Silicon plus: del Alamo's group builds the case for new microchip materials

December 9, 2009

A test chip used to evaluate the performance of indium gallium arsenide in logic circuits.  Image courtesy of the del Alamo group

EECS faculty member, Jesús del Alamo, Donner Professor of electrical engineering and principal investigator in the Microsystems Technology Laboratories (MTL) and his group are presenting their case--for upgrading the current silicon based electronics world--to the International Electron Devices Meeting held this week in Baltimore. They will be presenting four separate papers at what is considered the premier conference on microelectronics, as reported this week by the MIT News Office.

Del Alamo’s group works with compound semiconductors, which unlike silicon, are designed to take advantage of other materials that combine elements from columns III and V of the periodic table--such as gallium arsenide and indium gallium arsenide. Electrons travel through these so-called III-V materials much more rapidly than they do through silicon, and III-V semiconductors have been used for years in high-speed electronics, such as the devices that process data in fiber-optic networks. The significant issue that del Alamo and his group have been investigating is the issue of size--designing the new III-V optical components, currently several orders of magnitude larger than than those in computer chips, to behave at desired performance.

Last year, del Alamo’s group built a III-V transistor that set a world record for high-frequency operation, achieving amplification levels of higher-frequency signals than any previous transistor. Two of the four papers being presented in Baltimore assess properties of the transistor that better predict its performance as in this case, as a logic element.

"To measure those properties," del Alamo told the MIT News Office, "the group built chips with multiple transistors that were identical except for the length of their most critical element, called the gate. If a transistor is a switch, the gate is what throws it. When the gate is electrically charged, it exerts an electrostatic force on a semiconductor layer beneath it; that force is what determines whether the semiconductor can conduct electricity or not."

By comparing the performance of transistors with different gate lengths at different frequencies, del Alamo’s group was able to extract precise measurements of both the velocity of the electrons passing through the transistor and the electrostatic force that the gate exerted on the semiconductor layer.

"Electron velocity," del Alamo reports, "is the “key velocity that is going to set the performance of a future logic switch based on these kinds of materials, and we have obtained velocities that are easily two and a half times higher than the best silicon transistors made today.” While the electrostatic force exerted by the gate was lower than the researchers had hoped, measuring it so precisely allowed the group to develop better physical models of III-V transistors’ behavior. On the basis of those models, del Alamo says, he believes that the gate’s performance is a “manageable problem.”

The fourth paper addresses a different topic: it proposes a new design for III-V transistors that, del Alamo says, "will work better at smaller scales, because it permits a thinner layer of material to separate the gate and the semiconductor material beneath it."

While the MIT prototypes were built entirely with III-V materials--rarer materials that will cost more, del Alamo envisions “a silicon-like technology where just under the gate … you take silicon out and stick in indium gallium arsenide. It’s a minute amount of material that is required in every transistor to make this happen.” Indeed, “III-V is not really in competition with silicon,” Chau agrees. “It still will be a silicon transistor. It’s just that you’re using this non-silicon element to make it even better.”

Read more:

MIT News Office, December 8, 2009: "Life after silicon - Researchers in MIT’s Microsystems Technology Laboratories are making the case for using exotic materials to help microchips keep improving."