Joseph R. Flemish

Optimization of a Photonically Controlled Microwave Switch and Attenuator

A silicon-based photoconductive switch and attenuator for microwave signals has been demonstrated and optimized for low insertion loss and high attenuation response upon illumination from a standard infrared LED. This device is fabricated within a coplanar waveguide for easy integration into a planar antenna for photonic control of an array. A general tradeoff exists between the goals of low insertion loss and high attenuation range. However, design and process enhancements are found to improve the device performance. A device that offers up to 20-dB attenuation with insertion loss of only 0.6 dB at 2.0 GHz is demonstrated. These devices show a frequency range where the insertion phase shift upon attenuation is significantly less than 1°/dB.

Reconfigurable and Adaptive Antennas Using Materials with Variable Conductivity

This paper demonstrates the use of materials with variable conductivity in antenna design. These materials vary their conductivity in response to an electrical stimulus. Making part of an antenna from a variable conductive material provides some control over the resonant behavior of the antenna. This capability is useful for dynamically changing the operating frequency of the antenna or turning elements on and off for an adaptive array.

SF6∕O2 plasma effects on silicon nitride passivation of AlGaN∕GaN high electron mobility transistors

The effects of various plasma and wet chemical surface pretreatments on the electrical characteristics of AlGaN∕GaN high electron mobility transistors (HEMTs) passivated with plasma-deposited silicon nitride were investigated. The results of pulsed IV measurements show that samples exposed to various SF6∕O2 plasma treatments have markedly better rf dispersion characteristics compared to samples that were either untreated or treated in wet buffered oxide etch prior to encapsulation. The improvement in these characteristics correlates with the reduction of carbon on the semiconductor surface as measured with x-ray photoelectron spectroscopy. HEMT channel sheet resistance was also affected by varying silicon nitride deposition parameters.

Reliability of aluminum-bearing ohmic contacts to SiC under high current density

The degradation produced by high current density stressing of a contact to p-SiC consisting of an Al-bearing ohmic contact, a TiW diffusion barrier, and a thick Au overlayer was studied. The test structure allowed for vertical current stressing and the measurement of the specific contact resistance before and after stressing. A threshold current for contact failure was established for the Ti/Al and W/Al contacts, at which a large increase in specific contact resistance was measured and extensive voiding occurred in the ohmic contact region. The high current stressing generated a flux of Al from the ohmic contact layer, through the TiW barrier, to the surface to be oxidized, along with a flux of Au into the ohmic contact layer. The voiding in the ohmic contact layer, caused by the unequal fluxes of Al and Au, decreased the active area of the contact, consequently increasing the current density and the associated effects from electromigration and Joule heating, initiating a runaway event.

Adaptive Nulling Using Photoconductive Attenuators

Traditional adaptive antennas require receivers at each element in the array in order to form the correlation matrix and derive the adaptive weights. This approach is expensive and difficult to implement on many phased array architectures. An alternative is to minimize the total array output power by making only a subset of the elements adaptive. Nulls can be placed in the sidelobes with little effect on the main beam. This paper uses the later approach coupled with new photoconductive attenuators that serve as adaptive amplitude weights. The attenuators on some the elements in a phased array serve as adaptive weights while the remaining elements are uniformly weighted. A genetic algorithm controls the infrared signals to these elements in order to minimize the total output power. Experimental and computer results demonstrate the effectiveness of this approach.

Broadband Linear Array With Photoconductive Weights

This letter presents the design, construction, and testing of an eight-element broadband array with photoconductive weights at each element. The photoconductive weights are activated by an IR LED and vary between on and off with a small reflection coefficient and small insertion loss. Test results for the elements and array are presented. A low sidelobe array pattern is demonstrated by tapering the illumination of the photoconductive weights.

Prepassivation surface treatment effects on pulsed and dc I-V performance of AlGaN∕GaN high-electron-mobility transistors

This study has examined the effects that various prepassivation plasma surface treatments had on pulsed and dc I-V characteristics of AlGaN∕GaN high-electron-mobility transistors. Pulsed I-V current recovery data indicates that C2F6, NH3, O2, and Cl2 plasma treatments can be used in conjunction with plasma-enhanced chemical vapor deposited silicon nitride to significantly reduce rf dispersion. The different prepassivation surface treatments, however, produced little variation in dc I-V parameters such as gate leakage current, interdevice isolation current, and off-state breakdown voltage as compared to untreated passivated samples.

Investigation of polarity effects on the degradation of Pd/Ti/Pt ohmic contacts to p-type SiC under current stress

Degradation of low resistance Pd/Ti/Pt ohmic contacts to p-SiC under high current density stressing and the effects of polarity are presented. The Pd/Ti/Pt contact is compared to the conventional Ti/Al contact and provides a lower contact resistance, smoother surface morphology, lower required annealing temperature, and better stability under current stressing. A thick Au overlayer is added to both metallizations for continuous direct current (dc) and pulsed dc stressing. A 31% increase in total resistance is measured for the Ti/Al contacts after 20 h of continuous dc stressing at 200 mA for 5 μm radius contacts, compared to an increase of only 2.5% for the Pd/Ti/Pt contacts. Degradation of the entire anode and cathode was observed for the Ti/Al contacts, whereas damage to only the leading edge of the cathode of the Pd/Ti/Pt contacts was found. The degradation behavior of the Pd/Ti/Pt contacts is explained by a polarity effect, where the cathode reached a higher temperature than the anode under continuous...

Broadband monopole patch antenna optimization

The tab monopole is a very useful broadband microstrip antenna. This paper optimized the shape and size of the tab monopole at a lower frequency. A microstrip line fed version was also optimized.

Adaptive reflector antenna with smart materials

The paper describes a method for placing nulls in a reflector antenna pattern by incorporating elements which have a reflectivity that can be varied over a wide range using electrical or optical control.