Electrical control of glass-like dynamics in vanadium dioxide for data storage and processing

Metal–oxide–semiconductor junctions are the building blocks of modern electronics and can provide a variety of functionalities, from memory to computing. The technology, however, faces constraints in terms of further miniaturization and compatibility with post–von Neumann computing architectures. Manipulation of structural—rather than electronic—states could provide a path to ultra-scaled low-power functional devices, but the electrical control of such states is challenging. Here we report electronically accessible long-lived structural states in vanadium dioxide that can provide a scheme for data storage and processing. The states can be arbitrarily manipulated on short timescales and tracked beyond 10,000 s after excitation, exhibiting features similar to glasses. In two-terminal devices with channel lengths down to 50 nm, sub-nanosecond electrical excitation can occur with an energy consumption as small as 100 fJ. These glass-like functional devices could outperform conventional metal–oxide–semiconductor electronics in terms of speed, energy consumption and miniaturization, as well as provide a route to neuromorphic computation and multilevel memories.

Ab initio study of interface states at metal contacts to III-IV semiconductors

Thomas Maxisch

We present a theoretical study of the physical characteristics of metal/semiconductor junctions. Using first principle pseudopotential calculations, we have investigated the nature of electronic states with energies within the semiconductor band gap of representative abrupt, defect-free, anion-terminated metal/III-V interfaces. Namely, we focused on Al contacts to GaAs(001), AlAs(001) and cubic GaN(001) as well as on Al, Au and Cu junctions to cubic GaN(001). Recent advances in Schottky barrier concepts emphasize the possible relationship between interface states and the formation of the Schottky barrier. We aim at understanding the atomic-scale mechanisms responsible for interface states as well as their role in the Schottky barrier formation process. At As-terminated Al/GaAs(001) and Al/AlAs(001) junctions, resonant and localized interface states occur at the J point of the interface 2D Brillouin zone near the Fermi energy in the semiconductor midgap region. They correspond to intermetallic bonds between the outermost cation atoms of the semiconductor and the interfacial Al atoms of the metal. These interface states derive from an interaction between localized states of the Al(001) surface and semiconductor conduction band states, mediated by localized states of the unreconstructed, As-terminated semiconductor (001) surface. Our results indicate that interface states of the intermetallic, bonding-like kind could play an important role in the transport properties of metal/AlxGa1-xAs junctions. We have also investigated the electronic structure of Al, Au and Cu junctions to cubic, N-terminated GaN(001). The localized interface state reported for As-terminated Al/GaAs(001) and Al/AlAs(001) junctions occurs also at metal/GaN interfaces under the condition that atoms on the outermost atomic plane of the metal are placed in front of the outermost semiconductor cation. This indicates that the formation mechanism of this state is a very general one. In contrast to Al/AlxGa1-xAs junctions, these states occur at energy much larger than EF for the contacts to GaN. Thus, they are not expected to contribute significantly to the electronic transport of the latter interfaces. However, a large number of interface states attributed to d-type orbitals occur over a wide energy range including EF at contacts of noble metals to GaN.

EPFL2003

Modélisation physique d'un transistor de puissance IGBT

Serge Pittet

The IGBT transistor, associating the conduction advantages of the bipolar transistor and the switching advantages of the MOSFET transistor, is widely used in medium and high power applications with an operating voltage of 1.2kV to 4.5kV. New topologies, reducing the losses in the semi-conductor devices, are often proposed and have to be validated through a simulation tool. The finite element method (FEM) simulators accurately analyse losses in components, but the associated simulation time is so long that they cannot be used for complex topologies in power electronics. An equivalent transistor macromodel, correctly and quickly predicting currents and voltages on the element is needed. This thesis proposes an IGBT transistor model based on semiconductor physics and validated through a comparison with finite element simulator results. The model yields very good results regardless of transistor load and operating point. Base conduction is modelled using a novel equivalent charge approach. This results in a relatively simple macromodel with a good representation of the static and dynamic phenomena. The solving method of the Poisson equation has been revisited to fit recent changes of the internal structure of IGBT transistors. All approximations have been validated through fi- nite element analysis using an appropriate simulation set. The approximation of the internal MOSFET, even though based on existing models, takes into account the specificities of the IGBT transistor structure. It shows the degenerated properties of the internal MOSFET compared to the properties of a standard MOSFET, regardless of the operating point. Using intermediate potentials within the semiconductor structure allows a separate analysis of the different voltage contributions. A complete IGBT transistor macromodel has been developed and tested in different power electronics structures. Good results were obtained in all the tested structures with the proposed IGBT transistor macromodel. Parameter extraction can easily be automated using a mathematical tool such as Matlab and only requires few measurements on the device. The obtained results can then be quickly adapted to an other IGBT device.

EPFL2005

Semiconductor device

Serguei Okhonin

A semiconductor device, such as a memory device or radiation detector, is disclosed, in which data storage cells are formed on a substrate. Each of the data storage cells includes a field effect transistor having a source, drain, and gate, and a body arranged between the source and drain for storing electrical charge generated in the body. The magnitude of the net electrical charge in the body can be adjusted by input signals applied to the transistor, and the adjustment of the net electrical charge by the input signals can be at least partially cancelled by applying electrical voltage signals between the gate and the drain and between the source and the drain.