’ advance was to refine a lithographic process for indium-gallium-arsenide to produce sharp features at about 70 nanometers. Below that dimension, the “triangle starts to turn to a dot and you lose efficiency quite rapidly,” says Margala.
The researchers are in the process of testing a prototype. And while they don’t yet have finalized results, “it should work,” says Aimin Song, professor of electrical engineering at the University of Manchester. The prototype is based on the same principles and architecture, he says, that have proven successful in other circuit components. Previously, Song’s lab used a similar design to develop a rectifier, a device that converts alternating current to direct current.
Song adds that there would be some challenges in integrating this architecture into the semiconductor industry. The materials that the researchers used are considered exotic and are much more expensive than silicon. There is a more common semiconductor called gallium arsenide, which may offer a more feasible alternative. Gallium arsenide has a mean free path of about 150 nanometers at room temperature, he says, which could accommodate the ballistic deflection design. But again, the features would need to be made significantly smaller than the mean free path, which, according to Song, would put them “close to the limit of modern nanolithography.”
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