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MIT nears logic goal with transistor record

Reducing the gate length of PHEMTs with an InAs channel helps set the frequency standard in research part-funded by Intel.

Researchers at the Massachusetts Institute of Technology have produced the highest-ever current-gain cutoff frequency (fT) field-effect transistor, in researching towards a successor to silicon CMOS for computing.

In August s Electron Device Letters Jesús Del Alamo and Dae-Hyun Kim detailed 30 nm gate length InAs pseudomorphic high-electron mobility transistors (PHEMTs) with a 628 GHz fT.

They also described similar 50 nm gate length PHEMTs that they say have the best combination of ft and maximum oscillation frequency (fmax) of any transistor. In those devices fT was 557 GHz and fmax was 718 GHz.

The PHEMTs were grown on InP substrates, sandwiching an InGaAs/InAs/InGaAs multi-quantum well channel between InAlAs buffer and barrier layers. The overall structure is similar to devices that Kim and Del Alamo presented at the 2007 International Electron Devices Meeting in Washington, DC.

The latest paper sees the MIT pair raise the height of the gate stem separating the epitaxial layers from the top of the device s T-gate to reduce parasitic capacitances. Shrinking transistor gate length from 50 nm to 30 nm to deliver the fT record is also a recent achievement and a milestone on the way to a key goal.

“We are interested in making devices that approach a prototypical 15 nm node,” Del Alamo told compoundsemiconductor.net. “It is reasonable to expect that as we scale down size, while preserving short-channel effects, reducing footprint and parasitics, the frequency response of our transistors will continue to increase.”

“Our primary goal is future logic operation,” he said. “InGaAs-based FETs are one of the options that Intel is contemplating for a future post-silicon logic technology, at least for the n-channel device. It is of great interest to them.”

Although computer logic is the principal target, fmax and fT measurements are more commonly made for radio-frequency technology. However, Del Alamo said that transistors with high values for both could be considered the “ultimate device”.

“High fmax and ft is a reflection of a device design that has excellent intrinsic transport characteristics, very low short-channel effects and very low parasitic resistances and capacitances.”

“A device like this allows you to support a very rich set of applications on the same technology: millimeter-wave small signal, ultra-high data rate photonics and future logic applications.”

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