Hydrogen incorporation in n-type Si-doped GaAs layers results in the neutralization of the active dopants and a change of the conductivity along the growth direction. To characterize the active dopant concentration of doped GaAs layers containing hydrogen, we have used secondary ion mass spectroscopy and cross-sectional scanning tunneling microscopy. Spectroscopic measurements are performed as well as conductance images to visualize the variation of the conduction band-edge position. Such a variation, which is related to the concentration of Si–H complexes, allows the determination of the doping profile.

Under ultraviolet (UV) illumination of GaAs with photon energies above 3.5 eV, Si–H complexes are known to be efficiently dissociated at room temperature. Studying the dissociation kinetics of Si–H and Si–D complexes in GaAs, we have observed that, for a given incident UV photon density, the concentration of dissociated Si–D complexes is 10–20 times below the concentration of dissociated Si–H complexes. This strong isotope effect is discussed under the light of recent excitation models of Si–H(D) bonds at the surface of silicon and at the Si/SiO2 interface.

Room-temperature bias stress and annealing experiments have been performed on hydrogenated and deuterated Si doped n-type GaAs Schottky diodes. From a careful analysis of capacitance–voltage characteristics, we have studied the variation of the active doping concentration and the reactivation of neutralized dopants. In thermal annealing experiments at 250 °C, the Si–H complex dissociation rate does not vary significantly by incorporating deuterium rather than hydrogen. On the contrary, by applying high reverse bias voltages to the Schottky diodes at room temperature, a strong isotope effect is observed on the dissociation rate. In this case, the dopant reactivation should be due to hot electron excitations.

The premier applications of microwaves are presented in different chapters of this book; they are terrestrial and satellite links, radio and TV transmission (Chapters 1-3), radars and remote sensing (Chapter 4), countermeasures (Chapter 5), infrared detection (Chapter 6) and radioastronomy (Chapter 8). Other applications also exist and present a growing interest, which are related to the so-called industrial, scientific and medical (ISM) applications. The corresponding processes can be easily described once several basic properties related to interactions between microwaves and different kinds of material have been brought to mind.

Military and professional civil communications and radar systems have driven the state of the art of microwave technology and equipment. New applications, driven by commercial market, impose drastic changes in existing design and manufacturing. Apart well identified market like DBS, mobile radios and fkture applications around 60 GHz, it exists a plenty of microwave applications in niche markets, and not well known or still in development, but able to raise rapidly. This paper reviews technologies and principles able to solve the commercial market constraints such as MMIC, microwave smart cards, as well as new technological approaches and concepts. Some examples of commercial activities in France illustrate this survey.

Progress in the microelectronics industry is strongly coupled with the ability to make ever increasing numbers of smaller devices on a single chip. The advent of high-resolution electron and X-ray lithographic techniques is leading toward an era in which individual features sizes might well be fabricated on the scale of 10–20 nm. It will then become feasible to develop very small device structures where size and related effects may be as important as the bulk properties of the host semiconductor material. Moreover, it becomes obvious that we must now ask whether classical device modelling may be extrapolated down to the very small space and time scales usually encountered in sub-micron devices. It is the purpose of this paper to study in a first part what kind of new phenomena may occur in sub-micron devices, to suggest and to describe in a second part new methods of modelling which take them into account and to discuss in a third part what could be the future performance of submicronic logic or microwave devices.

This paper presents the realization of a GaAs monolithic broadband down convertor. The originality of this device consists in the use of only two field effect transistors (without d.c. bias for the first stage). As a consequence of the simplicity of our device, good reliability and high performances in microwave domain can be obtained. A typical result is a conversion gain of about 0dB in the frequency range up to 18 GHz. Finally, various microwave applications can be proposed for this original device. With this new type of circuit, many applications are possible such as frequency counter, broadband down convertor, network analysers, frequency stabilization and conversion. Two examples concerning these two early type of application are described.

A particle simulation of impact ionization taking into account most of the hot carrier transport phenomena (diffusion, relaxation) is proposed. The consideration of a statistical process to describe electron-hole creation permits a direct application to avalanche semiconductor devices. Results are given for a Si millimeter were double drift IMPATT diode.

Multilayer GaAs-AlxGa1−xAs avalanche and transit time devices have been studied theoretically using a particle time domain simulation of their operation. For structures derived from quantum well superlattices rather good performances were obtained at 100 GHz. It is shown that these results are in agreement with the interpretation of a bandedge-discontinuity-assisted impact ionization and a better defined avalanche region.

Progress in the microelectronics industry is strongly coupled with the ability to make an ever-increasing number of smaller devices on a single chip. The advent of high-resolution electron and x-ray lithographic techniques is leading toward an era in which individual features' sizes might well be fabricated on the scale of 0.01-0.1 gm. It will then become feasible to develop very small device structures characterized by higher and higher operating frequencies or shorter and shorter switching delays. In this type of device, carriers are often submitted to electric fields characterized by fast time variations and/or by strong spatial nonunifortuities. Due to these conditions, a "non-steady-state transport" will occur and the characteristics of such a transport can be very different from those obtained in the usual steady transport. Consequently, it becomes obvious that we must now ask whether classical device modeling may be extrapolated down to the very small space and time scales usually met in submicrometer devices. New phenomena may occur and it is the purpose of this chapter to study the specific features of electron transport in these conditions. First, the basic physics of carrier transport in semiconductors is briefly recalled. At the smallest size (feature sizes down to 0.01 gin), one does not know the extent to which the conventional effective-mass concept and band theory can be scaled down. In addition, the collisions acting on the carriers cannot be assumed to occur instantaneously either in space or in time [8.1]. At the most usual size (in the range 0.1-0.5 lam), which will be the only size studied in detail, the transport physics may still be based on the Boltzmann transport equation, but it will be shown that, even for this case, the features characterizing carrier transport can be very different from those related to steady-state and bulk transport. These new features occur either when the semiconductor samples are submitted to very fast time variation of the electric field E or when the electric field is characterized by small spatial scale. Time-dependent phenomena will be studied in Sects. 8.2-4. First, the case of high-frequency sinusoidal electric field superimposed to an applied steady field will be investigated for materials such as Si and III-V compounds. Transient phenomena when a time-pulse or a time-step configuration of E are applied to the whole semiconductor will then be analyzed. Drift and diffusion mechanisms will be discussed with particular attention to conditions in which the specific features of carrier transport could be used strongly to increase the average velocity of the carriers over the distance of the sample.

A simplified microscopic model for investigating energy relaxation effects in millimeter-wave IMPATT devices is presented. A statistical process is used to describe electron-hole multiplication by impact ionization from knowledge of the ionization coefficients. These coefficients are assumed to be functions of the individual energy of carriers (holes and electrons). A relaxation time formulation is used to calculate the energy of each carrier. Drift in the electric field and diffusion are modeled using the diffusive model proposed by Hockney. Simulations are carried out for silicon diodes. It is found that inclusion of the energy relaxation mechanisms modifies mainly the avalanche process for such material. The implications of these mechanisms on device performances are then discussed by calculating the large signal level dependence of the conversion efficiency and admittance for a typical double-drift structure at 100 GHz. The resulting calculations show good agreement with existing experimental data on these structures.

Studies of millimeter-wave IMPATT devices have demonstrated a drastic fall off of conversion efficiencies above 100 GHz. The saturation of ionization rates at very high electric field and the impossibility of maintaining the avalanche zone in a confined geometry are certainly two of the main fundamental causes of this degradation of performances [1].

Molecular dynamics calculations have been performed for fluid systems containing ellipsoidal particles which interact pair-wise with a modified Lennard-Jones potential. Assuming molecular parameters close to those of the CB7 compound, we have calculated both static and dynamic properties such as the internal energy, the entropy and specific heat changes at T NI, the compressibility factor, the temperature dependence of the orientational order parameters, the reorientational angular momentum and velocity time-dependent self-correlation functions, and the diffusion coefficients in both the nematic and isotropic phases. The thermodynamical data show a transition between isotropic and nematic phases and the results obtained are in qualitative agreement with the experimental data. The influence of a decenterd dipole along the molecular axis has been studied and partially bilayered smectic phases have been obtained.

The possibility of ballistic and overshoot transport in bulk semiconductor and submicronic devices are discussed using both simple analytical calculation and more exact Monte Carlo simulation. Assuming a homogeneous semiconductor, it appears possible, using a really ballistic motion of electrons to achieve over very short distances (a few tenths of a micron) higher velocities than in the overshoot motion. The influence of the doping concentration and of the operating temperature, as well as the improvement which could be achieved by using other materials, is also discussed. The potential of such ballistic motion and velocity overshoot in a real submicronic device is then assessed; it is shown that, due to spatial nonuniformities, additional phenomena occur which might considerably change, and often reduce, the velocity of the carriers.