Bibliothek

Notizen: Closed-form solutions for the transient conductivity and the electric field in optically controlled semiconductors have been obtained. The one-dimensional calculations include important effects such as field-independent electrode injection efficiency, displacement current, space charge, carrier trapping, and the influence of trapped charge on the field. The model is restricted to linear phenomena so that, for example, the carrier-velocity dependence upon electric field is neglected. Despite such limitations, the model provides much insight into the operation of the semiconductor switch and is particularly applicable under conditions of low carrier generation rate and velocity saturation. A significant result of the model reveals that even for small departures of the electrode injection efficiency from unity, nonuniform and very intense electric fields will develop, particularly near the electrodes. These large fields may be exaggerated, however, since the onset of a field-dependent injection efficiency will tend to limit such fields. A preliminary comparison of the theory and experiment for the current pulse was conducted for the case of a GaAs target, under conditions of low carrier generation rate. The comparison shows that the linear model is capable of explaining the observed current waveform under such conditions.

Notizen: A technique has been developed that allows single-shot measurement of the electric field over an extended area. A LiTaO3 crystal, covering the entire active area of the GaAs photoconductive switch, coupled the surface field with the polarization of an optical probe pulse. When imaged onto a two-dimensional detector array, this system produced snapshots of the surface field profile with 200-ps time resolution and 3-μm spatial resolution. The technique was used to monitor the collapse of the electric field in the GaAs as it was switched with a λ=1.06 μm optical pulse. The switching speed and generation of field enhancements were found to be dependent on the illumination configuration and the electric field. In particular, the switching efficiency was found to decrease with increasing field.

Notizen: A new technique for generating multikilovolt, nanosecond pulses was demonstrated. The technique relies on the use of a semi-insulating GaAs substrate, which performs the dual functions of energy storage and switching. The energy storage was realized by fabricating disk-shaped electrodes on a 7.5-cm-diameter GaAs wafer, thereby forming a radial transmission line. A mode-locked Nd:YAG laser was used to activate the switch, which is vertically fabricated in the central region of the substrate. This device generated and delivered about a 2 MW pulse with a rise time of less than 500 ps to a 50 Ω load.

Notizen: The reverse-biased performance of a molecular-beam-epitaxy-grown high-power optothyristor has been systematically characterized for pulsed power-switching applications. The device has a P+N-SI-PN+ thyristor-like structure with the bipolar junctions formed by AlGaAs. The semi-insulating (SI) GaAs used is liquid-encapsulated-Czochralski grown, undoped, and 650 μm in thickness. It is found that the reverse-biased optothyristor can be triggered by a light-emitting diode operated at 10−5 W, and miniature semiconductor lasers can trigger the switch with 132 A current using only a 1-mm-diam optical aperture. The reverse switching di/dt and the maximum peak current are reported as a function of blocking voltage. The effects of bipolar junctions on both sides of the SI-GaAs are also reported by comparing the bulk photoconductive current with the optothyristor switched current. It is shown that a laser beam of 0.05 μJ can be used to trigger on and switch about the same current as a 0.3 μJ laser beam, suggesting the possibility of integrating miniature semiconductor lasers and the optothyristors on the same chip to form a portable, compact, high-power solid-state pulser.

Notizen: We demonstrate the use of a magnetic field to switch and to control the direction and polarization of a THz beam radiated from a semiconductor emitter.

Notizen: We report measurement of the influence of electric and magnetic fields on optically induced THz electromagnetic radiation from semiconductors at normal incidence. The measurements show that electric and magnetic fields strongly influence the amplitude and phase of transient carrier-generated THz radiation, but do not affect THz optical rectification generated by the bulk second order nonlinear susceptibility, for static electric fields up to 104 V/cm.

Notizen: Background  The presence in a family of a child or children with epidermolysis bullosa (EB) may have profound psychological implications for other family members.Objectives  To assess the impact of the presence of EB in one or more children on the personal relationships between their parents.Methods  Standardized questionnaires were used.Results  In general, the presence of a child severely affected with EB had profound effects on many aspects of marriage. This included a lack of interest in participating in activities as couples [junctional EB (JEB), 45%; recessive dystrophic EB (RDEB), 25%], a lack of energy to invest in such pursuits (JEB, 82%; RDEB, 50%), limitations in opportunities for sharing nonintimate physical activities (reported by most parents having children with some type of generalized EB), and negatively altered parental sex life (JEB, 55%; RDEB, 39%). This is consistent with the fact that 10%, 64%, 25% and 36% of parents of an affected child with EB simplex (EBS), JEB, dominant dystrophic EB (DDEB) and RDEB, respectively, characterized their relationships as couples as revolving almost exclusively around the day-to-day care of their affected children. The severity of disease in an affected child clearly influenced parental decisions about having more children: 24% and 64% of parents of children with JEB and RDEB, respectively, chose not to have additional children, compared with 26% and 54% of parents with children having EBS or DDEB. This choice was most often pursued via tubal ligation; less often, alternative means of surgical sterilization were chosen. Divorce was common among parents of children with EB (range: 17% in EBS to 31% in JEB) and, with the exception of parents of children with EBS, was usually directly attributed by one or both parents to the profound impact that this disease had exerted on their marriage.Conclusions  Physicians caring for children with EB need to give more consideration to the many psychological factors that may contribute to their patients' well being. They may need to assist these children's parents in seeking support and counselling to prevent destruction of the family unit.