Thermal stability of Al/AlGaAs and Al/GaAs/AlGaAs (MBE) Schottky barriers

Outstanding stability has been observed in Al/Al/sub x/Ga/sub 1-x/As and Al/GaAs/Al/sub x/Ga/sub 1-x/As (x=0.25) Schottky barriers prepared by depositing Al in situ by MBE on annealing up to 400 degrees C. Conventionally evaporated barriers have been fabricated and compared with epitaxial ones. The changes in barrier height and ideality factor induced by annealing are reported.<>     

High-electric-field effects and degradation of AlGaAs/GaAs power HFETs: a numerical study

This work presents a numerical study of high-field degradation and reliability issues of AlGaAs/GaAs power HFETs. A commercial two-dimensional drift–diffusion tool is used to investigate electric-field distributions, the effects of electron capture at the device surface under hot-carrier conditions, and the impact of drain recess scaling on such effects. Wherever experimental data are available for direct comparison, a good match is observed with our simulations. The main results of this study are (1) a validation of the hypothesis that attributes the main high-field degradation effects to electron capture over the gate–drain access region, and (2) design indications pointing out to the possibility of a reverse correlation between the gate–drain breakdown voltage and the device hot-carrier reliability.     

Imaging with low-voltage scanning transmission electron microscopy: a quantitative analysis

A dedicated specimen holder has been designed to perform low-voltage scanning transmission electron microscopy in dark field mode. Different test samples, namely InGaAs/GaAs quantum wells, InGaAs nanowires and thick InGaAs layers, have been analysed to test the reliability of the model based on the proportionality to the specimen mass-thickness, generally used for image intensity interpretation of scattering contrast processes. We found that size of the probe, absorption and channelling must be taken into account to give a quantitative interpretation of image intensity. We develop a simple procedure to evaluate the probe-size effect and to obtain a quantitative indication of the absorption coefficient. Possible artefacts induced by channelling are pointed out. With the developed procedure, the low voltage approach can be successfully applied for quantitative compositional analysis. The method is then applied to the estimation of the In content in the core of InGaAs/GaAs core-shell nanowires.     

Fibre-top atomic force microscope probe with optical near-field detection capabilities

We present a fibre-top probe fabricated by carving a tipped cantilever on an optical fibre, with the tip machined in correspondence of the fibre core. When approached to an optical prism illuminated under total internal reflection conditions, the tip of the cantilever detects the optical tunnelling signal, while the light coupled from the opposite end of the fibre measures the deflection of the cantilever. Our results suggest that fibre-top technology can be used for the development of a new generation of hybrid probes that can combine atomic force microscopy with scanning near field optical microscopy.     

Test structures for dielectric spectroscopy of thin films at microwave frequencies

This work describes the application of two different test structures to execute broadband microwave measurements of the dielectric constant of ceramic thin films. Coplanar waveguide probes and vector network analyzer were used to measure the dielectric constant versus frequency of thin films of lead zirconate titanate and zirconium titanate, fabricated by sol gel methods. One-step lithography was used to produce planar metal-insulator-metal and interdigitated capacitor test patterns. The two test structures are compared for zirconium titanate films. The metal-insulator-metal method has been applied also to a lead zirconate titanate film and to show the capability of computing the dielectric tunability.     

Hot electron effects on Al0.25Ga0.75As/GaAspower HFET's under off-state and on-state electrical stress conditions

This work shows a detailed comparison of the degradation modes caused by off-state and on-state room temperature electrical stress on the DC characteristics of power AlGaAs/GaAs heterostructure field effect transistors (HFET's) for X- and Ku-band applications. The devices are stressed under DC bias conditions that result in electron heating and impact ionization in the gate-drain region. Incremental stress experiments carried out at gate-drain reverse currents up to 3.3 mA/mm (for a total of more than 700 h) show a remarkably larger degradation for the off state stress, due to more pronounced electron heating at any fixed value of gate reverse current. This represents an important piece of information for the reliability engineer when it comes to designing the accelerated stress experiments for hot electron robustness evaluation. The degradation modes observed, all of a permanent nature, include threshold voltage and drain resistance increase and drain current and transconductance reduction     

Evaluation of E2PROM data retention by field acceleration

This work presents an improved theoretical analysis of the physical mechanisms leading to charge loss from the floating gate of electrically erasable PROMs (E²PROMs). The analysis is applied to a simplified model of a FLOTOX cell in order to evaluate data retention characteristics using the applied field as an accelerating factor. A set of experimental results (obtained on virgin as well as on aged cells) is presented and degradation of data retention due to cell aging is examined.     

Analysis of the gate current as a suitable indicator for FET degradation under nonlinear dynamic regime

Electron device degradation, although not directly accounted for, represents a key issue in microwave circuit design. This is especially true when the particular applications involved (e.g., satellite, military, consumer) do not allow or strongly discourage any kind of maintenance. As a matter of fact, in order to account for device degradation in circuit design, a suitable electron device model is needed which is able to predict the performance degradation as a function of the actual electrical regime involved in the device operation. Such a kind of model is not available in literature. In this paper, quantitative results are provided for device degradation indicators which correlate DC and RF stress experiments. These results can be considered an important step toward the definition of a nonlinear model accounting for device degradation.     

On the correlation between drain-gate breakdown voltage andhot-electron reliability in InP HEMTs

By comparing devices with different recess widths, we show that the off-state drain-gate breakdown voltage (BV_DG) may give totally misleading indications on the reliability of lattice-matched InP HEMTs under hot-electron (HE) and impact ionization conditions, from both standpoints of gradual and catastrophic degradation. Since the hot-electron degradation effects observed in our HEMTs are quite common, we believe that our results should be considered as a general caveat whenever indications on HE HEMT robustness are inferred from BV_DG measurements     

Reliability physics of compound semiconductor transistors for microwave applications

This paper reviews the reliability problems of compound semiconductor transistors for microwave applications. These devices suffer from specific failure mechanisms, which are related to their limited maturity, with the exception of the GaAs MESFETs, which exhibit a stable technology and an assessed reliability. The metallizations employed in high electron mobility transistors (HEMTs) already benefit from this assessment. However, HEMT are affected by concerns related to hot carriers and impact ionization. The trapping of carriers and the generation of defects in the different layers are responsible for the observed instabilities. The stability of the base dopant is the main reliability concern for heterojunction bipolar transistors (HBTs). Beryllium outdiffuses from the base into the emitter and causes device degradation. Carbon has a lower diffusivity, but is affected by the presence of hydrogen, which prompts gain variations. Finally the hot carriers reliability concern in SiGe HBTs is briefly reviewed.