Scientific project

ULTIMATE is a three-year proposal for an ambitious research project focused on upper limit technology investigations mandatory to attain THz electronics. InP-based transistors are the world’s fastest three-terminal semiconductor devices. Regardless of whether bipolar or field-effect transistors are considered, current gain cutoff frequencies tend to stagnate in the range fT = 500-600 GHz (0.5-0.6 THz). Progress in device design so far relied on semi-qualitative electronic energy band structures interpolated from often incompletely known materials. While standard simulators can predict cutoff frequencies exceeding 1 THz, experiments fall far short from this goal, presumably because strong electric fields within nano-devices drive electrons to higher energy states (L, X valleys) not accounted for in traditional TCAD tools or even Monte Carlo simulators which neglect quantum mechanical transmission effects at barriers.

In the ULTIMATE project, for the first time, we will tackle the design of ultrahigh-speed transistors on a fundamental atomistic level, with Density-Functional Theory (DFT) for accurate band structure calculations and quantum transport simulations to break through the present bandwidth bottleneck and finally experimentally achieve 750-1000 GHz cutoff frequencies.

The partners of ULTIMATE project are two academic laboratories (IMS-Bordeaux and ETHZ), one industrial laboratory (Alcatel Thales III-V Lab) and a start-up (Xmod technologies). Each of them brings some specific contributions based on an excellent expertise and a high-level research in the fields of InP HBT technology, electrical characterization, Parameter extraction methodology, compact modeling and multi-scale simulation. Such gathering of complementary skills is particularly relevant to provide guideline for technology achievement in order to perform state of the art technology and measurement up to THz.

The way around the present fT cutoff frequency bottleneck requires securing and exploiting an intimate knowledge of the electronic structure of the involved materials. To our knowledge, no others have attempted to implement transistors from an atomistic level starting point. The work is unique because it synergistically builds on the leading device processing capabilities of III-V Lab and ETHZ mm-wave device fabrication together with the IMS-Bordeaux up-to date 500GHz S parameters measurements equipment and on the pioneering materials/device simulation expertise the ETHZ team. The entire project chain, from physical modelling to epitaxial growth, device fabrication and characterization is thus entirely within the France-Switzerland area.

Density-Functional Theory and quantum transport computations will be performed by ETHZ (Prof. Luisier team) for InP/InGaAs and InP/GaAsSb HBTs. Their fabrication will be identically realized using III-V Lab and ETHZ (Prof. Bolognesi team) fabrication processes respectively. Subsequently, their electrical characteristics will be verified against experimental data measured by IMS and extracted by XMOD. In particular, the band structure of strained heavily carbon-doped GaAsSb and InGaAs layers will be calculated to determine if higher lying valleys (L-minima) become populated and how they affect device dynamic performances. The transistor design will then be adjusted to optimize the injection efficiency of electrons from the GaAsSb or InGaAs base layer into the InP collector and enhance electron transport through both the base and the collector layers to reach cutoff frequencies in the range of 750-1000 GHz.

Atomistic simulations will provide new insights on the inner workings of ultrahigh-speed heterostructure transistors and enable the design of a new generation of THz devices. Success is expected in extending the life of transistor electronics to higher bandwidths, a critical outcome because no credible alternative to transistor technology has so far been developed.