69437 - NANOELECTRONICS M

Learning outcomes

The aim of the course is to provide the required background to address the study of nanoscale semiconductor devices. The complexity of the quantum equations of transport makes it difficult to work out compact models, and requires the adoption of numerical techniques for the solution of the Schroedinger equation wth either closed and open boundary conditions. Solution methodologies and numerical techniques are highlighted by the lectures. Devices to be studied include fully-depleted silicon-on insulator (SOI) field-effect transistors (FETs), quantum nanowire FETs and multi-gate devices (MG-FETs), embodying the current fabrication technology developed by the most important semiconductor manifactuters. The course will also examine hetero-structure devices based on III-V compound semiconductors, whose interest is currently growing for the development steep-slope turn-on components. Among them are band-to-band tunneling transistors and superlattice FETs, by which subthreshold swings below the traditional limit of 60 mV/dec become possible.

Course contents

The evolution of the microelectronic technologies has led to the fabrication of integrated systems containing some billions of elementary transistors with linear dimensions of a few tens of nanometer which, in many cases, are smaller than, or comparable with, the electron mean-free path. The miniaturization process of electron devices has thus reached a level which makes the classical treatments of carrier transport in semiconductors no longer adequate. The drift-diffusion transport model is in fact based on the opposite assumption, namely that the electric field within the device changes over a space scale much larger than the carrier mean-free path, and over a time scale much longer than the average time between collisions. Also, the structural confinement of the carriers is responsible for novel quantum-mechanical effects, such as energy quantization and, thus, the splitting of the conduction and valence bands into a multiplicity of sub-bands with a smaller dimensionality. Finally, direct source-to-drain and band-to-band tunneling effects are going to play an ever increasing role. In view of the previous considerations, a re-examination of the classical methodologies currently used for the analysis of electron devices becomes mandatory.

The course of Nanoelectronics aims to address such a need, and to investigate the properties of carrier transport in nanometric-scale structures, also referred to as mesoscopic systems. This term means that these systems are still large with respect to the atomic dimensions, so as to make it possible to use the concepts of band structure, Bloch waves, equivalent hamiltonian and effective mass but, at the same time, smaller or comparable with the electron mean-free path. The concept of local quasi-equilibrium is thus abandoned and so is the description of carrier transport via the concepts of mobility and diffusivity. The nature of the new constitutive equations becomes strongly non-local and the importance of the boundary conditions, by which the device under investigation is isolated from the neighboring circuit, increases.

The course of Nanoelectronics aims to provide the attending students the conceptual tools required to face the study of nanometer-scale electron devices. The complexity of the quantum equations makes it difficult the development of compact models, and forces us to adopt numerical techniques for their solution. Therefore, the course is going to include within its program, the study of the main numerical methods for the solution of the Schrödinger equations with either closed and open boundaries, for which the non-equilibrium Green's function (NEGF) formalism has become very popular.

The devices to be studied are going to include, due to their practical importance, ultra-thin body (UTB) silicon-on-insulator (SOI) transistors, silicon nanowire field-effect transistors (NW-FETs) and multi-gate (MG) FETs, announced by Intel as the basic components of their technology node at 22 nm and beyond. The course will examine as well heterostructure devices based on III-V semiconductors, the interest of which for logic applications is currently increasing at research level. A theme of great current interest is the development new device concepts for the fabrication of FETs with a steep transition between the off- and the on-state, with the aim to reduce the supply voltage and to cut down power consumption. Among these novel devices, the band-to-band tunnel transistors (BTB-TFETs) and the superlattice-based FETs make it possible the achievement of inverse subthreshold swings much smaller than (kBT/q) ln(10) = 60 mV/dec, due to their ability to filter out the high-energy electrons of the Boltzmann energy distribution, thereby effectively lowering the carrier temperature.

Readings/Bibliography

S. Datta: "Electronic Transport in Mesoscopic Systems", Cambridge University Press
S. Datta: "Quantum Transport: Atom to Transistor", Cambridge University Press
D. H. Ferry, S. M. Goodnick: "Transport in Nanostructures", Cambridge University Press
M. Lundstrom: "Fundamentals of Carrier Transport", Cambridge University Press
M. Rudan: "Physics of Semiconductor Devices", Springer

Teaching methods

Traditional lectures are delivered in the classroom, illustrating the most important physical concepts of the discipline. The required calculations leading to the main results are carried out at the blackboard. Occasionally, slides are used for a better presentation of images, not otherwise reproducible on the blackboard.

Assessment methods

The assessment of the student learning will occur via oral examination.

Teaching tools

A number of textbooks are suggested for consultation, and personal notes of the teacher are delivered to the students.

Office hours

See the website of Giorgio Baccarani