Naz E. Islam

Silicon carbide photoconductive switch for high-power, linear-mode operations through sub-band-gap triggering

The analysis of a 6H silicon carbide (SiC) photoconductive switch, designed and packaged for high-power, linear-mode operations, is presented. The switch, fabricated from semi-insulating compensated SiC, is triggered by an optical source with photon energy less than the band-gap energy. Simulation models incorporating the effects of vanadium trap and nitrogen dopant in the compensation material show I-V characteristics that agree with measured values. The photoconductive switch has improved rise-time characteristics as compared to a gallium arsenide (GaAs) switch. The analysis also shows that improved performance at high power is possible through passivation using high-permittivity dielectric near the contact-semiconductor interface and by placing a p+ layer next to the cathode.

Study of High-Power Wideband Terahertz-Pulse Generation Using Integrated High-Speed Photoconductive Semiconductor Switches

A 3-D finite-difference time-domain analysis of a photoconductive-semiconductor-switch-based terahertz (THz) source, integrated with a standard dipole and a large-aperture radiator, is presented. The simulation analysis is based on the coupling of semiconductor equations for charge transport with Maxwell electromagnetic equations. The simulation provides the transient field redistribution, carrier generation characteristics, and the field acceleration as a result of the bias voltage on the device, contributing to the nonlinear behavior of THz-pulse generation. A comparison of the radiation characteristics of the two antenna types shows that the large-aperture antenna produces approximately three times higher radiation amplitude and broader power spectrum than those produced by the dipole antenna.

Compensation mechanisms and the response of high resistivity GaAs photoconductive switches during high-power applications

Photoconductive semiconductor switches (PCSSs) made from semi-insulating (SI) GaAs are the primary switching component of one class of high-power, ultra-wideband (UWB) microwave sources. The high resistivity of the GaAs can be achieved through different processing techniques. The resultant device characteristics of the PCSS such as breakdown voltage, rise time, and turn-on delay will depend on the actual processing technique that was used for the material. Simulation studies comparing an intrinsic material and a high resistivity SI GaAs PCSS grown through the liquid-encapsulated Czochralski (LEC) process with a deep donor and shallow acceptor compensation mechanism highlight these differences. Simulations also elucidate the role of an n/sup +/-doped layer placed next to the cathode, which increases the breakdown voltage of the device. Extending the n/sup +/ layer length beyond the cathode does not yield further improvement but leads to current confinement along a narrow strip that can initiate local heating or burnout. The doping profile of the n/sup +/ layer also affects hold-off characteristics, a faster gradient ensuring better protection of the cathode against the substrate field, and electron injection. Doping the n/sup +/ region with a higher concentration of carbon impurities does not produce the same effect as doping the n/sup +/-SI interface. These material-related issues are critical to further extending the performance characteristics of PCSSs.

Design and characterization of silicon carbide photoconductive switches for high field applications

Characteristics of a silicon carbide photoconductive switch designed for high field applications have been studied. Analyses show that premature breakdown occurs primarily due to impact ionization and subsequent charge accumulation near the anode. For the shallow donor, deep-acceptor-type compensated material, a p+ layer next to the cathode results in field homogenization in the bulk. As a result, the blocking or hold off voltage of the switch could increase beyond the experimentally determined value of 18kV (0.45MV∕cm). Simulations also show that a minimum thickness, equivalent to a diffusion length, of the p+ layer is necessary to avoid premature breakdown. Following illumination, the photocurrent rise time of the switch would also improve by about 25% with the p+ layer near the cathode.

Methodology for interference analysis using electromagnetic topology techniques

For simulation codes based on the electromagnetic topology theory, a method to characterize electromagnetic interference and interactions between an external source and internal circuitry of a semi-shielded system is described. Simulation results based on this technique compare well with previously measured data. Analysis also shows that the high-frequency resonances on the cable voltage and current depend on the length of the cable and its termination impedances.

Growth characteristics of mung beans and water convolvuluses exposed to 425‐MHz electromagnetic fields

Effects of high-frequency, continuous wave (CW) electromagnetic fields on mung beans (Vigna radiata L.) and water convolvuluses (Ipomoea aquatica Forssk.) were studied at different growth stages (pre-sown seed and early seedling). Specifically, the effects of the electromagnetic sources power and duration (defined as power-duration level) on the growth of the two species were studied. Mung beans and water convolvuluses were exposed to electromagnetic fields inside a specially designed chamber for optimum field absorption, and the responses of the seeds to a constant frequency at various power levels and durations of exposure were monitored. The frequency used in the experiments was 425 MHz, the field strengths were 1 mW, 100 mW, and 10 W, and the exposure durations were 1, 2, and 4 h. Results show that germination enhancement is optimum for the mung beans at 100 mW/1 h power-duration level, while for water convolvuluses the optimum germination power-duration level was 1 mW/2 h. When both seed types were exposed at the early sprouting phase with their respective optimum power-duration levels for optimum seed growth, water convolvuluses showed growth enhancement while mung bean sprouts showed no effects. Water content analysis of the seeds suggests thermal effects only at higher field strength.

An FDTD Interaction Scheme of a High-Intensity Nanosecond-Pulsed Electric-Field System for In Vitro Cell Apoptosis Applications

A finite-difference time-domain analysis of a high-intensity nanosecond-pulsed electric-field (nsPEF) system, composed of a pulse-forming line (PFL) and a universal electroporation cuvette, is described. The simulation scheme is based on interactions of 1-D transmission-line equations for the PFL and 3-D Maxwells curl equations for the cuvette volume. Simulations incorporate system adjustment to facilitate maximum transfer of electrical energy from the PFL to the cuvette medium. Experimental validation of the voltage across the cuvette electrodes through the laboratory-constructed nsPEF system with an energy density of ~1 J/cm3 reveals an overall agreement with some discrepancies. The distribution profiles of the transient field inside the cell suspension area during the excitation of 5-kV 10-ns pulses would adequately account for the feasibility of using an integrated model as a design benchmark for the interaction physics of the generated nanosecond pulses and culture vessel. The observed nsPEF effects on cells include increased transmembrane potentials across organelle membranes without permanently damaging the cell membrane, increasing the probability of electric field interactions with intracellular structures.

Effects of dimensional nanoscaling on the optical and electrical properties of crystalline Si thin films

Thin film Si structures between 10 and 200nm in thickness and configured into two terminal metal-semiconductor-metal structures have been characterized for optical and electrical properties. Dark currents, spectral response, dc quantum efficiency, and ultrafast time response up to 400nm femtosecond laser illuminations at low fields have been studied. Dark currents and dc photocurrent measurements showed an increase in the film conductivity between 75 and 35nm, suggesting an increase in the carrier effective velocities due to confinement. An increase in the carrier effective velocity below 75nm was also confirmed through the transient response analysis. The measured spectral responses are in good agreement with Fresnel’s theoretical model for thin film coupling. The electron-limited transient signal has a full width at half maximum (FWHM) approximately 40ps for the 10nm Si film as compared to 490ps for a 200nm structure. For a hole-limited transit time signal the FWHM was about 82ps for the 10nm film as co...

Intersubband Transition in Asymmetric Quantum Well Infrared Photodetector

Intersubband transition in perfect rectangular quantum well is known to be limited by “selection rule” that results from certain symmetries of wavefunctions. This letter presents Greens function-based analysis that incorporates energy state broadening due to optical absorption and shows how stepped quantum well can introduce required “asymmetry” to defy the selection rule by breaking the bound state wavefunction symmetry and thus improve the overall optical absorption. Calculation for AlxGa1-xAs/GaAs/InyGa1-yAs stepped quantum well confirms that coupling among certain energy states increases with asymmetry and, as a result, so does the magnitude of optical absorption. The simulation approach presented is simple, physically intuitive, and extendable to any arbitrary shaped asymmetric quantum well.

A method to characterize the interactions of external pulses and multiconductor lines in electromagnetic topology based simulations

Electromagnetic topology based simulations have been carried out to characterize the response of a semishielded multiconductor line to external electromagnetic pulses. The concept of transfer function generation through an aperture for this simulation technique is explained and a methodology to apply the concept to a multiconductor transmission line is proposed. Using this simulation approach the effects of conductor length, its distance from the aperture, and the response of the cable at various frequencies have been studied. The results compare well with experiments. The length of individual cables, its terminating impedances, and the distance of the individual cable from the aperture influences the behavior of the cables under external threat. Analysis shows that the effects from an external electromagnetic pulse are more severe than from lightning.